Measuring Enthalpy Change

To find the Enthalpy change for a reaction what three things do you need to know?

To find the enthalpy change for a reaction, you only need to know three things

— the number of moles of the stuff that's reacting, the change in temperature, and how much stuff you're heating. Experiments that measure the heat given out by reactions are called calorimetry experiments. How you go about doing an experiment like this depends on what type of reaction it is.

For reactions that happen in solution , you just put the reactants in a container and use a thermometer to measure the temperature of the mixture at regular intervals. It's best to use a polystyrene beaker to reduce the amount of heat lost or gained through the sides.

How can you find out how much energy is given out by a combustion reaction?

You can find out how much energy is given out by a combustion reaction by measuring the temperature change it causes as it burns. To find the enthalpy of combustion of a flammable liquid, you burn it in a calorimeter.

As the fuel burns, it heats the water. You can work out the heat energy that has been absorbed by the water if you know the mass of the water, the temperature change (47), and the specific heat capacity of water (=4.18) g-' K-1) - see the next page for the details of how to do this.

Ideally, all the heat given out by the burning fuel would be absorbed by the water, allowing you to work out the enthalpy change of combustion exactly. In practice you always lose some heat to the surroundings, however well your calorimeter is insulated. This makes it hard to get an accurate result.

Also, when you burn a fuel, some of the combustion that takes place may be incomplete, meaning that less energy is given out. Flammable liquids are often quite volatile too, so you may lose some of the fuel to evaporation.

Calculating an accurate temperature change

The most obvious way of finding the temperature change in a calorimetry experiment is to subtract the starting temperature from the highest temperature you recorded. But that won't give you a very accurate value, because of the heat lost from the calorimeter to the surroundings. Instead, you can use a graph of your results to find a more accurate value. Here's what you do:

1. During the experiment, record the temperature at regular intervals, beginning a couple of minutes before you start the reaction.

2.Plot a graph of your results.

3.Draw two lines of best fit: one going through the points from before the reaction started and one going through the points from after it started.

4. Extend both lines so that they both pass the time when the reaction started

5.The distance between the two lines at the time the reaction started (before any heat was lost) is the accurate temperature change (47) for the reaction.

Using the equation q= mcAT

Calculating the standard Enthalpy change of combustion

To calculate the standard enthalpy change of combustion, 4.H°, using data from a laboratory experiment, follow these steps:

Step 1: Calculate the amount of heat lost or gained during the

combustion using q = mcAT and your measured or given values of m and AT. You'll then need to change the units of q from joules to kilojoules, because standard enthalpies of combustion are always given in units of kJ mol.

Step 2: Calculate the number of moles of fuel that caused this enthalpy change, from the mass that reacted. Use the equation:

n is the number of moles of fuel burned.

n = mass

Mr

M is the fuel's relative molecular mass.

Step 3: Calculate the standard enthalpy change of combustion, AH° (in kJ mol), using the actual heat change for the reaction, q (in kl), and the number of moles of fuel that burned, n. Use the equation:

Example

Measuring Enthalpy changes in solution

The same formula can also be used to calculate the enthalpy change for a reaction that happens in solution, such as neutralisation, dissolution (dissolving) or displacement.

Example : measuring an Enthalpy of neutralisation

The enthalpy of neutralisation is the energy change when one mole of water is formed by the reaction of an acid and an alkali.

To find the enthalpy change for a neutralisation reaction, add a known volume of acid to an insulated container (e.g. a polystyrene cup) and measure the temperature. Then add a known volume of alkali and record the temperature at regular intervals. Stir the solution to make sure it's evenly heated. When you've worked out the temperature change (using the method from page 113), you can work out the heat energy given out by the reaction using the formula from the previous page.

Rest of the example

Measuring the Enthalpy change of a dissolution reaction works in a very similar way.

Measuring the enthalpy change of a dissolution reaction works in a very similar way. You just add a known mass of the solid that you're dissolving to a known volume of water in the polystyrene cup and stir, recording the temperature regularly. Once you've found the temperature change, you can use it to work out the energy change for the reaction.

If you're asked to find the molar enthalpy change for this type of reaction, you need to find the enthalpy change per mole of solute dissolved (this is the enthalpy of solution). This means that you need to divide the heat energy change for the reaction by the number of moles of solid that you started off with.

Displacement reactions often mean mixing a solid (such as a metal)

with a salt solution. Some displacement reactions involve mixing two solutions though, so you can just use whichever method fits your reactants.