Calorimetry: Measuring Enthalpy Change (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about calorimetry and measuring enthalpy change.

Calorimetry works because a temperature change you can measure on a thermometer can be turned into an enthalpy change you cannot measure directly.

Calorimetry is the measurement of enthalpy changes in chemical reactions. You cannot read an enthalpy change straight off an instrument, but you can read a temperature change, and the equation q = m c delta T converts that temperature change into an amount of heat energy in joules. Dividing that heat energy by the number of moles that reacted gives the enthalpy change per mole.

We take those steps in order: first what calorimetry measures and what a calorimeter is, then the heat equation and the quantities in it, then the step from heat energy to an enthalpy change per mole.

Calorimetry is the measurement of enthalpy changes in chemical reactions. The apparatus is simple: a calorimeter can be made from a polystyrene drinking cup, a vacuum flask, or a metal can.

The diagram shows the polystyrene cup version, with the reaction mixture inside, a plastic lid on top, and a thermometer reading to 0.2 degrees Celsius through the centre. The temperature change of that mixture is the only quantity in the whole method that gets measured directly; everything else is calculated from it.

The energy transferred as heat is calculated with q = m c delta T.

q is the heat transferred, in joules. m is the mass of the water, in grams. c is the specific heat capacity, which is the energy needed to increase the temperature of one gram of a substance by one degree Celsius; for water it is 4.18 joules per gram per degree Celsius. Delta T is the temperature change.

Aqueous solutions of acids, alkalis and salts are assumed to be largely water, so you can just use the m and c values of water. This equation is the step that turns the temperature you read into a number in joules.

Heat energy in joules is not an enthalpy change yet. Enthalpy changes are quoted per mole, so the heat is divided by the number of moles that reacted: Delta H = minus q over n.

q is the heat transferred, in joules, and n is the moles of a reactant or product. Dividing by moles is what turns the temperature change you measured into an enthalpy change per mole. Minus q is used because an increase in temperature and therefore a positive temperature change would be an exothermic reaction and produce a negative enthalpy change.

When there is a rise in temperature, the value for delta H becomes negative, suggesting the reaction is exothermic. When the temperature falls, delta H becomes positive, suggesting it is endothermic.

You must also report calculations to an appropriate number of significant figures. Calculated results can only be reported to the limits of the least accurate measurement, so if the temperature is the least accurate value and it is quoted to two significant figures, the final answer is given to two significant figures as well.

Calorimetry is the measurement of enthalpy changes in chemical reactions.

q = m c delta T converts a measured temperature change into heat energy in joules.

Delta H = minus q over n converts that heat energy into an enthalpy change per mole.

A temperature change read off a thermometer is what gives you the enthalpy change.