Chem Rev Guide

Hwa Chong Institution H2 Chemistry 9647

C1 Promotional Examinations 2011

Prepared by Chee Yuan Jun

Name: ( ) CT Group: Tested Syllabus:

1) Atoms, Molecules and

Stoichiometry

2) Atomic Structure and

Periodicity

3) Chemical Bonding

4) Gaseous State

5) Chemical Energetics &

Thermodynamics

6) Reaction Kinetics

7) Chemical Equilibrium

8) Introduction to Organic

Chemistry

9) Alkanes

10) Alkenes

11) Arenes

12) Halogen Derivatives

(Halogenoalkanes and

halogenoarenes) (MCQ

only)

Test Format:

Paper Paper Type Marks Duration Remarks

1 MCQ 25 40 min 17 A, B, C, D Type

8 1, 2, 3 Type

2

Section A – Structured

Questions

45 70 min Planning included

Section B – Free Response

Questions

40 60 min 3 choose 2

The Chapters Section 1: Definitions

Section 2: Suggested Response Questions

Section 3: Stochiometry and Calculations

Section 4: Organic Chemistry

Section 5: Planning

How To Use This Resource

This is NOT a Revision Guide – this merely helps you consolidate certain pieces of information that

have appeared in your notes so as to help you prepare for Tuesday's examination. You should only

refer to this and use this ONCE you're done studying. Also note, for Section 2, not ALL the types of

questions are there – those are the more generic ones. You SHOULD be doing Tutorials, Performance

Tasks, TYS, PYP, Rainbow Series to get a larger exposure to questions.

Good Luck

Hwa Chong Institution C1 Promotional Examinations 2011 H2 Chemistry 9647


Section 1: Definitions Ensure that, before you take the promotional examinations, you know the correct chemical definitions of the following terms. These marks can make the difference eventually – every mark matters. Energetics Enthalpy Change of:

- Combustion - Reaction - Formation - Hydration - Fusion - Solution

Lattice Energy Bond Dissociation Energy Bond Energy First Ionization Energy Electron Affinity Equilibrium Le Chatelier's Principle Dynamic Equilibrium Organic Chemistry Optical isomers



Section 2: Suggested Revision Questions Before the promotional examinations, you should ensure that you are at the very least able to explain the following questions – these are the common type of questions that you should be able to explain – they are part of the learning objectives. Do NOT use this as a revision template – you should do that with your own lecture notes. This just tries to help you frame it for the examination. Chapter 2: Atomic Structure

Explain the general trend in the increase in first ionization energy across the period. Explain the two anomalies in the first ionization energy across the period. *Explain why the second ionization energy of selenium is higher than that of arsenic. Explain the reasons for the general trend in first ionization energy down the Group.

Chapter 3: Chemical Bonding

Using structure and/or bonding, explain why: - AlCl3 has a lower boiling point than AlF3. - Calcium has a higher boiling point than sodium - H2O has a higher melting point than HF. - FO2 cannot exist but ClO2 can. - CH4 is soluble in hexane, but not in water. - Bond Angle of NH3 > Bond angle of NF3 - Bond Angle of NH3 < Bond angle of CH4

- BP N2H4 < BP H2O2

- BP N2H4 > BP NH3 *Are you able to explain the different factors that affect b.p./m.p.? State Fajan's Laws and explain what causes atoms to be polarisable or have higher polarizing ability. Explain why F-F bonds have small bond length but small bond energy too. Deduce the identity of elements (using bonding). -> what must I explain? What are the conditions required for hydrogen bonding? Suggest and explain the difference in bp/mp between molecules that can undergo inter+intra H-bonding vs those that only undergo inter. Suggest reasons for solubility/insolubility between - Ion-dipole - Polar-polar - Non-polar-non-polar - Polar-non-polar.

**Suggest why Vitamin A is not soluble.



Gas Laws

Explain why gases behave more ideally under certain conditions. You are required to state these conditions. *The same reasons can be used to explain why there might be a difference in the number of moles of gas present and calculated – simply because that the gas has volume and IMF. State the few assumptions of gas molecules in the kinetic theory.

Chemical Energetics

Suggest reasons for the discrepancies in data you have calculated over: - Bond energy (vis-à-vis those in data booklet) - Enthalpy change of combustion/formation etc. Suggest reasons for a reaction's feasibility, using the formula ΔG = ΔH – TΔS. Study the Block Test question on the relationship between ΔHsol, L.E. and ΔHhyd.

Reaction Kinetics

Explain why it seems that reaction was 0 order with respect to reactant A. Suggest why the reaction between A+ and B+ is slow. What is an autocatalytic reaction? Draw a graph to determine the rate. Explain the mechanisms behind a homogenous and heterogeneous catalyst. ****Suggest ways in which you could monitor the rate of the reaction.

Chemical Equilibrium

Explain why the Kc/Kp values (or any visible observation) would still be the same if I doubled the volumes of solutions added? Why can't the same be said for gases? Suggest why certain conditions are used industrially despite it not obeying LCP.

Organic Chemistry

Explain, between cis- and trans- isomers, which has the higher boiling and melting points and why. Explain why benzene undergoes electrophilic substitution rather than electrophilic addition. Also explain the difference in structure between benzene and ethylene. (C2H4). Explain why benzene is a planar molecule but methylbenzene is tetrahedral w.r.t carbon on methyl group. Draw orbitals to illustrate bonding in the chiral carbon in CH3CH(OH)(Cl).



Section 3: Stochiometry and Calculations Stochiometry General reminders include: - Ensure that you actually multiple and construct equations properly. A lot of people tend to

forgot to divide or multiply by 10 when we withdraw only 25.0cm3 of solution from a 250cm3 one.

Practice Questions

1. A sample of impure magnesium was analysed by allowing it to react with excess HCl. After 1.32g of the impure metal was treated with 100cm3 of 0.750 mol dm-3 HCl, the remaining solution required 50cm3 of a 0.50mol dm-3 aqueous NaOH solution for complete neutralization. Assuming the impurities do not react with the acid, what is the percentage by mass of magnesium in the sample?

2. 1.60g of a metallic oxide MO was dissolved in 100cm3 of 1.0 mol/dm3 HCl. The resulting liquid was made up to 500cm3 of distilled water. 25.0cm3 of this solution required 21.05cm3 of a 0.1020 mol/dm3 NaOH solution for the neutralization. Deduce the relative formula mass of the oxide and hence deduce the identity of the metal.

3. A piece of cupronickel alloy containing nickel was to be tested for its percentage

composition by digesting it with dilute HCl before it is used to mint 10-cent coins. It is known that the only impurity is copper. 0.650g of the metal was dissolved in 200cm3 of 0.1mol/dm3 HCl, and the resulting solution was titrated with 24.0cm3 of 0.1mol/dm3 sodium hydroxide solution. Find the mass of nickel present and hence the percentage composition by mass of the cupronickel alloy.

4. Sulfur is formed when colourless solutions of Na2S2O3, sodium thiosulfate, and HCl, hydrochloric acid are mixed.

Na2S2O3 (aq) + 2HCl (aq) 2NaCl (aq) + SO2 (aq) + S (s) In an experiment, 50.0cm3 of 1.0mol/dm3 sodium thiosulfate is added to 50.0cm3 of 1.0mol/dm3 dilute hydrochloric acid. Find the mass of sulfur produced in the reaction.

Gas Laws Easily one of the easiest calculation type of questions. Ensure that all measurements given are in correct form. i.e. Pa, m3, mol, K. Make sure you change it accordingly if it's not. You MUST also know how to use P1V1/T1 = P2V2/T2 Energetics Ensure that you are able to: - Construct an energy cycle. - Calculate the enthalpy change of formation via bond-breaking/bond-forming method. - Explain situations of feasibility and conditions. - Explain relationships between different enthalpies, or explain discrepancies!



Kinetics Ensure that you are able to compare concentrations so as to determine order of reaction. Also ensure that you are able to plot graphs using given data. Remember to state if the catalyst is homogenous or heterogeneous! Equilibrium REMEMBER Le Chatelier's Principle, and how you NEED to explain it. REMEMBER for Kp, you need to illustrate that this is a PARTIAL pressure. i.e., your calculation must reflection mole fraction. REMEMBER for Kc, you need to divide by the volume present. Also remember that solids and liquids (unless the entire reaction is in liquid form) do NOT appear in your calculations. REMEMBER that a catalyst can increase the rate of BOTH the forward AND the backward reaction.



Section 4: Organic Chemistry – Structural Elucidation, Reaction Mechanisms A lot of Chemistry students cannot seem to get the reaction mechanisms part correct. Free Radical Substitution Remember that the C-H bond does NOT break under uv light conditions – the formation of CH3 radical only occurs after Cl radical is formed. In that respect, C-Cl bond can easily be broken. Also note the explanation why people don't generally like to use FRS. (i.e. hard to control substitution). Free Radical Substitution Mechanism



Reaction of Alkenes If the carbocation is trigonal planar and is chiral, you're more than likely to end up with a racemic mixture, this is so because it has no preference to being attacked from the top or bottom I the second step. Therefore, in an asymmetric alkene, it has equal chances of producing 2 optical isomers. Reaction Mechanisms General rules: - Markonikov's Rule – you DO NOT use this to explain mechanism. Use the idea of "electron

donating, more stable carblecation" etc. The rule is to merely help you remember. - Remember that there are certain groups which are electron withdrawing.

Outline the following reaction mechanisms, stating the reaction conditions required. State the organic compounds formed, and indicate if it is a major or minor product.

1. + NaCl (aq)

2. + Br2 (aq)

3. + HBr

4. + NaBr (l)

5. + conc. HNO3 (aq)

6.

Cl

1-Chloro-butane + NaOH (aq)

7.

Cl

+ NaOH (aq)



8.

Cl Cl

+ Cl2 (aq)

9.

F

+ Br2 (aq)

10. F

F

+ Br2 (methanol)

11. + HBr

12.

Cl

+ Br2 (aq)

Oxidation What happens if I add Hot, acidified, dilute KMnO4 vs Hot, alkaline, dilute KMnO4 Cold, acidified, dilute KMnO4 vs Cold, alkaline, dilute KMnO4 Hot, acidified, concentrated KMnO4 vs Hot alkaline concentrated KMnO4 Cold, acidified, concentrated KMnO4 vs Cold, acidified, concentrated KMnO4

Structural Elucidation is a tricky question – leave it for last if you can. If you're still boggled, at least write down deduction statements – these can earn you marks. Don't spend more than 7 minutes on this question if you really ARE stuck – come back to it later! Also remember that when you're explaining observations to distinguish two organic compounds, explain what you actually observe! Don't just explain it in chemical theory – how do I know how theory works in the lab? Speaking of which, if they ask for simple chemical tests, this means no reflux, and should be done in the laboratory.



Section 5: Planning Question At the A Level Examinations, the Planning Question is worth 12 marks and carries 5% weightage for the candidate's overall A Level Grade. The Topic for the Planning Question for the Promotional Examinations is likely to be Kinetics. (since they've already tested Energetics for Lecture Test) The Practical, specifically, would be SIMILAR (i.e. NOT THE SAME) as Practical 11.

Pre-requisities: A SOLID understanding of - The methods that can be used (initial rate method? Excess concentration?) refer to your Kinetics

notes for more details. - How to observe the rate of reaction (i.e. how to tell when the reaction has stopped) – do I use

pressure to gauge, or colour, or what? Learning Points of Practical 11 - How to plot the graphs. - Suggest why rate is 1000/t - Suggest why volume of a solution is proportionate to concentration.

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Chem Rev Guide