Enthalpy change — AQA A-Level Chemistry
Test yourself on Enthalpy change with AQA A-Level practice questions.
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Enthalpy change explained
Every reaction breaks bonds and makes bonds.
Read the full explanation
Breaking bonds takes energy in, making bonds gives energy out, and the sign of ∆H records the balance. If more energy is released forming the new bonds than was needed to break the old ones, the system loses energy to the surroundings, the mixture warms up and ∆H is negative: combustion and neutralisation behave this way. If bond breaking dominates, energy is drawn from the surroundings, the mixture cools and ∆H is positive, as when ammonium nitrate dissolves in water. On an enthalpy profile the products sit below the reactants for an exothermic change and above them for an endothermic one, and both profiles still show an activation energy hump, because energy is always needed to start breaking bonds.
Enthalpy change (∆H) is the heat energy change measured under conditions of constant pressure.
Enthalpy, H, is the heat content of a system at constant pressure, and ∆H is the difference H(products) − H(reactants). The constant pressure condition matters because a reaction that produces gas does work pushing back the atmosphere; measure the heat in a sealed rigid vessel and you get a slightly different quantity. Reactions done in an open beaker or a polystyrene cup happen at constant atmospheric pressure, so the heat measured there is the enthalpy change directly. Enthalpy itself cannot be measured, only the change, which is why every value is quoted relative to a defined starting point and per mole of a specified substance, in kJ mol⁻¹. Examiners accept heat change at constant pressure but refuse heat change or energy change on its own.
Standard enthalpy changes refer to standard conditions, Ɵ ie 100 kPa and a stated temperature (eg ∆H ).
A standard enthalpy change is quoted under agreed conditions so values from different sources can be added and compared. AQA fixes the pressure at 100 kPa and requires the temperature to be stated, normally 298 K, and every substance must be in its standard state, the physical state it adopts under those conditions: water liquid, bromine liquid, iodine solid, oxygen gaseous, carbon as graphite. Solutions are taken at 1.00 mol dm⁻³. The symbol is written with a superscript plimsoll, as in ∆H⦵, and subscripts name the type: ∆fH⦵ for formation of one mole of a compound from its elements in their standard states, ∆cH⦵ for the complete combustion of one mole in excess oxygen. Because states are fixed, the enthalpy of formation of an element in its standard state is zero by definition.
Your focus
- Deduce the sign of ∆H from a measured temperature change in a reaction mixture, stating whether heat left or entered the system.
- Explain the sign of ∆H for a named reaction by comparing the energy needed to break the reactant bonds with the energy released forming the product bonds.
- Sketch an enthalpy profile for an exothermic and an endothermic reaction, labelling reactants, products, ∆H and the activation energy.
Show all 12 objectives
- Account for a measured enthalpy of combustion being less exothermic than the data book value, quoting incomplete combustion or heat loss.
- Define ∆H in a form that includes heat energy, the constant pressure condition and the per mole basis.
- Convert a measured heat change q in joules into an enthalpy change in kJ mol⁻¹ for a stated substance.
- Explain why a reaction run in an open polystyrene cup gives ∆H directly, while the same reaction in a sealed rigid container does not.
- Distinguish between the total heat released by a sample and the enthalpy change per mole, using an example in which the amounts are doubled.
- State the conditions AQA treats as standard and identify the standard state of water, bromine, iodine and carbon at 298 K.
- Write an equation with state symbols whose enthalpy change is the standard enthalpy of formation of a named compound, using fractions where needed.
- Explain why the standard enthalpy of formation of an element in its standard state is zero.
- Identify which of a set of given equations does not represent a standard enthalpy of combustion, and say why.
Enthalpy change exam tips
Marking Points
- one mark for stating that a reaction is exothermic because more energy is released making bonds than is taken in breaking them
- one mark for linking a temperature rise in the surroundings to a negative ∆H, or a temperature fall to a positive ∆H
- one mark for an enthalpy profile with products drawn below the reactants for an exothermic change, with ∆H labelled between the two levels
- one mark for describing an experimental combustion value as less exothermic or less negative, rather than just smaller, when comparing with a data book value
- one mark for defining ∆H as the heat energy change measured at constant pressure
- one mark for stating that ∆H is the enthalpy of the products minus the enthalpy of the reactants
- one mark for quoting the change per mole with units kJ mol⁻¹ and a sign
- one mark for recognising that a reaction carried out in an open vessel is at constant pressure, so the heat measured is ∆H
- one mark for quoting a pressure of 100 kPa and a stated temperature, usually 298 K
- one mark for adding that all substances are in their standard states at those conditions
- one mark for defining a standard enthalpy of formation as the formation of one mole of a compound from its elements in their standard states
- one mark for state symbols written correctly in the equation that accompanies a standard definition
Examiner Tips
- 💡Fix the sign by asking which way heat travels: out of the system means negative, into the system means positive.
- 💡If a question asks how a measured value differs from a data book value, answer less exothermic and give a reason such as incomplete combustion or heat loss to the surroundings.
- 💡Always give an enthalpy change a sign and the units kJ mol⁻¹; an unsigned number can be read either way and loses the mark.
- 💡Learn the phrase at constant pressure; it is the marking point that separates a full definition from a half one.
- 💡Check units before you write a value: joules for a heat change q, kilojoules per mole for an enthalpy change ∆H.
- 💡State what the mole refers to, for example per mole of methane burned, when the equation contains several substances.
- 💡When a definition question follows, write the equation with state symbols underneath: it often carries a mark of its own.
- 💡Fractions are allowed in formation equations so that exactly one mole of product is made, for example H₂(g) + ½O₂(g) → H₂O(l).
- 💡Say formed from its elements in their standard states, not made from its elements, which examiners treat as incomplete.
Common Mistakes
- writing that bond breaking releases energy and bond making absorbs it, which reverses the sign of every answer that follows
- calling a reaction exothermic because the flask felt cold, confusing the system with the surroundings
- quoting an exothermic enthalpy change without the minus sign, so −890 kJ mol⁻¹ is written as 890 kJ mol⁻¹
- saying an endothermic reaction is impossible because it takes energy in, rather than checking the entropy term
- assuming an exothermic reaction needs no activation energy, so no hump is drawn on the profile
- defining ∆H as simply the heat change or the energy change, which is refused because the constant pressure condition is missing
- writing reactants minus products, which flips the sign of every answer
- treating enthalpy as something a substance possesses a measurable absolute amount of, rather than something only measurable as a change
- omitting per mole, so a heat output in joules is offered as an enthalpy change
- assuming ∆H changes if the amounts are doubled, rather than the total heat released doubling while ∆H per mole stays fixed
- quoting 1 atm or 101 kPa instead of the 100 kPa the specification uses
- calling 298 K a fixed part of the definition when the specification only requires the temperature to be stated
- writing H₂O(g) in an equation for a standard enthalpy change, when the standard state of water at 298 K is liquid
- writing an equation for enthalpy of formation that makes two moles of the compound, so the value no longer matches the definition
- giving carbon as diamond rather than graphite in a formation equation