It is the time of the year again.

GCE A-Level Examination has started in Singapore for academic year 2018 with the science practicals.

Students in Singapore who are taking the A-Level H2 Chemistry syllabus (code: 9729) will be taking their Practical Exam (Paper 4) this Thursday 25th October 2018.

During the last few lessons in our JC2 A-Level H2 Chemistry tuition classes, I have recapped with my students on the key points to take note of for their upcoming Chemistry practical examination. I also realised that a lot of their friends intend to start preparing for their Practical Exam only on the eve of the exam i.e. 24th October 2018.

Today, I would like to share with you readers on some of the key points you should also take note of.

Format of Paper 4 Practical:

First of all, we must know that Paper 4 Practical is worth a total of 55 marks and this constitute 20% weightage of your overall H2 Chemistry grade. The time allocated to finish your practical paper is 2 hours and 30 minutes.

This practical paper consists of a variable number of compulsory practical questions. Students must attempt all the questions and should not skip anyone of them.

One, or more, of the questions will incorporate assessment of Planning (P) and require candidates to apply and integrate theoretical knowledge and understanding from different sections of the H2 Chemistry syllabus. Planning used to be inside Paper 2 but have been shifted to Paper 4 Practical since the start of the new syllabus (code: 9729) in 2017. So this is the 2nd year we have Planning in Paper 4 Practical. Note that the assessment of Planning will have a weighting of 5% and the rest will have a weighting of 95%.

One very interesting point to note about Planning questions is that all these (in the older syllabus), it is just tested as a written practical theory questions. No actual hands-on practical is required for this particular question. However, i just learned from my students that the Planning question this year in H2 Biology Practical requires them to not only write out the practical theory BUT also to carry out the experiment. Could this trend or phenomenon be extended to 2019 H2 Chemistry Practical (Paper 4) this year!? Be mentally prepared at least so that you wont get a shock if it really happens. Let’s condition ourselves mentally and emotionally first, as part of our preparation of revising for Practical exam.

Do also note that the assessment may include questions on data-analysis which do not require practical equipment and apparatus at all. It requires the treatment of given experimental data in drawing relevant conclusion and analysis of proposed plan. As such, students should revise their theory and know their concepts well before going to their practical exam.

Candidates are not allowed to refer to notebooks, textbooks or any other information during the assessment.

A copy of the Notes for Qualitative Analysis will be given to students for reference, which includes the procedures and observations when carrying out tests to test for (a) Cations, (b) Anions, (c) Gases and (d) Colour of Halogens.

 

[Source: https://www.seab.gov.sg/content/syllabus/alevel/2017Syllabus/9729_2017.pdf]

Topics and Skills to be tested:

Before going to the Chemistry Practical exam, you must first be very sure of the type of questions (and thus the hands-on practicals) that you will be tested. You should have some experience by now, based on those Chemistry practicals sessions you went through with your school teachers. Check your practical worksheets to revise.

You must be able to use appropriate apparatus/equipment to record a range of measurements such as mass, length, time, volume and temperature.

In addition, you will be expected to handle a range of experimental techniques such as:

1) Titration

• acid-base titration (with suitable indicators such as methyl orange, screened methyl orange, thymolphthalein and thymol blue)
• redox titration e.g.acidified KMnO₄ or acidified K₂Cr₂O₇ or iodine-thiosulfate titration
• iodimetric titrations which are used to quantify reducing agents. In this type of titration the species whose quantity is to be determined is titrated with iodine. In the reaction, iodine gets converted into iodide, which can be detected using starch solution as indicator. So it is a direct method.
• indirect titration, also known as back titraion where the concentration of an analyte is determined by reacting it with a known amount of excess reagent. The remaining excess reagent is then titrated with another, second reagent. The second titration’s result shows how much of the excess reagent was used in the first titration, thus allowing the original analyte’s concentration to be calculated. You may be asked to prepare standard solutions.
• precipitation titration e.g. Mohr’s titration where you will be asked to determine [Cl^–] by titration against AgNO₃ using K₂CrO₄ as indicator
• other types of titrations may also be required, where appropriate, sufficient working details will be given.

2) Gravimetric analysis

• volatilisation gravimetry whereby the amount of an analyte (the ion being analysed) is determined through the measurement of mass i.e. comparing the masses of two compounds containing the analyte
• e.g. use heat to decompose a compound to give solid residue and gaseous product (resulting in change in mass), and the difference in mass account for mass lost

3) Gas collection

• determine the molar volume of hydrogen by measuring the volume of hydrogen gas produced when zinc is reacted with an excess of dilute hydrochloric acid. You may be asked to use PV=nRT to calculate the mole of gas collected.

4) Thermochemistry

• use of styrofoam cup and thermometer for simple
• calculate the heat capacity of the water-beaker calorimeter
• determine the enthalpy change of reaction such as metal displacement reaction i.e. Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
• determine the enthalpy change of reaction between sodium hydrogencarbonate, NaHCO₃(aq) and hydrochloric acid, HCl(aq)
• thermometric titration whereby the maximum temperature reached during the reaction between two reagents e.g. hydrochloric acid and sodium hydroxide solution is measured . The volumes that have reacted at the highest temperature, represent the ‘end point’ of the titration

5) Chemical kinetics i.e. to measure the reaction rate and subsequently to determine the rate equation

(a) continuous method:

• usually one experiment
• many readings collected
• physical methods which involve the continuous measurement of a physical property such as colour intensity, volume of gas produced of the reactants or products per unit time e.g. follow the rate of production of carbon dioxide during the thermal decomposition of metal carbonate
• chemical methods which involve the continuous measurement of [A] at various time intervals by sampling, quenching and titration e.g. follow the rate of the acid-catalysed iodine-propanone reaction using a titrimetric method (sodium thiosulfate as titrant)

(b) initial rate method:

• at least two experiments
• one reading from each experiment
• measure time taken for a particular observation
• physical method such as colorimetry
• clock reaction methods such as those which uses sodium, potassium or ammonium peroxodisulfate to oxidise iodide ions to iodine. Sodium thiosulfate is used to reduce iodine back to iodide before the iodine can complex with the starch to form the characteristic blue-black color.  Iodine is generated: 2 I⁻ + S₂O₈²⁻ → I₂ + 2 SO₄²⁻ and is then removed: I₂ + 2 S₂O₃²⁻ → 2 I⁻ + S₄O₆²⁻. Once all the thiosulfate is consumed the iodine may form a complex with the starch.
• thiosulfate-acid reaction to study the effect of concentration of reactants and temperature on the reaction rate i.e. Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + SO₂(g) + S(s)

6) Qualitative inorganic analysis as detailed in the Qualitative Analysis Notes

• test for 11 cations
• test for 9 anions
• test for 6 gases

Reactions involving ions not included in the Qualitative Analysis Notes may be tested: in such cases, you will not be expected to identify the ions but only to draw conclusions of a general nature.

7) Qualitative organic analysis requiring a knowledge of simple organic reactions and you will be tested on observation skills and drawing general conclusions only

• test-tube test for the presence of unsaturation (C=C)
• test-tube test for alcoholic (-OH) group
• test-tube test for phenolic group
• test-tube test for carbonyl (C=O) group
• test-tube test for carboxyl (-COOH) group
• test-tube test for amino groups

8) Simple organic synthesis and purification, including use of water bath, setting up and use of reflux and distillation apparatus

9) Identifying Transition Metal Cations in Aqueous Solutions

A potential questions could be like this: You are provided with five 1 cm³ aqueous solutions, each containing a different metal cation. The aim is to identify the cation present in each solution by carrying out simple chemical tests.

• First, you need to record the colour of each aqueous solution.
• Test 1: Add a few drops of sodium hydroxide solution. Record your observations. Then add an additional 5 cm³ of sodium hydroxide solution, shake well and record your observations.
• Test 2: Add a few drops of ammonia solution. Record your observations. Then add an additional 5 cm³ of ammonia solution, shake well and record your observations.
• Test 3: Add concentrated HCl gradually until 5 cm³ have been added. Shake well and record your observations.
• Test 4: Add a few drops of sodium carbonate solution. Record your observations.

Using your understanding of the essential concepts in the topic of Transition Metals, you will be required to write balanced chemical equations to explain the phenomenon observed.

You may find the following links from chemguide.co.uk useful for your quick revision:

• The reactions of the hexaaqua ions with hydroxide ions
• The reactions of the hexaaqua ions with ammonia
• The reactions of the hexaaqua ions with carbonate ions

It is also useful to know the specific Chemistry of some of the Transition Metal Ions such as: vanadium, chromium, manganese, iron, cobalt and copper.

Common Reagents Available:

It is also good to know what are the common chemical reagents which are usually made available for practical exam.

According to Singapore Examinations and Assessment Board (SEAB), they are:

• hydrochloric acid (approximately 2.0 mol / dm³)
• nitric acid (approximately 2.0 mol / dm³)
• sulfuric acid (approximately 1.0 mol / dm³)
• aqueous ammonia (approximately 2.0 mol / dm³)
• aqueous sodium hydroxide (approximately 2.0 mol / dm³)
• aqueous barium nitrate (approximately 0.2 mol / dm³)
• aqueous silver nitrate (approximately 0.05 mol / dm³)
• limewater (a saturated solution of calcium hydroxide)
• aqueous potassium manganate(VII) (approximately 0.02 mol / dm³)
• aqueous potassium iodide (approximately 0.1 mol / dm³)
• aluminium foil
• red litmus paper
• blue litmus paper
• Universal Indicator paper
• hexane

For GCE A-Level H2 Chemistry Examination (code: 9729), there will still be Paper 2 (Structured Questions) with 30% weightage, Paper 3 (Free Response Questions) with 35% weightage and Paper 1 (MCQ Paper) with 15% weightage after Paper 4 (Practical Paper). As such, it is important that students continue to revise their theories and concepts all the way until the last paper.

A quick note that we will be conducting our annual Score 100% A-Level H2 Chemistry MCQ Workshop between 22nd-24th November 2018 to help students to score full marks for their Paper 1 (MCQ Paper) with 15% weightage. We have consistently received positive feedbacks and glowing reviews for this annual workshop and this will be the 6th consecutive year that we are conducting it. Join us if you are committed to UP your overall H2 Chemistry grade.

“Sean was engaging throughout the MCQ workshop from 9am-5pm. He was very thorough when going through the questions and made lessons fun despite the workshop being so long. I am glad that i joined the workshop as it force me to study whereas if i had not signed up, i will be probably sleeping and resting already.”

Dana Lim, Raffles Institution (Junior College)

To Your Chemistry Success,

Sean Chua