Physical Chemistry
8 marks · NormalA student investigates the temperature change when dilute hydrochloric acid reacts with excess solid magnesium carbonate. She collects the following data:
| Volume of HCl added (cm³) | Initial temperature (°C) | Maximum temperature (°C) |
|---|---|---|
| 25.0 | 21.0 | 27.0 |
Separately, she wants to know how fast the reaction is at the beginning.
Use the formula Q = mcΔT to calculate the heat evolved, Q, in joules during this experiment. Assume the final solution volume is 25.0 cm³, density is 1.0 g/cm³, and c = 4.18 J/g°C.
Calculate the average rate of reaction in °C per minute if the temperature rose from 21.0°C to 27.0°C in 2.5 minutes.
If a second experiment is done with half the concentration of hydrochloric acid, predict and explain (using collision theory) how the rate of temperature rise will change, assuming all other conditions are identical.
State one source of error in this type of calorimetry experiment and suggest a way to minimise it.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) 627 J
(b) 2.4 °C per minute °C/min
(c) The rate of temperature rise will be slower because lower concentration means fewer HCl particles per unit volume, so fewer collisions per second between reactants (according to collision theory). · must show fewer collisions per second or collision frequency, rate slower due to concentration
(d) Heat loss to the surroundings is a source of error. To minimise this, use a lid or more insulation on the container (polystyrene cup). · must show names error and suggests minimisation
Common mistakes: Uses wrong units for Q; Confuses initial and maximum temperature; Does not mention frequency of successful collisions; Gives incomplete error minimisation
Show mark scheme and worked solution
Marking points
A1 (a) · 2 marks · Correct use of Q = mcΔT and correct value (allow 5 J error).
A2 (b) · 2 marks · Correct rate calculation (temp change/time) with correct units.
B1 (c) · 2 marks · Sound prediction and collision theory explanation (both required).
B2 (d) · 2 marks · States suitable error and effective minimisation method.
Worked solution
a) Q = mcΔT. Mass of solution = 25.0 g. ΔT = 27.0 – 21.0 = 6.0°C. Q = 25.0 × 4.18 × 6.0 = 627 J.
b) Average rate = (temp change) / time = (27.0 – 21.0) / 2.5 = 6.0 / 2.5 = 2.4 °C per minute.
c) Lower concentration means fewer acid particles per cm³, so there will be fewer collisions per second. The rate of temperature rise will be slower (collision theory).
d) One source of error is heat loss to the surroundings. This can be minimised by insulating the cup (e.g., with a lid or extra insulation).