Topic 5: Energy changes

Every chemical reaction involves a transfer of energy with the surroundings, usually felt as heat. In this topic you will learn to classify reactions as exothermic or endothermic, sketch and read reaction profiles, and use bond energies to work out how much energy a reaction releases or takes in. The chemistry of electrical cells, batteries and hydrogen fuel cells is also covered.

Exothermic and endothermic reactions

An exothermic reaction transfers energy from the reacting chemicals to the surroundings, so the temperature of the surroundings rises. An endothermic reaction takes in energy from the surroundings, so the temperature of the surroundings falls. The total amount of energy stays the same overall, it is simply moved between the chemicals and their surroundings, which is the law of conservation of energy. A simple way to tell them apart in the lab is to measure the temperature of the reaction mixture before and after: a rise means exothermic, a fall means endothermic.

Everyday examples

Exothermic changes are very common. Combustion (burning fuels), most oxidation reactions such as rusting, and neutralisation of an acid by an alkali all give out heat. Self-heating cans for drinks and reusable hand warmers use exothermic reactions to keep you warm. Endothermic changes take in heat and are less obvious. Thermal decomposition, such as breaking down calcium carbonate by heating, is endothermic, and the reaction between citric acid and sodium hydrogencarbonate (the fizz in some sweets and sherbet) feels cold. Instant cold packs used to treat sports injuries rely on an endothermic process to draw heat away from the skin.

Reaction profiles and activation energy

A reaction profile is a diagram showing the energy of the chemicals as a reaction proceeds, with energy on the vertical axis and progress of reaction on the horizontal axis. For an exothermic reaction the products sit at a lower energy than the reactants, and the difference between the two levels is the energy released. For an endothermic reaction the products sit higher than the reactants. In both cases the line rises to a peak before settling at the product level. The height of this peak above the reactant level is the activation energy: the minimum energy that colliding particles must have for a reaction to happen. A larger activation energy means the reaction is harder to start.

Bond breaking and bond making

During a reaction the bonds in the reactants must be broken and new bonds must be formed in the products. Breaking a chemical bond always requires energy, so bond breaking is an endothermic process. Making a new bond always releases energy, so bond making is an exothermic process. The overall energy change of a reaction depends on the balance between the two. If more energy is released making bonds than is needed to break bonds, the reaction is exothermic overall. If more energy is needed to break bonds than is released making them, the reaction is endothermic overall.

Bond energy calculations (Higher)

The energy needed to break (or released on making) one mole of a particular bond is called the bond energy, measured in kJ/mol. The overall energy change of a reaction is calculated as: energy change = (total energy to break all bonds in reactants) minus (total energy released making all bonds in products). A negative answer means an exothermic reaction, and a positive answer means an endothermic reaction. Worked example: for the reaction H-H + Cl-Cl giving 2 H-Cl, use bond energies H-H = 436 kJ/mol, Cl-Cl = 242 kJ/mol and H-Cl = 431 kJ/mol. Energy in to break bonds = 436 + 242 = 678 kJ/mol. Energy out making two H-Cl bonds = 2 x 431 = 862 kJ/mol. Overall energy change = 678 - 862 = -184 kJ/mol. The negative value confirms the reaction is exothermic, releasing 184 kJ/mol.

Chemical cells and batteries

A simple chemical cell can be made by connecting two different metals (electrodes) in a solution that conducts electricity (an electrolyte). Because the metals react at different rates, a voltage is produced and a current can flow. The bigger the difference in reactivity between the two metals, the larger the voltage produced. A battery is two or more cells connected together in series to give a higher total voltage. In non-rechargeable cells the chemical reactions stop once one of the reactants is used up, so the cell goes flat and cannot be reused. Rechargeable cells and batteries can be recharged because the chemical reactions are reversed when an external electrical current is passed through them.

Fuel cells (Higher)

A fuel cell uses the energy from the reaction of a fuel with oxygen to produce a voltage continuously, as long as fuel is supplied. The hydrogen fuel cell is an important example. Overall, hydrogen reacts with oxygen to produce only water: 2 H2 + O2 gives 2 H2O. At the negative electrode hydrogen loses electrons (it is oxidised), and at the positive electrode oxygen gains electrons (it is reduced), with the electrons flowing through the external circuit to deliver electrical energy. Hydrogen fuel cells have advantages over rechargeable batteries: they do not need recharging, they are lighter for the energy stored, and the only waste product is water. Drawbacks include the difficulty of storing flammable hydrogen and the energy needed to produce hydrogen in the first place.

Comparing fuel cells with rechargeable batteries

Both hydrogen fuel cells and rechargeable batteries can power electric vehicles, but they work differently. A battery stores a fixed amount of energy and must be plugged in and recharged when flat, which can take a long time and means the battery slowly loses capacity over many cycles. A fuel cell keeps producing electricity as long as hydrogen is fed to it, so refuelling is quick, but it relies on a supply of hydrogen and on the infrastructure to make, transport and store that gas safely. Considering the whole life cycle, both options aim to reduce the pollutants released compared with burning petrol or diesel directly.