Physical Chemistry
8 marks · NormalIn an experiment, the enthalpy change of neutralisation between sodium hydroxide and hydrochloric acid is determined using calorimetry. The rate of the reaction is also studied using different concentrations of hydrochloric acid.
Describe, in outline, how you would carry out a calorimetry experiment to measure the heat change for the neutralisation reaction.
A student carries out the experiment twice: first with 1.00 mol/dm³ HCl and then with 0.50 mol/dm³ HCl, using the same volume and concentration of NaOH each time. State and explain the effect (if any) changing the concentration of HCl has on (i) the total heat evolved and (ii) the rate of temperature rise during the experiment.
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(a) Add known volumes and concentrations of NaOH and HCl to an insulated (polystyrene) cup. Measure initial temperatures separately, mix the solutions, stir, and record the maximum temperature reached. Use the temperature change and volumes to calculate heat evolved (using Q = mcΔT). · must show use insulated cup, measure and mix solutions, record temperature change, calculate Q
(b) i) If the same volume and concentration of NaOH are used, and hydrochloric acid is always in excess, then decreasing the acid concentration can reduce the total amount of reaction, so less heat is evolved if not all NaOH is neutralised. If enough HCl is still present to neutralise all the NaOH, the total heat evolved remains the same, because the same amount of neutralisation occurs. · must show explains with/out limiting reagent, total heat may not change if acid is still in excess
(b) ii) Lower acid concentration slows the reaction, so the temperature rises more slowly (rate is reduced). This is because there are fewer H⁺ ions per cm³, so collisions and therefore successful neutralisations occur less frequently. · must show rate of temperature rise slower, collision frequency/collision theory
Common mistakes: thinking more concentrated acid increases heat evolved when acid is in excess; not distinguishing between total heat and rate
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Marking points
A1 (a) · 1 mark · Describes use of insulated calorimeter (cup, beaker, etc.) for reaction.
A2 (a) · 1 mark · States need to measure starting and maximum temperature.
A3 (a) · 1 mark · Mentions calculation of Q = mcΔT or use of solution mass/volume.
B1 (b) · 2 marks · Explains effect on total heat with reference to limiting reagent; states total heat unchanged if all NaOH neutralised.
B2 (b) · 1 mark · States the rate of temperature rise is slower with lower acid concentration.
B3 (b) · 1 mark · Explains slower rate using collision theory (fewer H⁺ ions, fewer collisions per second).
B4 (b) · 1 mark · Correctly distinguishes between total heat and rate of reaction.
Worked solution
a) To measure the enthalpy change for neutralisation, use a polystyrene (insulated) cup as the calorimeter. Measure equal volumes of sodium hydroxide and hydrochloric acid of known concentrations. Record their initial temperatures, mix them in the cup, stir, then record the highest temperature. The heat given out (Q) is calculated using Q = mcΔT (mass is the total solution mass).
b) (i) If HCl is always in excess and all NaOH is neutralised, the total heat evolved will be the same for both concentrations because the same amount of reaction occurs. If the lower concentration of acid is insufficient to neutralise all NaOH, less heat is evolved. (ii) At lower concentration of HCl, the rate at which the temperature rises will be slower. This is because there are fewer H⁺ ions per cm³, so successful neutralisations and corresponding heat release per second are fewer (collision theory).