IP Chemistry – Chemical Equilibrium and Le Chatelier’s Principle
First of all, this topic on Chemical Equilibrium is not being tested for GCE O-Level Pure Chemistry (syllabus code 6092) in Singapore. It is only being tested in IP Chemistry, IGCSE Chemistry and IB Chemistry.
At the end of the blog post, you will be able to answer three important questions:
- Q1: What is meant by ‘dynamic equilibrium’?
- Q2: How is equilibrium being maintained?
- Q3: How can experimental conditions be manipulated to obtain the maximum yield?
Just like all the Chemistry tuition classes and holiday revision workshops which I have been personally conducting over the last 16+ years, I would like you to have an overview in what you be learning today.
The key points in this post are:
- A) Reversible Reactions
- B) Dynamic Equilibrium
- C) Le Chatelier’s Principle & Position of Equilibrium
- D) Haber Process (Case Study)
A) Reversible Reactions
Many chemical reactions can proceed in one direction only. i.e. they cannot be reversed and they go towards completion.
e.g. Neutralisation reaction between potassium hydroxide & hydrochloric Acid
KOH(aq) + HCl(aq) → KCl(aq) + H2O(l)
Some chemical reactions can be reversible .i.e. reactions can go either directions and they reached an equilibrium, instead of going towards completion.
At equilibrium, the forward and backward reactions do not stop; they continue, but at the same speed. Hence, there is no overall change in the amounts of reactants and products.
At the end of reaction, a mixture of reactants and products is present and they are known to be in equilibrium.
e.g. Haber Process: N2(g) + 3H2(g) ⇌ 2NH3(g)
By altering conditions of temperature, pressure and use of catalysts, these reactions can be adjusted to favour more reactants or more products.
Whether a reaction is reversible or irreversible depends on activation energy.
If the activation energy of the reverse reaction (i.e. Eb) is exceptionally high, then this reaction will be unfavourable and the reaction is described as irreversible.
B) Dynamic Equilibrium
Let’s consider a reversible reaction whereby:
A + B ⇌ C + D
When the mixture of A and B reacts to become C and D, the concentrations of A and B (reactants) decrease with time, while the concentrations of C and D (products) increases with time.
As the concentrations of reactants decrease, the rate of the forward reaction also decreases with time
At the start, the rate of backward reaction is zero because there is no C and D.
As the reaction proceeds, concentrations of C and D increase. Hence, rate of backward reaction also increases.
After a period of time, Dynamic Equilibrium is reached.
Dynamic Equilibrium refers to a reversible reaction in which the rates of forward and reverse reactions have become equal and there is no change in the concentrations of the reactants and the products.
C) Le Chatelier’s Principle & Position of Equilibrium
Summaries the effect of external factors (changes in temperature, concentration or pressure) on a system at equilibrium.
Le Chatelier’s Principle states that if a change is made to a system in equilibrium, the system reacts in such a way as to tend to oppose the change, and a new equilibrium is formed.
Basically, whatever is done to a system in equilibrium, the system does the opposite to it!
- If something is added to a system at equilibrium, the system will behave as to remove it
- If something is removed from the system, the system will behave so as to put it back
Let’s consider 4 external factors: concentration, pressure, temperature and addition of a catalyst.
C.1 Changes in Concentration
If reactants are added (or products removed) in an equilibrium, a new equilibrium is produced which contains a higher proportion of the products i.e. position of equilibrium shifts to the right hand side.
At the new equilibrium, concentrations of the reactants and products are not the same as those in the previous equilibrium.
C.2 Changes in Pressure
Only applicable for systems containing gas(es) only.
If the pressure of an equilibrium mixture is increased (or volume decreased), the mixture will try to reduce the pressure by reducing the number of moles (or molecules) of gases.
- ↑ Pressure favours reaction which produces fewer moles of gas
- ↓ Pressure favours reaction which produces more moles of gas
C.3 Changes in Temperature
This is linked to the concepts you have learned in the topic of Energy Changes (also known as Chemical Energetics or Thermodynamics).
- If temperature is increased, system tries to decrease it by ‘absorbing’ extra heat energy i.e. favours Endothermic reactions
- If temperature is decreased, system tries to increase it by ‘producing’ extra heat energy i.e. favours Exothermic reactions
C.4 Presence of Catalyst
This is linked to the concepts on catalysts which you have learned in the topic of Energy Changes as well as Rate of Reaction.
- When a catalyst is added to an equilibrium, it increases both the forward and reverse reaction rates by the same extent
- The activation energies for both the forward and reverse reactions are lowered by the same extent
- Catalysts shortens the time needed to attain the same final equilibrium concentrations
- It does not affect the position of equilibrium nor the equilibrium composition
- Only increase the rate of reaction so that equilibrium is reached faster
D) Haber Process (Case Study)
N2 (g) + 3H2( g) ⇌ 2NH3(g) ∆H = – 92 kJmol-1
Note that we have covered Manufacturing of Ammonia using Haber Process previously in this blog, which touch on the key concepts and associated keywords necessary for students taking GCE O-Level Pure Chemistry (syllabus code: 6092). We have described how nitrogen gas (from fractional distillation of liquefied air), and hydrogen gas (from cracking of crude oil), as raw materials for producing ammonia in the Haber Process. We have also looked into the essential /optimum conditions for the process. We shall not talk about those here today, so, it’s really good that you refer to that previous blogpost.
Instead, we shall look at how chemical engineers and chemists can use Le Chatelier’s Principle to maximise the yield of ammonia in the Haber Process, by manipulating the conditions in it.
Conditions (Optimum)
1. Temperature: 450 oC
- Le Chatelier’s Principle predicts that lower temperature gives a higher yield of NH3 (Adv)
- If temperature is too low, reaction rate will be too slow (Disadv)
2. Pressure: 200 atm
- Le Chatelier’s Principle predicts that higher pressure gives a higher yield of NH3 (Adv)
- Too high a pressure is dangerous and involve higher cost of maintaining equipment (Disadv)
3. Catalyst: Finely divided Fe catalyst
- Increased the rate of production of NH3 but not the yield of NH3
4. Continual removal of ammonia
- Removal of NH3 shifts position of equilibrium to the RHS i.e. increasing yield of NH3
- Done by cooling the reaction mixture to -50oC to liquefy NH3 formed
- BP of NH3 is around -50oC, while N2 & H2 have lower BPs; will remain as gases. Only NH3 will be liquefied.
I hope you find the content easy for your understanding and if you have any questions, leave me a comment below. Feel free to share this blog post with your friends.
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Mole Concept and Chemical Calculations: Difference between Relative Atomic Mass, Relative Molecular Mass, Relative Formula Mass and Molar Mass
Many students are confused when it comes to the difference between the following terms commonly used in Mole Concept & Chemical Calculations, namely:
- Relative Atomic Mass
- Relative Molecular Mass
- Relative Formula Mass
- Molar Mass
In order to differentiate them (as well as see how they are connected to each other), we have to know their definitions first.
Let’s take a look at them one at a time…..
A) Relative Atomic Mass, Ar
The relative atomic mass of an atom is the average mass of one atom of that element compared to 1/12 of the mass of one carbon-12 atom.
Basically, it is not practical for scientists to use actual masses of atoms in scientific calculations since atoms have very small masses.
As such, scientists compare masses of different atoms with reference to the carbon-12 atom (which is an isotope of carbon).
To be more precise, the masses of all elements (in terms of atom) listed in The Periodic Table are always compared to 1/12 of the mass of one atom of carbon-12.
The symbol for relative atomic mass is Ar.
Relative atomic mass is a ratio and therefore has no unit.
If you refer to the Periodic Table, you will notice that the relative atomic masses of some elements are not whole numbers. These elements exist as a mixture of isotopes (which have different mass numbers).
The relative atomic masses that you see in the Periodic Table are calculated based on the relative percentage abundance of the isotopes. So, relative atomic mass is basically an average value after considering all the different isotopes of the element.
For GCE O-Level Pure Chemistry students (syllabus code 6092) in Singapore, the Periodic Table is not as precise as the one used by the GCE A-Level H2 Chemistry students (syllabus code 9729).
The following Periodic Table is used by GCE O-Level Chemistry students.
Notice that chlorine (atomic number 17) has a relative atomic mass, Ar of 35.5 which is not a whole number. In fact, most elements in the Periodic Table exist as isotopes and thus their relative atomic mass is not a whole number.
If you look at the Periodic Table used by more advanced Chemistry syllabus, such as GCE A-Level H2 Chemistry, most of the elements are not whole numbers. They are presented with 1 decimal place.
Let’s take a look at a classic example on how relative atomic mass is being calculated.
Example:
Chlorine exists as chlorine-35 (% abundance of 75%) and chlorine-37 (% abundance of 25%).
Hence, the relative atomic mass of chlorine = (75/100 x 35) +(25/100 x 37) = 35.5 (as shown in the Periodic Table)
Recap on the definition of Isotopes:
- Atoms of the same element
- Same number of protons and electrons
- Different number of neutrons
A) Relative Molecular Mass, Mr
The relative molecular mass of a molecule is the average mass of one molecule of that element or compound compared to 1/12 of the mass of one carbon-12 atom.
Many elements and compounds exist as covalent molecules.
The mass of a molecule (element or compound) is measured in terms of its relative molecular mass.
The symbol for relative molecular mass is Mr.
Relative molecular mass is also a ratio and therefore has no unit.
To calculate the relative molecular mass of a molecule, you need to add together all the relative atomic masses of all the atoms in its chemical formula.
Due to isotopic effects, actual molecular masses could be different when the atoms of each element present are isotopes.
Let me give you an example.
Carbon is known to exist in two isotopes: carbon-12 and carbon 13.
Oxygen is known to exist in three isotopes: oxygen-16, oxygen-17 and oxygen-18.
As such, we can have carbon dioxide, CO2 molecules with different molecular masses.
Note that for mole calculations, we tend to just use the relative atomic masses (average values) of the elements (as seen in the Periodic Table) and calculate the relative molecular masses (average values).
C) Relative Formula Mass, Mr
The relative formula mass of an ionic compound is the average mass of one unit of that ionic compound compared to 1/12 of the mass of one carbon-12 atom.
Ionic compounds such as sodium chloride and magnesium oxide do no exist as molecules.
As such, it is confusing to use the term relative molecular mass for ionic compounds.
Instead, we use the term relative formula mass.
Relative formula mass is given the same symbol of Mr and have no unit.
It is calculated in the same way as relative molecular mass for molecules, by looking at the chemical formula and adding up the relative atomic masses of atoms.
D) Molar Mass
Molar mass refers to the mass of one mole of a substance (which could be an element or a compound).
- The molar mass of an element (in terms of atom) is equal to its relative atomic mass (Ar) in grams.
- The molar mass of a molecular substance is equal to its relative molecular mass (Mr) in grams.
- The molar mass of an ionic compound is equal to its relative formula mass (Mr) in grams.
Example:
- The relative atomic mass of magnesium is 24. This means that one mole of magnesium atoms has a mass of 24 g. The molar mass of magnesium will thus be 24 g/mol.
- The relative molecular mass of carbon dioxide is 44. This means that one mole of carbon dioxide molecules has a mass of 44 g. The molar mass of carbon dioxide will thus be 44 g/mol.
In terms of Mole Concepts and Chemical Calculations, do note that there is really “no difference” between Molar Mass and Relative Atomic Mass (Ar) or Relative Molecular/Formula Mass (Mr). When it comes to calculation, you will end up with the same numerical answer.
And for this reason, i have seen educators (teachers and professional tutors) use the two terms interchangeably and causally.
Let me give you an example….
Chemists count particles by using a unit known as the Mole (SI unit is “mol”), which are used to determine chemical formulae of substances when they perform calculations.
One mole of any substance contains 6 x 1023 particles. This is known as Avogadro’s constant or Avogadro’s number.
You will probably come across two similar looking formulae involving Mole and Mass when you are googling and searching online.
A) Mole of Element (mol) = Mass of Element (g) / Relative Atomic Mass of Element
Example: Say we have 24 g of magnesism
Answer: Mole of Mg = (24 g) / (24) = 1 mol
B) Mole of Element (mol) = Mass of Element (g) / Molar Mass of Element (g/mol)
Example: Say we have 24 g of magnesism
Answer: Mole of Mg = (24 g) / (24 g/mol) = 1 mol
As you can see, based on SI unit, Formula B is the more precise one because the units will cancel off nicely to give you “mol”.
However, you can also see that both formulae will give you the same answers of “1 mol”.
It also sometimes depends on the syllabus content given by their country’s Ministry of Education. School teachers tend to follow very closely. As a professional Chemistry specialist tutor in Singapore, i also follow it closely, so that i don’t have to argue with students who are attending my GCE O-Level and IP Pure Chemistry Weekly Tuition classes, whenever they say that the formula they saw in their school is slightly different.
Let me show you what i mean.
“Chemistry Matters” Textbook by Marshall Cavendish Education publisher is one of the two approved Pure Chemistry textbooks by Ministry of Education Singapore (MOE) which all Sec 3 and 4 Chemistry students have to get as a reference guide.
The formula to calculate the number of moles of substances is slightly different in 1st Edition (before year 2013) and in 2nd/3rd Edition (2013 onwards).
Formula A is printed in the 1st Edition (for use before Year 2013).
Formula B is printed in the 2nd Edition (for use from 2013-2017). The newest 3rd edition (2014 onwards) also follow this version.
So for all my customised Chemistry notes, YouTube videos and chemistry blogs before 2013, i have been using Formula A. I started to use Formula B from 2013 onwards.
You can see that i was using Formula A (use of relative molecular mass, Mr instead of Molar Mass) in one of my Chemistry YouTube Videos produced in April 2009 and has about 15,000 views. Wow, time really flies, this video is already 11 years old!
Hope the above is clear and you know the differences as well as the connection between the four common terms used in Mole Concept and Chemical Calculations.
Enjoy learning Chemistry with understanding! Get it right the 1st time!
If you have any questions, leave me a comment below. Feel free to share this blog post with your friends.
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PS: Under related articles below, there are several blog post discussions and questions related to Mole Concept and Chemical Calculations. You can also do a keyword search using the search box at the top right hand corner.
O-Level & IP Pure Chemistry: Examination Tips on Elements, Compounds & Mixtures
Matters has been classified as 3 states of matter, namely Solid, Liquid & Gas.
Matters can also be classified as Elements, Compounds & Mixtures.
So, how do we tell if a substance is an element, a compound or a mixture?
You need to know their definitions and properties very well in order to do so.
Let’s get started ………
A) Elements
It is a pure substance that cannot be split into two or more simpler substances by chemical means.
For example, carbon, hydrogen and oxygen are known as elements since they cannot be broken down further into simpler substances.
Elements are represented by chemical names & chemical symbols. The names and chemical symbols of some common elements are listed below:
- Calcium (Ca)
- Carbon (C)
- Chlorine (Cl)
- Hydrogen (H)
- Oxygen (O)
- Iron (Fe)
- Neon (Ne)
- Silicon (Si)
- Sodium (Na)
Very often, you can guess the chemical symbol from the first two letters of the element’s chemical name. You should refer to the Periodic Table for the full list of elements.
Elements can be classified in different ways:
(i) By State – Solids, Liquids & Gases
(ii) As Metals, Non-Metals & Metalloids
Metalloids have properties of both metals and non-metals. e.g. Si and Ge
Elements can exist as atoms or molecules.
An atom is the smallest particle of an element that has the chemical properties of an element.
Each element contains only one type of atom.
Group 0 Noble Gases such as helium, neon, argon, kryton and xenon are known as monoatomic elements i.e. they exists as individual atoms.
A molecule is a group of two or more atoms that are chemically combined together.
Diatomic molecules refers those that are formed by the combination of two atoms. Elements which exists as diatomic elements are hydrogen (H2), oxygen (O2), nitrogen (N2), chlorine (Cl2), etc.
Polyatomic molecules are those that contain three or more atoms. Common example will be ozone (O3) which is triatomic.
B) Compounds
A pure substance that contains two or more elements chemically combined together in a fixed ratio.
A compound can be decomposed by chemical processes such as thermal decomposition and electrolysis (which is the process of using electricity to break down a compound).
A compound may be made up of molecules (covalent compound) or ions (ionic compound). For example, carbon dioxide is a compound made up of molecules while sodium chloride is a compound made up of positive sodium ions and negative chloride ions.
Compounds have a chemical name which indicate the types of elements present.
| Compound | Elements present |
| sodium chloride (common salt) | sodium and chlorine |
| carbon dioxide | carbon and oxygen |
| copper (II) sulfate | copper, sulfur and oxygen |
| magnesium oxide | magnesium and oxygen |
Compounds can also be represented by chemical formulae.
The chemical formula of a compound can be determined by putting the formula of the elements that made up the compound.
The chemical formula shows us the
- types of elements (i.e. atoms) present in the compound
- ratio of the different atoms present in the compound
Tips on writing chemical formula of compound:
| Rules | Examples |
| For compounds that contain both Metallic & Non-Metallic elements, Symbol of Metallic Element is written 1st | Sodium chloride (NaCl) |
| No of atoms of elements is written as subscripts | Water (H2O and not H2O) |
| Not necessary to write subscript “1” | Water (H2O and not H2O1) |
| Oxygen atom is usually written at the end | Carbon dioxide (CO2 and not O2C) |
| Reduce the “subscript” to the lowest term | MgO and not Mg2O2 |
C) Mixtures
A mixture is made up of two or more substances that are not chemically combined.
Mixtures do not have a chemical formula to represent it. This is different from elements and compounds.
Mixtures can be made up of elements or compounds.
The composition of a mixture are not fixed i.e. they can be present in any ratio.
We can have a mixture of:
a) Two elements e.g. helium (He) and oxygen (O2)
b) Two compounds e.g. water (H2O) and carbon dioxide (CO2)
c) Element and compound e.g. oxygen (O2) and carbon dioxide (CO2)
The components of a mixture can be separated by physical separation techniques such as filtration, simple distillation and sublimation, etc.
An example of a widely used mixture is alloys which is defined as a mixture of a metal with other element(s) (can be metals or non-metals). e.g. Steel is made up iron and carbon.
It is important for students to know the four main differences between a mixture and a compound, in terms of:
- Separation
- Properties
- Energy Changes (such as heat/light energy is involved)
- Composition
In order to remember the four main differences easily, we told our Sec 3 and 4 GCE O-Level and IP Pure Chemistry Tuition Classes to remember the mnemonics S.P.E.C.
Difference Between Compounds and Mixtures:
| Compounds | Mixtures | |
| Separation | Can only be broken down into its elements or into simpler compounds by chemical means i.e. thermal decomposition or electrolysis | Components can be separated by physical processes i.e. filtration, simple distillation and sublimation |
| Properties | Physical and chemical properties of a compound are different from that of its constituent elements | Chemical properties of a mixture are the same as that of its components |
| Energy change | A chemical reaction tales place when a compound is formed i.e. energy change which involves heat/light is involved | No chemical reaction takes place when a mixture is formed i.e. no energy change is involved |
| Composition | Elements in a compound are always combined in a fixed ratio | Components of a mixture can be mixed in any proportion |
Let us take a look at an example on how we can apply this table of differences between compounds and mixtures.
Example: Water (H2O) is formed when hydrogen (H2) and oxygen gas (O2) are chemically combined.
| Water (a compound) | Mixture of hydrogen and oxygen | |
| Separation | Can be broken down into hydrogen and oxygen by electrolysis (you will learn this in more detail under the topic of Electrolysis in Sec 4). | Can be separated by physical process such as fractional distillation (you will learn this in more detail under the topic of Separation Techniques in Sec 3). |
| Properties | Water is a colourless liquid while hydrogen and oxygen are colourless gases at room temperature and pressure. Unlike oxygen which supports combustion, water does not. | Uniform mixture of colourless gases at room temperature. Mixture of hydrogen and oxygen support combustion, just like the mixture of hydrogen and oxygen. |
| Energy change | An energy change is observed when water is formed. | No energy change when the mixture is formed. |
| Composition | Ratio of hydrogen atom : oxygen atom is 2:1. | Ratio of hydrogen gas to oxygen gas can vary. |
Let me give you an exam-based multiple choice question (MCQ) on this topic of Elements, Compounds & Mixtures to test your understanding of the key concepts.
Question:
X and Y combine to form XY2. Which of the following statements is true?
A) Ratio of X:Y is 2:1
B) An energy change occurs when XY2 is formed
C) XY2 will have a density in between those of X and Y
D) XY2 will have the properties of both X and Y
Do leave your answers in the comments section below. You can also state your reasons for choosing that particular option (A, B, C or D).
I hope you find the content easy for your understanding and if you have any questions, leave me a comment below. Feel free to share this blog post with your friends.
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PS: Under related articles below, there are several blog post discussions and questions related to Elements, Compounds and Mixtures. You can also do a keyword search using the search box at the top right hand corner.
O Level and IP Pure Chemistry Online Tuition Lessons
Greetings to all students, parents, fellow educators and subscribers.
I hope everyone is doing fine and holding up well in this covid-19 pandemic environment.
Over here in Singapore where i am based, our government has taken the necessary measures and try to contain the situation. Not an easy task i must say. Every few days, there will be new measures and guidelines being introduced.
Lessons in Ministry of Education (MOE) schools have also been affected and many have turned to “online-based learning” which varies from school to school. Some have recorded lectures while others give students the printed out notes for self-studying. At the point of writing, schools have started, stopped and re-started proper live-streaming lectures (according to our students) using Zoom technology. Many students have also feedback to us that they are quite lost about some of the Chemistry topics since the pandemic started. As such, some students are already worried about their upcoming exams, which has been confirmed by Ministry of Education (MOE) Singapore to go ahead as per planned, including the PSLE, GCE O-Level, GCE N-Level and GCE A-Level national examinations.
Currently, we are inside the extension of the Circuit Breaker aka “mini lockdown” and students are having their mid-year school holiday month long break, which is being pushed forward by a month i.e. May holiday instead of the usual June holiday.
Students will start to go back to school starting 2nd June but in a controlled manner in order to practice social distancing. Most levels will go back to school on alternate weeks. Which means one week of school, one week of home based learning, etc etc.
Tuition centres and enrichment centres have also been suspended from physical lessons since the start of the circuit breaker. Initially, we were suspended until 30th April 2020 but it has been further suspended until notice.
With the suspension of centre-based tuition lessons, Winners Education Centre Singapore (where i am teaching Chemistry for the last 16 years) have quickly moved all our Chemistry lessons to Online Live! Learning (OLL) TM system.
It has been about 8 weeks (since early April) with the transition and i am glad that lessons are as “usual” for all our Chemistry students in the Sec 3 and Sec 4 O-Level Pure Chemistry & IP Chemistry Tuition Classes as well as our JC 1 and JC 2 A-Level H2 Chemistry Tuition Classes.
In fact, we did a survey in mid April and received 100% Positive Feedback for our Online Chemistry lessons.
We also have quite a number of students who joined our weekly Chemistry Tuition Classes during this period of compulsory online learning and we are glad they are enjoying the lessons.
We started the OLLTM during the March holidays to test out the best approach for online lessons so that our students continue to understand clearly with the change of teaching medium.
Some of our student’s parents have even commented that their child is learning Chemistry from us in a more focused manner since they are not distracted by their friends or peers. Personally, i have also noticed that our Chemistry students are asking me more questions during the online 2 hours class plus the complimentary 1 hour additional Q & A session, as compared to the physical lessons. I have also received a 5-fold increase in the number of questions being directed to my personal WhatsApp which i helped them to clarify their doubts within 1-2 days.
Personally, the transition from physical lessons in the tuition centre classrooms to online teaching is quite smooth and easy for me. As you may already know, i have been producing and sharing Chemistry Videos on YouTube for more than 10 years and have close to 4,000 subscribers at SimpleChemConcepts YouTube channel (O-Level Pure Chemistry & IP Chemistry Videos) and ALevelH2Chemistry YouTube channel (A-Level H2 Chemistry Videos).
Students, parents as well as educators have to embrace this change in teaching and learning. For us at Winners Education, we are committed to support all our students to continue learning effectively (with crystal clear understanding of Chemistry concepts & application skills).
In Winners Education, to allow our students to adapt to the ‘new’ way of learning, we work harder (even on our rest day now) by having an additional 1 hour Q & A Monday session, complimentary for all our current students. This session will allow students to be exposed to more answering techniques and at the same time clarify their doubts.
Our Online Live! Learning (OLL)TM system is designed to be very similar to our physical lesson where it involves:
- Whiteboard teaching of key concepts
- Exam-based questions discussions
- Ask any questions freely without restrictions
- YouTube Chemistry Videos + Chemistry Blog Posts as additional learning resources
- 24/7 WhatsApp Support for clarification of doubts, just like what many students are already doing
- Graded assignments are to be submitted via online for marking after each topic is completed to ensure consistency and improvement
Our online chemistry tuition lessons currently includes the following levels / syllabus:
- Sec 3 and 4 O-Level Pure Chemistry Online Tuition
- Year 1, Year 2, Year 3 and Year 4 IP Chemistry Online Tuition
- JC 1 and JC 2 A-Level H2 Chemistry Online Tuition
We are grateful that students and parents are embracing this change as we move along with the development of the Covid-19 situation.
Nobody knows when the pandemic situation will be over and things go back to near normal. However, one thing is for sure…the GCE O-Level Examination, GCE A-Level Examination as well as school’s examination will continue as per planned (according to Ministry of Education MOE Singapore).
The all important GCE O-Level Examination and GCE A-Level Examination are just 5 and 6 months away respectively.
Students should stay calm and continue learning Chemistry with interest and understanding.
We are certain that we will all emerge stronger 💪after this episode #SGunited #GLOBALunited
(Online Live!) Lessons as usual at Winners Education. If your child needs help with his GCE O-Level Pure Chemistry, IP Chemistry or GCE A-Level H2 Chemistry, call us today at 88290998 to join the rest of 100+ students to continue learning effectively and Pass Chemistry with Distinction🌈.
To Your “A” in Chemistry,
Sean Chua
O-Level & IP Pure Chemistry: Reaction of Metals with Water, Steam and Dilute Acid
In the previous blog post, we have discussed on the importance of the Reactivity Series of Metals as well as the mnemonic (super memory technique) to remember the sequence of metals in terms of their relative reactivities. In case you have missed that blog post which comes with a learning Chemistry Youtube Video, do check it out.
In the Reactivity Series, metals are arranged from the most reactive to the least reactive.
So how is the order of reactivity of metals being determined?
The order of reactivity of metals are determined by the scientists based on the:
- Reaction of metals with cold water or steam
- Reaction of metals with dilute acids such as hydrochloric acid
Determining the Order of Reactivity of Metals
A) Reaction of Metals with Cold Water or Steam
The more reactive metals tend to react with cold water to form metal hydroxide (alkaline solution) and hydrogen gas.
This reaction can be easily represented by the following word equation:
Metal + Water → Metal Hydroxide + Hydrogen Gas
Note that a more reactive metal will react more violently with cold water.
Some metals such as zinc and iron, do not react with cold water but they do react with steam. Such metals will react with steam to form metal oxide and hydrogen gas.
This reaction can be easily represented by the following word equation:
Metal + Steam → Metal Oxide + Hydrogen Gas
Note that a more reactive metal reacts violently with steam.
Check out the table below on the observations and chemical equation for the reaction of metals with cold water and/or steam.
| Metal | Reaction with Cold Water / Steam |
| Potassium | 2K(s) + 2H2O(l) → 2KOH(aq) + H2(g) violent reaction |
| Sodium | 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g) violent reaction |
| Calcium | Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g) reacts readily |
| Magnesium | Mg(s) + 2H2O(l) → Mg(OH)2(aq) + H2(g) very slow reaction Mg(s) + H2O(g) → MgO(s) + H2(g) violent reaction |
| Aluminium | Al(s) + 2H2O(l) → No Reaction 2Al(s) + 3H2O(g) → Al2O3(s) + 3H2(g) reacts readily |
| Zinc | Zn(s) + H2O(l) → No Reaction Zn(s) + H2O(g) → ZnO(s) + H2(g) reacts readily |
| Iron | Fe(s) + H2O(l) → No Reaction 3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g) reacts slowly |
| Tin | Sn(s) + H2O(l) → No Reaction Sn(s) + H2O(g) → SnO(s) + H2(g) reacts readily |
| Lead | Pb(s) + H2O(l) → No Reaction Pb(s) + H2O(g) → No Reaction |
| Copper | Cu(s) + H2O(l) → No Reaction Cu(s) + H2O(g) → No Reaction |
| Silver | Ag(s) + H2O(l) → No Reaction Ag(s) + H2O(g) → No Reaction |
| Gold | Au(s) + H2O(l) → No Reaction Au(s) + H2O(g) → No Reaction |
B) Reaction of Metals with Dilute Hydrochloric Acid
Most metals react with dilute acids to form a salt solution and hydrogen gas.
This reaction can be easily represented by the following word equation:
Metal + Dilute Acid → Salt Solution + Hydrogen Gas
The reactions of the metals with the dilute acids will also indicate how reactive the metals are and this is used to place them in the Reactivity Series.
A more reactive metal will react more violently with the dilute acid.
Check out the table below on the observations and chemical equation for the reaction of metals with dilute acid.
| Metal | Reaction with HCl(aq) |
| Potassium | 2K(s) + 2HCl(aq) → 2KCl(aq) + H2(g) reacts explosively |
| Sodium | 2Na(s) + 2HCl(aq) → 2NaCl(aq) + H2(g) reacts explosively |
| Calcium | Ca(s) + 2HCl(aq) → CaCl2(aq) + H2(g) reacts violently |
| Magnesium | Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) reacts rapidly |
| Aluminium | 2Al(s) + 6HCl(aq) → 2AlCl3(aq) + 3H2(g) reacts rapidly |
| Zinc | Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g) reacts moderately |
| Iron | Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g) reacts slowly |
| Tin | Sn(s) + 2HCl(aq) → SnCl2(aq) + H2(g) reacts slowly |
| Lead | “No apparent reaction” |
| Copper | No Reaction |
| Silver | No Reaction |
| Gold | No Reaction |
Note that lead should react easily with dilute acids since it is higher than hydrogen in the Reactivity Series.
However, do note that when hydrochloric acid is being used, the initial reaction between lead and hydrochloric acid will form an insoluble layer of lead (II) chloride. This becomes a protective layer and prevents further reaction of the hydrochloric acid with the underlying lead metal. As such, reaction slows down and eventually stops. Hence, lead does not appear to react with hydrochloric acid. This is being heavily covered in my Sec 3 GCE O-Level and IP Pure Chemistry Tuition Classes.
The chemical equation with state symbols for the reaction:
Pb(s) + 2HCl(aq) → PbCl2(s) + H2(g)
Similarly, the above applies when sulfuric acid is being used. Lead does not appear to react with sulfuric acid because of the insoluble layer of lead (II) sulfate coated onto the underlying lead metal.
The chemical equation with state symbols for the reaction:
Pb(s) + H2SO4(aq) → PbSO4(s) + H2(g)
Thus, based on the reactions of metals with cold water, steam and dilute hydrochloric acid, we can place metals in order of their reactivity i.e. Reactivity Series of Metals.
YouTube Video Tutorial on Reaction of Metals with Water, Steam and Dilute Acids
You can watch the YouTube Video below to have a greater understanding of the reactions between metals and water, steam as well as dilute acids.
Click on the following link for the video on O-Level Chemistry . IP Chemistry: Reaction of Metals with Water, Steam and Dilute Acids.
Length of Video: 11.27 minutes
Before we end the post, let me give you a quick check multiple-choice question to test your understanding and applications skills. I have given this question previously to my Sec 3 O-Level Pure Chemistry and IP Chemistry Tuition Classes for discussion.
Question:
Which of the following metal will displace hydrogen from aqueous solutions of acids but not from cold water?
A) Calcium
B) Copper
C) Sodium
D) Zinc
Do write your answers in the Comment section below. Have fun!
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