Let’s check out a question that was emailed to me by one of the student that read this blog.

Hi

I just want to clarify whether the explanation is correct.

Q) Explain why graphite has a lower melting point than diamond. (1 mark).

The answer is “Graphite has fewer covalent bonds than Diamond.”

What about this answer:

“Graphite has weak van der wals forces between the layers. Lesser energy is required to overcome such forces. As a result, it has a lower melting point than diamond”.

Is this answer acceptable?

From my own understanding, diamond has 4 covalent bonds between the carbon atoms while graphite has 3 covalent bonds between the carbon atoms and weak van der wals forces. More energy is required to break the extra covalent bond in diamond than the weak van der wals forces between the layers of atoms in graphite.

Regards
Weijie

The suggested answer “Graphite has fewer covalent bonds than Diamond.” is INCORRECT and is one of the common mistakes made by students batch after batch every year. Not sure where he found this suggested answer though.

Weijie is correct in his own suggested answer. =)

Graphite  are made up of layers of hexagonal rings that are held together by weak intermolecular  van der Waals’ forces of attraction. Lesser energy is required to overcome such weak forces. As a result, it has a lower melting point than diamond which is made of carbon atoms that are covalently bonded strongly together to form an extensive 3-D network with tetrahedral arrrangement. A large amount of energy is required to break these strong covalent bonds.

Tips:

Covalent Bonds (an intramolecular bond) is much stronger than van der Waals’ forces (an intermolecular bond)

Remember to master this topic on Chemical Bonding if you want to excel in Chemistry. : )Related Articles:

• Chemistry Phenomenon: Diamond, A Giant Macromolecule
• More Error in Marshall Cavendish GCE ‘O’ Level “Chemistry Matters” Textbook – Part 2
• O Level Chemistry: Energy Changes (Exo/Endo) & Bond Energy
• O Level Chemistry Question: Chemical Bonding & Structural Properties
• O Level Chemistry: Which has lower Melting Point

Following are some quick questions that my O-Level Chemistry students asked me last two days as they revise their work and need to clarify some doubts before their exams.

I thought it would be great to share with all my readers (especially if you are one of those going for the exam also). Questions are posted in BLACK while my suggested answers / comments are posted in BLUE.

If you find the post useful to your friends, feel free to share the link with them. =)

Questions from Elizabeth:

Hi Mr Chua, I have 21 questions for you, could you kindly respond accordingly? Thanks ((:

1) What is the precision of measuring cylinder & pipette?

Measuring cylinder is to 1 cm³ accuracy.

Pipette normally is not used for measurement. Instead it is used to transfer specific volume of liquid/solution.
E.g 10 cm³, 25 cm³, 50 cm³, etc.

2) What exactly is calcium chloride?

It is an ionic compound / salt.

3) Is metal sulphate a salt?

Yes. Metal sulfate is a general term. So many examples you can easily think of are: Sodium sulfate, calcium sulfate, magnesium sulfate, etc.
They are all ionic compound / salt.

4) During O’s, for the test for sulphate, do I write:
-add nitric acid, then barium nitrate
OR
-add acidified barium nitrate?

Both can. And you will see either form in the questions in O-Levels. Adding nitric acid means to acidify barium nitrate solution.

5) To test for chlorine, do I use damp litmus paper OR damp blue litmus paper?

Given a choice, we would prefer to write damp blue litmus paper.
It turns from blue to red and then bleached.

6) What exactly happens when liquid freezes?

Liquid state changes to solid state.

Refer to Kinetic Particle Theory topic to understand how the ARRANGEMENT and MOVEMENT changes from L to S state.

7) What is the definition of ‘volatile’?

It refers to liquid that easily changes to gaseous state at room temperature.
E.g. Ethanol in perfume.

Sometimes, they apply it to solids that sublimes also. E.g. Solid iodine sublimes to become gaseous iodine at room temperature.

9) Does concentration affect cations?

Don’t understand this question.

10) For O’s,are these the only soluble hydroxides we need to know- Zinc, Aluminium & Potassium?

We talked about this before. I disagree with your answer above.
We usually remember Group I metal hydroxides are soluble in water (NaOH, KOH, LiOH)
as well as Ba(OH)₂ for Group II metal hydroxides. Ca(OH)₂ is sparingly soluble.

Zinc hydroxide and Alumnium hydroxide are INSOLUBLE! Refer to QA, test for Cations.

11) What does energy level on the energy profile diagram represent?

It shows 3 things.

1. Reactants higher or lower energy than Products
2. Enthalpy change is negative or positive
3. Presence of Activation Energy

12) In exo reactions, why do products have lower energy?

In EXO, enthalpy change is NEGATIVE.

Refer to the formula of:  Enthalpy Changes = H_B-B + H_B-M

Use the formula and reason out the thought process.

H_B-B is endo (+ve) whereas H_B-M is exo (-ve).

You will get it. Refer to textbook if you still dont. They give very good examples to understand.

13) Which is the definition of catalyst?

- Provide an alternative route for the reaction with lower activation energy?
- substance which speeds up the rate of reaction

Yes. Combine both sentence into one answer.

14) For the oxidation of ethanol, for the chemical equation, do I write 2[O] or O₂?

If due to bacterial oxidation, we should write O₂.

If used acidified potassium dichromate (VI) in lab, we normally write as 2[O].

Check textbook to re-affirm.

15) Is Group 1 hydroxides soluble?

Answer above.

16) For Group 1(Apart from SPA) salts, do I use titration or Excess method?

Theoretically, use TITRATION.

17) What are found ‘free’ in the ground?

The more common elements are Gold, Platinum and maybe silver.

18) For reduction by carbon, when do you know it will give Carbon Monoxide/ Carbon Dioxide?

I mentioned this before.

The equation required in your O-Level Pure Chemistry (code: 5072) is showing the use of carbon monoxide as the reducing agent.

Thus, the products from the reduction of iron (III) oxide in Blast Furnace should be MOLTEN IRON and CARBON DIOXIDE.

Forget about the carbon monoxide that your teacher told you it will happen for the prelim papers.

It is not in syllabus.

If O-Level it comes out, they will give you alot of clues if they want you to write the product as MOLTEN IRON and CARBON MONOXIDE.

19) Can Silver and Gold be reduced by Carbon and Hydrogen?

You meant Silver oxide? They can, but it is not necessary. You can simply heat it to get Silver metal.

Gold usually are found uncombined in the earth.

20) Do I write ‘Calcium Sillicate’ OR ‘slag’ for my answer during O level?

It is Calcium Silicate. You have made a spelling error.

Both should be fine if they asked for substance.

But if they ask for chemical name, then you should write Calcium Silicate.

If they asked for Chemical Formula, then you write it as CaSiO₃.

21) When I add bromine to an AlkAne, do I say it decolourise slowly OR there is no observation?

I supposed you meant adding aqueous bromine, Br₂(aq).

Alkanes are saturated hydrocarbons and will not react with aqueous bromine. Thus, no changes. Aqueous bromine remains reddish-brown.

Only alkenes will decolourised the reddish-brown aqueous bromine since they are unsaturated.

Thanks in advance for answering (:

Sent from my iPhone

Questions from Lay Teng:

Her questions are in fact from the Pass With Distinction book that i wrote for Shinglee Publisher.

1) When questions ask about compare & contrast, do we have to mention both Similarity & Differences?

Yes. You need to. Let’s say questions ask us to compare and contrast Evaporation & Boiling process, you are required to mention the similarity and differences.

Similarity: Both involves the change of state from liquid to gas of water.

Differences:

A. Boiling is fast while evaporation is slow

B. Boiling is throughout the whole liquid while evaporation only at the surface

C. Boiling occurs at 100 ^oC (BP of water) while evaporation occurs below the boiling point

2) How come the book state Caesium atom and Caesium ion has different number of shells? And why the atom and ion has the same mass?

The answers given in the book is correct.

Caesium atom has 6 electron shells. When it gives away one electron, it forms Caesium ion which has only 5 electron shells.

Mass number is only determined by the sub-atomic particles present in the nucleus. i.e. protons + neutrons.

Since when atom becomes ion, only electrons are transferred, the mass will stay the same.Related Articles:

• Chemistry Phenomenon – Sublimation Process
• O Level Chemistry – Physical & Chemical Properties of Alkalis
• O Levels Chemistry 2008 Exam Question: Organic Chemistry in Paper 1 (MCQs)
• O Level Chemistry: Assigning State Symbols in Chemical Equations (Exercise)
• O Level Chemistry SPA: Color of Copper Hydroxide Vs Copper Carbonate Vs Soluble Copper Salts

Today, i shall discuss with you an exam-based multiple choice question (MCQ) that test you on your concepts on Group Trends.

Below is an exam-based question send in by one of Sec 3 (2011) O-Level Pure Chemistry student:

Question:

HELLO MR CHUA;
i have chem P2 on friday and i did my school’s past year paper. i asked my teacher this qn, but i don’t understand D:
the ans is D but i don’t understand why (:
Which is most likely to be the most stable compound?
A. NaCl
B. RbCl
C. NaF
D. RbF                             ( D )

Suggested Solution:
Before we start, you need to understand which topic they are testing you on. On the first look, it seemed to be question on Chemical Bonding topic.

But once you realised they asked you about “stability” of the compounds, you need to know they are in fact asking about the chemical reactivity of Group I and VII elements, and thus the stability of the ionic compound that is formed.

The most stable compound must be made up of the most reactive metal and the most reactive non-metal for an ionic compound.

Group I Trend: Reactivity increases down the Gp I i.e. Rb is more reactive than Na

Group VII Trend: Reactivity decreases down the Gp VII i.e. F is more reactive than Cl

As such, RbF (D) will be the most stable compound among the four choices.

Hope you are learning something useful here. Feel free to forward this blog post to your friends if you think they can benefit from it. Keep sharing =)Related Articles:

• O Level Chemistry: Metals / Gases Question
• O Levels Chemistry Questions – Periodic Table & Group Trends
• O Level Chemistry – Periodic Table Mini Series (Part I)
• O Level Chemistry – Periodic Table Mini Series (Part II)
• O Level Chemistry – Periodic Table Mini Series (Part IV)

Many substances, especially ionic compounds, are soluble in water. In order to show the reaction of such substances in water, Ionic Equation which is a simplified chemical equation is being used.

Let’s take a look at the video to see easy it is to write an ionic equation:

Youtube link to video: http://youtu.be/JgOImRN7-_I

As you can see from the video, the general steps to write an ionic equation are:

1. Write the balanced chemical equation
2. Include the state symbols of all the reactants & products
3. For reactants or products that are in aqueous state (aq), split them into their ions
4. Cancel out the spectator ion (s) i.e. ion (s) that appear on both the LHS & RHS of equation
5. Write your Ionic Equation

Now, let’s look at another example on writing ionic equation.

Youtube link to video: http://youtu.be/cgV7m3tXNf8

Hope you are learning something here.

If you find the video useful, feel free to forward it to your friends to benefit from it. Keep Sharing, Keep Learning!

Subscribe to my videos on Chemistry Tips and Phenomenon hereRelated Articles:

• O Levels Chemistry Questions – Mole Calculations / Mole Concepts
• Chemistry Questions – Mole Calculations
• O Level Chemistry – Determination of Ionic Equation & Spectator Ions
• O Level Chemistry: Mole Calculations – Balanced Chemical & Ionic Equations
• O Level Chemistry: Group Trends of Halogens (Gp VII Elements)

In the YouTube video below, i will discuss on the Group VII Elements (commonly known as the Halogens) in terms of their Physical & Chemical Properties. I will also highlight to you what are the possible questions that examiners like to ask in Chemistry examinations.

Direct link to video: http://youtu.be/jChTEHAvrBg

Note: Colour of Iodine Solids is commonly stated to be Purplish-Black (in the video i mentioned it is Black in colour)

Hope you learned something important to UP your Chemistry grades and interests. Do try out the question that was posted at the end of the video. You can leave your comments/solutions in the Comments Section below. Adding a balanced chemical equation for that Halogen Displacement Reaction would show that you are cool!

If you think the concepts discussed are useful to your friends, feel free to forward this blog post to them.

Subscribe to my videos on Chemistry Tips and Phenomenon hereRelated Articles:

• O Level Chemistry: Videos on Writing Ionic Equations
• O Levels Chemistry Questions – Mole Calculations / Mole Concepts
• O Level Chemistry – Periodic Table Mini Series (Part II)
• O Level Chemistry – Periodic Table Mini Series (Part III)
• O Level Chemistry: Periodic Table & Group Trends