Enthalpy Changes & Energy Level Diagrams (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching chemistry, and this video is about enthalpy changes and energy level diagrams.

An enthalpy change is the heat energy a reaction releases or absorbs, and an energy level diagram is the picture of that change.

Enthalpy is the total chemical energy contained within a substance. When a chemical reaction happens that energy changes, and the change is written as delta H. An energy level diagram shows the energies of the reactants, the transition state and the products as the reaction proceeds, so the sign and the size of delta H can be read straight off it.

The first part is what an enthalpy change is, and what a negative or a positive delta H tells you about a reaction. The second is the standard conditions those values are measured under, and the named enthalpy changes that go with them. The third is the diagram itself: the transition state, and the activation energy needed to reach it.

The total chemical energy contained within a substance is called its enthalpy, or its heat content. When a chemical reaction occurs there is a change in chemical energy, and therefore a change in enthalpy. That change is represented by delta H, and it can be either positive or negative.

A reaction is exothermic when the products have less energy than the reactants. Heat energy is released to the surroundings, so the temperature of the environment increases, and you can measure that with a thermometer. Energy is transferred out of the system, so the energy of the system decreases, the enthalpy decreases, and delta H is negative.

A reaction is endothermic when the products have more energy than the reactants. Heat energy is absorbed from the surroundings, so the temperature of the environment decreases. The energy of the system increases, the enthalpy increases, and delta H is positive.

The graph on the right shows that for an exothermic reaction: the reactants sit on one energy level, the products on a lower one, and the drop between them is the enthalpy change.

To allow a meaningful comparison of enthalpy changes between reactions, thermodynamic measurements are carried out under standard conditions. Those conditions are a pressure of 100 kilopascals, a temperature of 298 kelvin, which is 25 degrees Celsius, and all substances in their standard physical states, solid, liquid or gas.

There are four named enthalpy changes to know. The standard enthalpy change of reaction is the heat energy absorbed or released during a chemical reaction at constant pressure. The standard enthalpy change of formation is the heat energy change when exactly one mole of a compound is formed from its constituent elements in their most stable standard states. The standard enthalpy change of combustion is the heat energy released when one mole of a substance is completely burned in excess oxygen. The standard enthalpy change of neutralisation is the energy released when an acid and a base react to form one mole of water.

Reaction and formation can be exothermic or endothermic. Combustion and neutralisation are always exothermic. Each of these is an enthalpy change, so each one has a sign, and each one can be drawn as an energy level diagram.

An energy level diagram shows the energies of the reactants, the transition state and the products of a reaction as the reaction proceeds, along the axis labelled extent of reaction.

The transition state is a stage during the reaction at which chemical bonds are partially broken and partially formed. It is very unstable: a molecule in the transition state cannot be isolated, and it is higher in energy than both the reactants and the products. That is the peak on the diagram.

The activation energy, Ea, is the energy needed to reach the transition state. It is the minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction, and on the diagram it is the height from the reactants up to the peak.

In an exothermic reaction the reactants are higher in energy than the products, so they are closer in energy to the transition state, which means exothermic reactions have a lower activation energy than endothermic ones. In an endothermic reaction the reactants are lower in energy than the products, further in energy from the transition state, so the activation energy is higher.

The diagram on the right is hydrogen reacting with chlorine to form hydrogen chloride gas. Reactants, transition state and products, with the activation energy and the enthalpy change both marked on the same picture.

Two things to remember. First, always specify the physical states of each species when you write an equation involving an enthalpy change, because changes of state can involve very large enthalpy changes. Solid sodium chloride going to aqueous ions has a delta H of plus 4 kilojoules per mole. Gaseous sodium chloride going to gaseous ions is plus 500.

Second, label the axes of your energy level diagrams. Energy in kilojoules per mole up the y axis, extent of reaction along the x axis.

An enthalpy change is the heat energy released or absorbed when a reaction happens, and it is written as delta H. A negative delta H means the reaction is exothermic: the products are lower in energy than the reactants, and heat goes out to the surroundings. A positive delta H means the reaction is endothermic: the products are higher in energy, and heat is absorbed from the surroundings.

Standard enthalpy changes are measured at 100 kilopascals and 298 kelvin with every substance in its standard state, which is what makes values from different reactions comparable.

An energy level diagram is where all of that becomes visible at once: the energies of the reactants and the products, the enthalpy change between them, the transition state, and the activation energy needed to reach it.