2024 XMS SEC4 WA2 MS

1 Marking Scheme Sec 4 Chemistry (Pure) WA2 2024 Section A – MCQ 1 2 3 4 5 A C D D C Section B – Structured Questions Qn Marking Scheme Marks 6a Na and K 1 6b Br 1 6c F 1 6d Ti and Ni Overall minus 1 mark if the chemical name was used instead. 1 [Total: 4] 7ai experiment is there corrosion observed on iron metal? what is observed on surface of the iron metal? A yes red-brown deposits B no no red-brown deposits formed A: no deposit, no visible change, no change, no reaction/nothing C yes red-brown deposits formed Any 2 correct 1 mark 1 1 7aii 1. Experiment B. Method: Sacrificial protection. 2. Zn is more reactive than Fe. Hence, it corrodes preferentially / reacts more readily / loses electrons more readily / reacts in place of iron. If Experiment B is not stated, students must mention the Zn/Fe combination in their answers to be awarded the full mark. R: Galvanising. 1 1 7b 1. The oxidation state of iron increases from 0 (in Fe) to +2 in (FeSO4) hence oxidation occurs. 2. At the same time, the oxidation state of copper decreases from +2 (in CuSO4 ) to 0 (in Cu), hence reduction occurs. 1m – increase or decrease in oxidation states 1m – correct calculation of oxidation states 1 1 Mark scheme will use these abbreviations / + R A Ig ref ecf AW AVP ORA OWTTE underline ( ) alternatives statements on both sides of the + are needed for that mark reject accept (for answers correctly cued by the question) ignore as irrelevant with reference to error carried forward alternative wording (where responses vary more than usual) alternative valid point or reverse argument or words to that effect actual word given must be used by candidate (grammatical variants excepted) the word / phrase in brackets is not required but sets the context

2 [Total: 6] Section C – Free Response Questions Qn Marking Scheme Marks 8a 1. The total energy absorbed to break the bonds (in H2 and N2) 2. is less than [1] 3. the total energy released to form the bonds (in NH3) ORA 1&3: 1m 8b Shape – 1m Axis and chemical symbols – 1m Correct Ea and H – 1m (ECF from shape) 3 [Total: 5] 9ai Energy and wavelength are inversely proportional / As wavelength increases, energy decreases. 1 9aii 1. Similarity: colours produced in both fireworks and neon signs are due to the electrons being ‘excited’ / promoted to higher energy level / ‘jumping’ from ground state to a higher energy level (and falling back down, releasing energy as light). 2. Difference: Heat is used to excite the electrons in fireworks, while in neon signs, electricity is used instead. R: If answers do not describe how the colours are formed. 1 1 9aiii Yes (must state but no marks) 1. Down the group, the wavelengths of the emission spectral lines generally decreases. 2. This can be seen from the wavelengths of helium between 667 – 706