Measuring Enthalpy Change & Reaction Rate (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about AQA Required Practicals 2 and 3: measuring an enthalpy change, and measuring the rate of a reaction.

Neither practical measures its answer directly. In both, you record readings as the reaction proceeds and then get the quantity you actually want off a graph.

Calorimetry measures the enthalpy change of a reaction through a temperature change. The rate practical measures how quickly a reaction happens by timing it. In neither case is the number you are after read straight off the instrument: for enthalpy, the true temperature change has to be recovered by extrapolating a cooling curve, and for rate it comes from the gradient of a concentration-time graph, or from a plot of one over time. Processing the graph is the skill both practicals are testing.

We'll take calorimetry and the temperature correction graph, then combustion calorimetry, then measuring rate from a graph, and finish with the disappearing cross experiment.

Calorimetry is a technique used to measure changes in the enthalpy of chemical reactions, and a calorimeter can be as simple as a polystyrene drinking cup, a vacuum flask or a metal can.

The energy transferred as heat is q equals m c delta T: mass, specific heat capacity and temperature change. Specific heat capacity is the energy needed to raise the temperature of one gram of a substance by one kelvin, and for water it is 4.18 joules per gram per kelvin. For a reaction in solution, you carry it out with an excess of one reagent and record the temperature over the course of a few minutes. The calculation assumes the solution behaves like pure water, that the container itself absorbs nothing, that the reaction is complete, and that heat losses are negligible.

For reactions that are not instantaneous there is a delay before the maximum temperature is reached, and during that delay the substances are already losing heat to the surroundings, so the true maximum is never actually reached.

Graphical analysis recovers it. Take temperature readings for a few minutes before adding the reactants to get a steady value, add the second reactant and continue recording, then plot the graph and extrapolate the cooling part backwards until it intersects the time at which the second reactant was added. The temperature change you read off there is the one the calculation needs, and it is not a value the thermometer ever showed you.

The second type of calorimetry is combustion. The principle is to use the heat released by a burning fuel to raise the heat content of water, with a simple calorimeter measuring the temperature change.

Record the starting and final temperature, and the starting and final mass of the spirit burner, so the mass of fuel burned can be worked out and converted to moles. Not all the heat produced reaches the water: some is lost to the surroundings and some is absorbed by the calorimeter. Keeping the copper can close to the flame, putting a lid on it, and shielding it from draughts all reduce that. The main sources of error in this experiment are heat losses and incomplete combustion.

Rate of reaction is the change in the amount of reactants or products divided by time. As a reaction proceeds, the concentration of the reactants decreases and the concentration of the products increases, so the rate is not the same throughout the reaction: it changes.

That is why one number will not do. The rate at a particular moment is found from a concentration-time graph by drawing a tangent at that point on the curve, and the tangent gives the rate at that specific time. It is the same principle as extrapolating the cooling curve, in that the answer comes from the graph rather than from a reading.

The disappearing cross experiment shows how rate changes with temperature. A fixed volume and concentration of sodium thiosulfate is warmed to a set temperature, preferably in a water bath so the heating is more even. The acid is added, and the time taken for a cross underneath to disappear is noted. The experiment is repeated for a range of temperatures.

It works because one of the products is sulfur, a solid, which precipitates and makes the solution opaque. Temperature is the independent variable, the time for the cross to disappear is the dependent variable, and the volumes, concentrations, depth of solution and the cross and observer are controlled. Sulfur dioxide is toxic and an irritant, so work in a well-ventilated room or a fume cupboard. Plotting one over time against temperature gives the rate.

In q equals m c delta T, use the mass of the water or solution, not the mass of the fuel or the solid. Check whether you have converted joules to kilojoules before quoting delta H, because if a question specifies the units you can lose marks by leaving the answer in joules.

Forgetting to give delta H a negative sign for an exothermic reaction is a common way to lose marks. And when you calculate a temperature change, do not add 273: a rise of one degree Celsius is a change of one kelvin.

Calorimetry measures an enthalpy change through a temperature change, using q equals m c delta T with the mass of the water. Where the reaction is slow, the true temperature change is recovered by extrapolating the cooling line back to the moment the reactant was added. Rate is found from a concentration-time graph by drawing a tangent, or from the disappearing cross by plotting one over time against temperature. Both answers come off a graph, which is the skill both practicals are really testing.