enthalpy change

Enthalpy changes and standard states

methane gas burning in a Bunsen burner The enthalpy change (ΔH) for a reaction represents the amount of heat energy released when the reactants are changed into products during a chemical reaction, its value is expressed in kilojoules (kJ) or kilojoules per mole (kJ mol-1). As an example consider the combustion of methane gas in a Bunsen burner to form carbon dioxide and water. Two equations are shown below for this combustion reaction.

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)     ΔH=-803kJ
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)     ΔH=-891kJ
Each of the two equations is balanced. When we write equations to represent an enthalpy change (ΔH) it is important that a number of other factors are made clear, these include:

To ensure that all enthalpies are recorded in a similar way and to make it possible to compare enthalpy changes for different reactions chemists use a set of conditions called standard condition. Standard condition are:

When all these conditions have been met then the enthalpy change of the reaction (ΔH) is called the standard enthalpy change and is given the symbol (ΔHo)

The standard enthalpy change(ΔHo) for a reaction is the enthalpy change under standard conditions (298K, 100kPa,) with all reactants and products being in their standard states.

Under these conditions the standard or most stable state of water is a liquid at 298K and 100kPa, while methane, oxygen and carbon dioxide are all gases. So this means that the second of the two equations above, that is:

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)     ΔH=-891kJ
Could represent the standard enthalpy of combustion of methane(ΔcHo) as long as the enthalpy change of -891kJ was calculated under standard conditions. The standard enthalpy of combustion of a substance (ΔcHo) is defined as:

The standard enthalpy of combustion of a susbstance is the enthalpy change that occurs under standard conditions, when 1 mol of a substance is completely burned in oxygen, with all reactants and products being in their standard states.

Standard enthalpy of formation (ΔfHo)

There are many enthalpy changes you will will learn about as you complete your A-level chemistry course. The standard enthalpy of formation is simply one of these. In the example above using standard enthalpies of combustion, one mole of a susbstance was completed burned in oxygen. Well as the name suggests the enthalpy of formation is the enthalpy change when 1 mole of a substance is formed under standard conditions from its elements, with all reactants and products being in their standard states. For example the equation to show the standard enthalpy of formation (ΔfHo) of methane would be:

C(s) + 2H2(g) → CH4(g)     ΔfHo =-75kJ/mol

The element carbon exists in two forms or allotropes, diamond and graphite. The most stable allotrope of carbon is graphite which will be the standard state used in enthalpy change calculations. You may see this written as Cgraphite in some webpages and textbooks.

You should be aware that when you come to carry out enthalpy calculations using enthalpies of formation that by definition the standard enthalpy of formation of an element is by defintion 0. This may not at first seem obvious but if you think about what an enthalpy change of formation is then it should be apparent. The enthalpy of formation (ΔfHo) is the enthalpy change (amount of heat released at constant pressure) when a substance is formed from the most stable form of its elements. Or you can think of the reactants as the initial state and the product as the final state in the enthalpy change taking place. However with an element there is NO enthalpy change taking place because the reactants and products are the same. The element is already formed so no enthalpy of formation is possible, the element cannot react to form itself! Setting the enthalpy of formation for an element to O is a good choice since we cannot measure the enthalpy of substances anyway, we can only measure enthalpy changes. So the setting of the enthalpy of formation of an element to 0 is an obvious choice for setting a relative reference or starting point.

The same but different

If you were asked to write an equation to show the standard enthalpy of combustion (ΔcHo) of carbon you would write:

C(s) + O2(g) → Co2(g)
However if you were asked to write an equation to show the standard enthalpy of formation of carbon dioxide gas, what would you write? Well the equation below show the standard enthalpy of formation of carbon dioxide:
C(s) + O2(g) → Co2(g)
The 2 equations are clearly identical. It may seem tedious but you should make an effort to learn the definitions for the many enthalpy changes you will meet in your chemistry course because you will come across many such similarities.

Key points

Practice questions

Check your understanding - Questions on enthalpy and standard states

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