Question 1

Why is q = mcΔT alone not enough for an exothermic reaction?

Accepted Answer

The solution is not the only heat absorber. The calorimeter walls, lid and thermometer also gain heat, so the reaction heat is calculated as the negative of the total heat absorbed by the solution, calorimeter and thermometer.

Question 2

What is the difference between coffee-cup and bomb calorimetry?

Accepted Answer

A coffee-cup calorimeter works approximately at constant pressure, so the measured heat is q_p and is associated with ΔH. A bomb calorimeter operates at constant volume, so the measured heat is q_v and is associated with ΔU when only pressure–volume work is involved.

Question 3

How is the enthalpy of solution of an ionic solid interpreted?

Accepted Answer

Separating ions from the crystal lattice requires energy, while hydration of the separated ions releases energy. The overall enthalpy of solution is the sum of these two contributions and may be positive or negative.

Question 4

Why does heat loss make the measured enthalpy less exothermic?

Accepted Answer

Some heat escapes to the room instead of warming the solution, calorimeter and thermometer. The observed temperature rise is therefore smaller, so the calculated magnitude of q and ΔH is also smaller.

Question 5

What remains constant during fusion or vaporisation of a pure substance?

Accepted Answer

During a change of state, added or removed heat changes the phase without changing the temperature. The heat involved is Q = mL, where m is mass and L is latent heat.

Calorimetry Lab — Measure the Heat of a Reaction

About this lab

Practice