Calometry and Enthalpy change

Q: What is calorimetry?

A: Measuring heat energy transferred during a reaction by measuring the temperature change of the surroundings.

Q: What is the formula for heat energy?

A: q = mcΔT.

Q: What does q represent?

A: Heat energy transferred, measured in joules.

Q: What does m represent?

A: Mass of the substance being heated or cooled, in grams.

Q: What does c represent?

A: Specific heat capacity, measured in J g⁻¹ °C⁻¹.

Q: What does ΔT represent?

A: Temperature change, calculated as final temperature minus initial temperature.

Q: What is the specific heat capacity of water?

A: 4.18 J g⁻¹ °C⁻¹.

Q: What is the formula for moles from concentration and volume?

A: n = cV, with volume in litres.

Q: What is the formula for moles from mass?

A: n = m/M.

Q: What is the formula for molar enthalpy change from calorimetry?

A: ΔrH° = −q/n, where q is in kJ and n is in mol.

Q: Why is there a negative sign in ΔrH° = −q/n?

A: q measures the heat gained or lost by the surroundings, so the reaction has the opposite sign.

Calculation steps

Calculate ΔT.

Use q = mcΔT.

Convert q from J to kJ.

Calculate moles of the reacting substance.

Use ΔrH° = −q/n.

Include the correct sign and unit.

Common mistake

If the water warms up, q is positive for the surroundings, but the reaction is exothermic, so ΔH is negative.