WARNING: This is a very lengthy post which covers comprehensively the key concepts for students taking the 2019 O-Level Pure Chemistry (code: 6092) Paper 2 Structured & Free Response. The links in blue when clicked will show the relevant YouTube videos or blog posts and will further reinforce your understanding.

For students based in Singapore, you would have taken the O-Level Pure Chemistry syllabus (code: 6092) Paper 3 Practical Exam on Wednesday 10th October 2019. Hope you benefitted from the Tips for O-Level Pure Chemistry Paper 3 Practical Exam 2019 blog post. Several readers told me that they were glad to have read my previous blogpost as I have reminded them on some of the key concepts which will be tested in practicals (and THEY CAME OUT!).

Most of my students in my Secondary 4 weekly tuition classes found the practical to be easy as the three questions asked were quite basic and straight forward. We even managed to kind of spot some of these common possible tests as we were doing our topical revision.

• Question 1 was on Acid-Base Neutralisation and Energy Changes (use of thermometer to determine the temperature of solution). Mole calculations was being tested, as usual.
• Question 2 was on Thermal Decomposition of Sodium Hydrogencarbonate and to find the Percentage by Mass of Sodium Hydrogencarbonate present in an Impure Sample as well as to Identify the Impurity. QA anion tests for chloride, Cl^– and sulfate, SO₄^2- ions were involved. Students were also tested on why excess dilute nitric acid must be added before the addition of AgNO₃(aq) and Ba(NO₃)₂(aq) to test for chloride, Cl^– and sulfate, SO₄^2- ions respectively. All these were covered comprehensively in our weekly revision tuition classes.
• Question 3 was on Planning and students were asked to outline a method to obtain crystals of sodium hydrogensulfate using 1.0 mol/dm³ sulfuric acid and aqueous sodium hydroxide of known concentration.

No matter how you have fared for the Practical Paper 3, you need to look forward to the other two papers which makes up the bulk of GCE O-Level Pure Chemistry syllabus. Paper 2 Structured & Free Response and Paper 1 MCQ have a combined 80% weightage. Scoring well in them will still give you a Distinction A-grade, regardless of the Practical Paper 3.

During the lessons in our Sec 4 Pure Chemistry and IP Chemistry tuition classes, I have shared with my students on a 15 pages checklist on the concepts which will be tested.

Today, I would like to share with you, my readers on some of the key concepts (summarised) you may be tested for your upcoming Paper 2 Structured & Free Response Exam on 30th October 2019.

Format of Paper 2 Structured & Free Response Exam:

First of all, we must know that Paper 2 is worth a total of 80 marks and this constitute 50% weightage of your overall Chemistry grade. The time allocated to finish this paper is 1 hour and 45 minutes.

According to Singapore Examinations & Assessment Board (SEAB), this paper consists of two sections.

Section A will carry 50 marks and consists of a variable number of compulsory structured questions.

Section B will carry 30 marks and consists of three questions:

• The first two questions are compulsory questions, one of which will be a data-based question requiring candidates to interpret, evaluate or solve problems using a stem of information. This question will carry 8–12 marks.
• The last question will be presented in an either/or form and will carry 10 marks.

A copy of the Periodic Table of Elements will be printed as part of this paper and students should make full use of it.

Key Concepts to be tested:

Listed by topics, students must know the following key concepts well before their Paper 2 Exam.

1) Experimental Techniques

• suitable apparatus to measure time, temperature, mass and volume
• suitable apparatus to collect gases and measure rates of reaction

2) Separation Techniques

• separating techniques such as filtration, chromatography, simple distillation, fractional distillation, separating funnel, crystallisation, evaporation to dryness and sublimation.
• interpret paper chromatograms including comparison with ‘known’ samples and the use of R_f values
• use of locating agents in the chromatography of colourless compounds

3) Qualitative Analysis (QA)

• identification of 8 cations
• identification of 5 anions
• identification of 6 gases

NOTE: We might be expecting some more marks in Paper 2 (as well as Paper 1) this year for QA. Do remember the 3 tables of tests! Be prepared!

4) Kinetic Particle Theory

• changes of states and the energy involved
• evidence for the movement of particles in liquids and gases
• explain everyday effects of diffusion in terms of particles
• effect of molecular mass and temperature on the rate of diffusion

5) Atomic Structure

• relative charges and approximate relative masses of a proton, a neutron and an electron
• describe, with the aid of diagrams, the structure of an atom as containing protons and neutrons (nucleons) in the nucleus and electrons arranged in shells
• define proton (atomic) number and nucleon (mass) number
• work on questions involving atomic number and nucleon number
• define the term isotopes

6) Chemical Bondings and Structural Properties

• determine the chemical formula of ionic compounds
• describe the formation of ionic bonds between metals and non-metals, e.g. NaCl; MgCl₂
• state that ionic materials contain a giant lattice in which the ions are held by electrostatic attraction, e.g. NaCl
• relate the physical properties of ionic compounds to their giant crystal lattice structures
• determine the chemical formula of covalent molecules
• describe, using ‘dot-and-cross’ diagrams, the formation of covalent bonds between non-metallic elements, e.g. H₂; O₂; H₂O; CH₄; CO₂
• relate the physical properties of covalent substances to their structure and bonding
• compare the structure of simple molecular substances, e.g. methane; iodine, with those of giant molecular substances, e.g. poly(ethene); sand (silicon dioxide); diamond; graphite in order to deduce their properties
• compare the bonding and structures of diamond and graphite in order to deduce their properties such as electrical conductivity, lubricating or strength
• describe metals as a lattice of positive ions in a ‘sea of electrons’
• relate the electrical conductivity of metals to the presence of mobile electrons in their structures

You may also find my YouTube video on Chemical Bonding Overview useful towards your final revision.

7) Mole Concepts and Chemical Calculations

• construct chemical equations, with state symbols, including ionic equation
• define relative atomic mass, A_r
• define relative molecular mass, M_r
• calculate relative molecular mass (and relative formula mass) as the sum of relative atomic masses
• calculate percentage mass of an element in a compound
• calculate empirical and molecular formulae
• calculate stoichiometric reacting masses and volumes of gases
• calculations regarding limiting and excess reactants
• apply the concept of solution concentration (in mol/dm³ or g/dm³) to process the results of volumetric experiments
• calculate % yield and % purity

For those who are not strong in writing ionic equations yet, fret not because it is actually very easy to do so. You just have to follow a series of standard steps (at least that’s how all my students learned effectively from me) and you will ALWAYS get the answer. You can watch my YouTube videos on Writing Ionic Equations Part 1 and  Writing Ionic Equations Part 2 to master it.

I also have 2 YouTube videos on solving Mole Calculations Part 1 and Mole Calculations Part 2. You can also do a search in this blog for all the relevant Mole Calculations discussion of exam-based questions.

8) Electrolysis

• basic concepts of electrolysis (as in an electrolytic cell)
• describe electrolysis as the conduction of electricity by an ionic compound (an electrolyte), when molten or in aqueous solution, leading to the decomposition of the electrolyte
• describe, in terms of the mobility of ions present and the electrode products, the electrolysis of molten ionic salt such as sodium chloride, using inert electrodes
• apply the idea of selective discharge (with the use of inert electrodes) based on:

1. cations: linked to the metals reactivity series
2. anions: halides, hydroxides and sulfates e.g. aqueous copper(II) sulfate, dilute sodium chloride solution, etc
3. concentration effects (as in the electrolysis of concentrated and dilute aqueous sodium chloride)

• predict the likely products of the electrolysis of a molten binary compound as well as aqueous solution
• construct ionic equations for the reactions occurring at the electrodes during the electrolysis
• describe the electrolysis of aqueous solution which involves the use of reactive electrodes
• describe the electrolysis of aqueous copper(II) sulfate with copper electrodes as a means of purifying copper
• describe the electroplating of metals, e.g. copper plating, and state one use of electroplating
• describe the production of electrical energy from simple cells (i.e. two difference metal electrodes in an electrolyte) linked to the reactivity series and redox reactions (in terms of electron transfer)

9) Energy Changes

• describe the meaning of enthalpy change in terms of exothermic (negative enthalpy change) and endothermic (positive enthalpy change) reactions
• represent energy changes by energy level diagrams, including reaction enthalpy changes
• represent energy changes by energy profile diagrams, including reaction enthalpy changes and activation energies
• describe bond breaking as an endothermic process and bond making as an exothermic process
• explain overall enthalpy changes in terms of the energy changes associated with the breaking and making of covalent bonds
• describe hydrogen, derived from electrolysis of water or cracking of hydrocarbons, as a potential fuel, reacting with oxygen to generate electricity directly in a fuel cell i.e. hydrogen-oxygen fuel cell (note that details of the construction and operation of a fuel cell are not required)
• state the advantages and and disadvantages of hydrogen as an alternative fuel

10) Rate of Reactions

• describe the effect of concentration, pressure, particle size and temperature on the rates of reactions and explain these effects in terms of collisions between reacting particles
• define the term catalyst and describe the effect of catalysts (including enzymes) on the speeds of reactions
• explain how pathways with lower activation energies account for the increase in speeds of reactions
• state that some compounds act as catalysts in a range of industrial processes and that enzymes are biological catalysts
• questions involving rate of reaction experiments and its rate graphs

You may find my YouTube video on the 5 Factors that affects the Rate of Reaction useful towards your final revision.

11) Redox Reactions

• define oxidation and reduction (redox) in terms of oxygen/hydrogen gain/loss, electron transfer and changes in oxidation state
• use of aqueous potassium iodide in testing for oxidising agents from the resulting colour changes
• use of acidified potassium manganate(VII) in testing for reducing agents from the resulting colour changes

You may find my blog post on Tests for Oxidising Agents and Reducing Agents useful towards your final revision. Cambridge examiners love to test students on their concepts.

12) Acids & Bases

• define acid and alkali in terms of the ions they produce in aqueous solution and their effects on Universal Indicator
• how to test hydrogen ion concentration and hence relative acidity using Universal Indicator and the pH scale
• difference between strong and weak acids in terms of the extent of ionisation or dissociaton i.e. strength of acids and alkalis
• basicity of acids
• chemical properties of acids as in reactions with metals, bases and carbonates
• uses of sulfuric acid in the manufacture of detergents and fertilisers, and as a battery acid
• ionic equation for acid-alkali neutralisation is H⁺(aq) + OH^–(aq) → H₂O(l)
• importance of controlling the pH in soils and how excess acidity can be treated using calcium hydroxide
• chemical properties of bases in reactions with acids and with ammonium salts
• classify oxides as acidic, basic, amphoteric or neutral based on metallic/non-metallic character as well as state their chemical properties

You may find my YouTube video on Strength versus Basicity of Acids useful towards your final revision. Cambridge examiners love to test students on their concepts.

13) Salts

• solubility for common salts such as nitrates, chlorides (including silver and lead), sulfates (including barium, calcium and lead), carbonates, hydroxides, salts of Group I cations and ammonium salts
• 3 methods to prepare salts are (a) Titration (b) Ionic Precipitation (c) Acid + Excess Insoluble Substances
• describe the techniques used in the preparation, separation and purification of salts

14) Ammonia

• use of nitrogen (from air) and hydrogen (from the cracking of crude oil) in the manufacture of ammonia
• essential conditions for the manufacture of ammonia by the Haber process
• displacement of ammonia from ammonium salts with the use of strong alkalis

You may find my recent blog post on Ammonia and The Haber Process useful towards your final revision. Cambridge examiners love to test students on their concepts.

15) Periodic Table and Group Trends

• describe the Periodic Table as an arrangement of the elements in the order of increasing proton (atomic) number
• relationship between group number and the ionic charge of an ion of an element
• similarities between the elements in the same group of the Periodic Table in terms of their electronic structure
• change from metallic to non-metallic character from left to right across a period of the Period Table
• relationship between group number, number of valency electrons and metallic/non-metallic character
• Group I alkali metals are relatively soft, low density metals showing a trend in melting point and in their reaction with water
• Group VII halogens are diatomic, non-metals showing a trend in colour, state and their displacement reactions with solutions of other halide ions
• Group 0 noble gases are monatomic elements that are chemically unreactive (full electronic configuration) and hence important in providing an inert atmosphere industrially, e.g. argon and neon in light bulbs; helium in balloons; argon in the manufacture of steel
• transition elements are metals having high melting point, high density, variable oxidation state and forms coloured compounds
• transition elements and/or their compounds are often able to act as catalysts

16) Metals

• metals have high melting and boiling points, malleable, good conductors of heat and electricity
• alloys is a mixture of a metal with another element, e.g. brass; stainless steel
• explain why alloys have different physical properties to their constituent elements (metals)
• metals reactivity series in terms of decreasing reactivity: potassium > sodium > calcium > magnesium > aluminium > zinc > iron > tin > lead > (hydrogen) > copper > silver > gold
• understand the reactions, if any, of the metals with water, steam and dilute hydrochloric acid
• understand the reduction, if any, of their oxides by carbon and/or by hydrogen
• metal displacement reactions in terms of a more reactive metal to displace a less reactive metal in aqueous solution
• metal displacement reactions in terms of a more reactive metal to displace a less reactive metal in a metal oxide
• action of heat on the carbonates of the listed metals and relate thermal stability to the reactivity series
• ease of extracting metals from their ores by relating the elements to their positions in the reactivity series
• describe metal ores as a finite resource and hence the need to recycle metals, e.g. recycling of iron
• discuss the social, economic and environmental issues of recycling metals
• essential reactions in the extraction of iron using haematite, limestone and coke in the blast furnace
• describe steels as alloys which are a mixture of iron with carbon or other metals and how controlled use of these additives changes the properties of the iron, e.g. high carbon steels are strong but brittle whereas low carbon steels are softer and more easily shaped
• uses of mild steel, e.g. car bodies; machinery
• uses of stainless steel, e.g. chemical plants; cutlery; surgical instruments
• essential conditions for the corrosion (rusting) of iron are presence of oxygen and water
• prevent rusting by barrier protection e.g. painting, greasing, plastic coating, galvanising
• prevent rusting by sacrificial protection e.g. underground water pipes have a piece of magnesium attached to them

17) Air & Atmosphere

• volume composition of gases present in dry air is approximately 78% nitrogen, 21% oxygen and the remainder being noble gases (with argon as the main constituent) and carbon dioxide
• name some common atmospheric pollutants, e.g. carbon monoxide; methane; nitrogen oxides (NO and NO₂); ozone; sulfur dioxide; unburned hydrocarbons
• state the sources and adverse effects of these pollutants, as well as how to reduce or prevent them
• redox reactions in catalytic converters to remove combustion pollutants
• use of calcium carbonate to reduce the effect of ‘acid rain’ and in flue gas desulfurisation
• importance of the ozone layer and the problems involved with the depletion of ozone by reaction with chlorine-containing compounds, chlorofluorocarbons (CFCs)
• describe the carbon cycle in simple terms, to include (a) the processes of combustion, respiration and photosynthesis (b) how the carbon cycle regulates the amount of carbon dioxide in the atmosphere

18) Organic Chemistry

• natural gas (mainly methane) and petroleum as sources of energy
• petroleum aka crude oil is a mixture of hydrocarbons and its separation into useful fractions by fractional distillation
• name the seven fractions in the order of increasing boiling points (petroleum gas, petrol aka gasoline, naphtha, kerosene aka paraffin, diesel oil, lubricating oil, bitumens) and state their uses
• describe the issues relating to the competing uses of oil as an energy source and as a chemical feedstock
• an introduction to Organic Chemistry and Organic Compounds
• homologous series refers to a group of compounds with a general formula, similar chemical properties and showing a gradation in physical properties as a result of increase in the size and mass of the molecules, e.g. melting and boiling points; viscosity; flammability
• alkanes refers to the homologous series of saturated hydrocarbons with the general formula C_nH_2n+2
• draw the structures of branched and unbranched alkanes, C1 to C4, and name the unbranched alkanes methane to butane
• define isomerism and identify isomers
• alkanes (exemplified by methane) are generally unreactive but does undergo combustion and substitution reaction by chlorine or bromine in the presence of UV light
• alkenes refers to the homologous series of unsaturated hydrocarbons with the general formula C_nH_2n
• draw the structures of branched and unbranched alkenes, C2 to C4, and name the unbranched alkenes ethene to butene
• manufacture of short chain alkenes, short chain alkanes and hydrogen by cracking hydrocarbons
• cracking is essential to match the demand for fractions containing smaller molecules from the refinery process
• difference between saturated and unsaturated hydrocarbons in terms of their molecular structures
• test for unsaturation using aqueous bromine
• alkenes (exemplified by ethene) undergo combustion, addition polymerisation and the addition reactions with bromine, steam and hydrogen
• meaning of polyunsaturated when applied to food products
• manufacture of margarine by the addition of hydrogen to unsaturated vegetable oils to form a solid product
• alcohols refers to the homologous series containing the –OH group with the general formula of C_nH_2n+1OH
• draw the structures of alcohols, C1 to C4, and name the unbranched alcohols, methanol to butanol
• alcohols undergo combustion and oxidation reaction (to form respective carboxylic acids)
• formation of ethanol by (a) the catalysed addition of steam to ethene known as hydration, and (b) by fermentation of glucose
• uses of ethanol such as: as a solvent, as a fuel and as a constituent of alcoholic beverages
• carboxylic acids refers to the homologous series containing the –CO₂H group with the general formula of C_nH_2n+1COOH
• draw the structures of C1 to C4 carboxylic acids, methanoic acid to butanoic acid, and name them
• carboxylic acids are weak acids, reacting with carbonates, bases and some metals
• formation of ethanoic acid by (a) the oxidation of ethanol by atmospheric oxygen or (b) acidified potassium manganate(VII)
• describe the reaction of a carboxylic acid with an alcohol to form an ester
• commercial uses of esters such as perfumes, flavourings and as solvents
• macromolecules are large molecules (also known as polymers) built up from small units known as monomers
• addition polymer such as poly(ethene) and poly(propene), from their monomers ethene and propene respectively
• uses of poly(ethene) as typical plastic such as plastic bags, clingfilm and water pipes
• condensation polymers such as nylon (a polyamide) and Terylene (a polyester)
• typical uses of man-made fibres such as nylon and Terylene are clothing, curtain materials, fishing line, parachutes and sleeping bags
• pollution problems caused by the disposal of non-biodegradable plastics e.g. land and air pollution

It is important that students go through all the bullet points stated above (as well as ALL my related YouTube videos) and make sure that they know the Chemistry concepts involved. Do take note of all the necessary keywords and chemical equations too.

Feel free to share this blog post with all your friends who are taking GCE O-Level Pure Chemistry (and IP Chemistry).

I would like to sign off by wishing everyone (my own students as well as those I have not met before) the best in their O-Level Pure Chemistry Paper 2!

PS: A quick note that we will be conducting our Score 100% O-Level Pure Chemistry MCQ Workshop on 9th November 2019 to help students to score full marks for their Paper 1 (MCQ Paper) with 30% weightage. We have consistently received positive feedbacks and glowing reviews for this annual workshop and this will be the 11th consecutive year that we are conducting it. Join us if you are committed to UP your overall Chemistry grade.

To Your Chemistry Success,

Sean Chua | Winners Education

“This programme tells me the topics that I am weak in and it emphasises the importance of getting full marks for Chemistry MCQ. I have learnt to manage my time wisely, which is crucial in doing Paper 1 as I usually could not finish the paper. I learnt more about the strategies of doing MCQ and revise on key concepts. I hope I am able to get full marks and I will recommend this to anyone taking O-Levels.”  Cher Yi Jia, Nan Hua High School