9709/12/M/J/13 Questions

10. A student mixed 25cm³ of 0.10moldm^–3 sodium hydroxide solution with 25cm³ of 0.10moldm^–3 hydrochloric acid and noted a temperature rise of 2.5°C. What is the enthalpy change of the reaction per mole of NaOH?

A –209 kJ mol^–1

B –104.5 kJ mol^–1

C –209 J mol^–1

D –522.5 J mol^–1

Reference: 9701/12/M/J/13 Q10

Solution:

Answer: A

Q = mcΔT, where m is mass of solution.

Q = 50 x 4.18 x 2.5 = 522.5 J mol^-1

ΔH = Q/n, where n is the no. of moles of NaOH.

No. of moles of NaOH = MV = 0.10 x 25/1000 = 0.002500 mol

ΔH = 522.5/0.0025 = 209 000 J mol^-1

Since the temperature increases, it is an exothermic reaction. Thus, ΔH = -209 000 J mol^-1 = -209 kJ mol^-1

14. In which row of the table are all statements comparing the compounds of calcium and barium

correct?

	solubility of calcium hydroxide	solubility of barium hydroxide	thermal stability of calcium carbonate	thermal stability of barium carbonate
A	higher	lower	higher	lower
B	higher	lower	lower	higher
C	lower	higher	higher	lower
D	lower	higher	lower	higher

Reference: 9701/12/M/J/13 Q14

Solution:

Answer: D

Going down Group II (magnesium, calcium, strontium, barium), the solubility of hydroxides increases and the thermal stability of carbonates increases.

Thus,

(a) solubility of calcium hydroxide < solubility of barium hydroxide

(b) thermal stability of calcium carbonate < thermal stability of barium carbonate

25. The compound shown is menthol, a naturally-occurring alcohol found in peppermint oil.

When menthol is heated with concentrated sulfuric acid it reacts. The products that form include

compound T.

What could be the structure of compound T?

Reference: 9701/12/M/J/13 Q25

Solution:

Answer: D

Concentrated sulphuric acid is a dehydrating agent. Thus, menthol undergoes dehydration to form an alkene. Thus, B is incorrect.

H is removed from the adjacent carbon atom together with the –OH group. There are two ways:

D is the only compound that is formed based on the diagram above. Thus, D is the answer. 

31.  In which pairs do both species have the same number of unpaired p electrons?

1 Al^2– and O⁺

2 N and Cl²⁺

3 C and Cl⁺

Reference: 9701/12/M/J/13 Q31

Solution:

Answer: B (1 and 2 only correct)

Statement 1:

Al^2- has 15 electrons. The electronic configuration is 1s²2s²2p⁶3s²3p³.  

O⁺ has 15 electrons. The electronic configuration is 1s²2s²2p⁶3s²3p³.  

Looking at the outer electrons only,

both Al^2- and O⁺ have three unpaired p electrons. Thus, statement 1 is correct.

Statement 2:

N has 15 electrons. The electronic configuration is 1s²2s²2p⁶3s²3p³.  

Cl²⁺ has 15 electrons. The electronic configuration is 1s²2s²2p⁶3s²3p³.  

Looking at the outer electrons only,

both N and Cl²⁺ have three unpaired p electrons. Thus, statement 2 is also correct.

Statement 3:

C has 14 electrons. The electronic configuration is 1s²2s²2p⁶3s²3p².  

Cl⁺ has 16 electrons. The electronic configuration is 1s²2s²2p⁶3s²3p⁴.  

Looking at the outer electrons only,

C has two unpaired p electrons but Cl⁺ has one unpaired p electron. Thus, statement 3 is false.

40. An organic compound, Z, will react with calcium metal to produce a salt with the empirical formula CaC₄H₆O₄.

What could be the identity of Z?

1 ethanoic acid

2 butanedioic acid

3 methylpropanedioic acid

Reference: 9701/12/M/J/13 Q40

Solution:

Answer: D (1 only is correct)

Statement 1: Ethanoic acid is CH₃COOH.

Ca + 2CH₃COOH à (CH₃COO)₂Ca + H₂

The empirical formula for the salt is CaC₄H₆O₄. Thus, statement 1 is true.

Statement 2: Butanedioic acid, HO₂CCH₂CH₂COOH.

Ca + HO₂CCH₂CH₂COOH à (OOCCH₂CH₂COO)Ca + H₂

The empirical formula for the salt is CaC₄H₄O₄. Thus, statement 2 is false.

Statement 3: Methylpropanedioic acid, HO₂CCH(CH₃)CO₂H

Ca + HO₂CCH(CH₃)CO₂H à (OOCCH(CH₃)COO)Ca + H₂

The empirical formula for the salt is CaC₄H₄O₄. Thus statement 3 is false.