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Date	May 2017	Marks available	2	Reference code	17M.3.sl.TZ1.5
Level	SL	Paper	3	Time zone	TZ1
Command term	Outline	Question number	5	Adapted from	N/A

Question

A class was determining the concentration of aqueous sodium hydroxide by titrating it with hydrochloric acid, whilst monitoring the pH of the solution. The sodium hydroxide solution was added into a glass beaker from a measuring cylinder and the hydrochloric acid added using a burette. One group of students accidentally used a temperature probe rather than a pH probe. Their results are given below.

Volume of aqueous NaOH = 25.0 ± 0.5 cm³

Concentration of HCl = 1.00 ± 0.01 mol dm⁻³

The graph of temperature against titre can be used to calculate the concentration of alkali without knowing the concentration of the hydrochloric acid, using the enthalpy of neutralization.

Explain how the concentration may be calculated in this way.

[2]

Heat losses would make this method less accurate than the pH probe method. Outline why the thermometric method would always give a lower, not a higher, concentration.

[2]

Suggest how heat loss could be reduced.

[1]

State one other assumption that is usually made in the calculation of the heat produced.

[1]

Suggest why scientists often make assumptions that do not correspond to reality.

[1]

Outline why the thermochemical method would not be appropriate for 0.001 mol $\,$ dm⁻³ hydrochloric acid and aqueous sodium hydroxide of a similar concentration.

[1]

Markscheme

heat change/evolved can be calculated from the «maximum» temperature increase and the mass of solution
OR
q = mcΔT

heat «evolved» gives the number of moles «of both acid and alkali present when neutralisation occurs»
OR
$n = \frac{q}{{\Delta {H_{neut}}}}$

volume «of acid and the volume of alkali required to just neutralise each other» can be used to calculate the concentration«s of both»
OR
$\left[ {{\text{NaOH}}} \right] = \frac{n}{V}$

[2 marks]

smaller temperature increase/ΔT
OR
heat released would «appear to» be less

amount of substance/n calculated is smaller

[2 marks]

using «expanded» polystyrene cup
OR
insulating beaker
OR
putting a lid on beaker

Do not accept calorimeter by itself.

Accept any other reasonable suggestion.

[1 mark]

«specific» heat capacity of the beaker/container/thermometer is ignored
OR
density of the solutions is assumed as 1.00 g $\,$ cm^–3/same as water
OR
specific heat capacity of the solutions is assumed as 4.18 J g^–1 $\,$ K^–1/same as water

Accept “reaction goes to completion”.

Accept “reaction is conducted under standard conditions”.

Accept “no evaporation occurs”.

Accept any other relevant valid assumption.

Do not accept “heat is not released from other reactions”.

[1 mark]

allows simple theories to be applied to real life situations
OR
enables us to start to understand complex situations
OR
gives answers that are accurate to the required order of magnitude
OR
simplifies the calculations involved

Do not accept “to simplify the situation” without further detail.

Accept “errors do not have a major impact on the results”.

[1 mark]

temperature rise would be too small «to be accurately measured»

Accept “heat released would be too small «to be accurately measured»”.

[1 mark]