Measuring energy changes

Burning fuels is a great way of introducing energy changes. Candles on birthday cakes, fireworks on 5 November, Formula One racing cars and even the family barbecue provide excellent contexts to engage students and to link experiments with their everyday lives.

Underpinning chemistry and progression

Awareness that chemical change usually involves energy change is first introduced at age 7–11 while looking at properties and change of materials. The next step, at 11–14, is to equip students with a vocabulary they can use to describe these changes. Finally, at 14–16, we provide them with opportunities to measure the energy changes that occur.

Developing practical skills and progression

At 11–14 students should have the opportunity to carry out, and see demonstrated, a range of experiments so they share a common experience with teachers. Refer to these experiments while developing appropriate vocabulary and theoretical frameworks to describe what is happening. Experiments should include both exothermic and endothermic changes.

Reusable hand warmers are a great example of using exothermic reactions. One common type makes use of the exothermic crystallisation of a supersaturated solution of sodium ethanoate. The pack is recharged by heating it in a pan of boiling water, which dissolves the crystals back into a small volume of water.

At 14–16, students move from observing energy changes to measuring them. All the exam boards include measuring energy changes in their lists of GCSE practical activities. The skill of using a thermometer to measure temperature is fairly straightforward. What makes these experiments more demanding is the sequence of steps and the dexterity required in burning fuel experiments.

Practical problems and suggested solutions

Measuring energy changes in solution

Quite accurate values can be obtained for energy changes in solution using simple apparatus such as an expanded polystyrene coffee cup. These cups are easily knocked over so it is a good idea to support them in a glass beaker. A lid on the cup with a hole for a thermometer also helps as it will cut down heat loss from the surface of the liquid.

Simple experiments such as adding acid to alkali, reactive metal to acid or dissolving a salt in water are ideal opportunities for students to devise their procedure for themselves so that they learn not only what steps to take but think through why they carry them out in a particular sequence. Asking students questions about what amounts of reactants to use and why stirring is helpful can provide useful scaffolding.

Energy changes of reactions in solution can also be measured with a reduced scale technique using a glass vial surrounded by cotton wool as an insulating material. Targeted questions can be used to bring out the advantages of this method in terms of reduced amount of materials to use and dispose of.

Measuring energy changes when fuels are burned

A popular experiment uses different alcohols in spirit burners to heat up water in a glass beaker, conical flask or metal calorimeter.

It is better to use a different spirit burner for each alcohol rather than using the same burner for different alcohols as this avoids the need for burners to be emptied and refilled. Take care to keep the stock bottles of alcohols well away from the area where the burners are used and to make sure that the burners are allowed to cool down after use before adding more fuel.

This type of experiment provides an opportunity to ask students rich questions that will really probe their understanding:

• What is the black deposit that forms on the underside of the calorimeter and how might it affect the accuracy of the experiment?

The deposit is soot, which suggests incomplete combustion of the fuel has taken place. This will reduce the value of the energy change.

• Why is it a good idea to weigh the spirit burner as soon as the flame is extinguished?

To minimise the loss of fuel through evaporation before the burner is reweighed.

• Why is it better to compare the mass of different fuels that have to be burned to produce the same temperature rise rather than simply burning different fuels for the same time?

So that we are comparing the same amount of heat transferred to the water each time.

• What assumption do we make when comparing the energy changes associated with burning different alcohols?

That the proportion of the energy from burning the fuel that is used to heat up the water in the calorimeter is the same each time.

• What steps could we take to reduce the heat losses?

Shield the flame using Bunsen burner mats and use a copper calorimeter with a lid instead of a glass flask.

Interpreting information from energy change experiments

At 14–16 students should be encouraged to use their graphical and arithmetic skills to make sense of their measurements.

Although an expanded polystyrene cup reduces heat losses to the surrounding air in an exothermic reaction, it does not eliminate them – so the maximum temperature recorded will be slightly lower than it would be without the heat loss. One way to get round this problem is to take temperature measurements of the solution at regular intervals from just before the reactants are mixed until after the temperature begins to fall. A graph of temperature plotted against time can be used to estimate what the maximum temperature would have been if there had been no energy loss to the air.

When burning different alcohols, students can compare the mass of fuel needed to produce the same temperature change. They can also calculate a value for the energy transferred by using the equation:

Energy (J) = mass of water heated (g) x temperature change (°C) x 4.2 (specific heat capacity of water)

This value can be converted into energy per gram or energy per mole to lead to an interesting discussion about which value is most useful.