OCR AS Chemistry

Revision Notes

3.4.1 Enthalpy Changes

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Endothermic & Exothermic Enthalpy Changes

  • The total chemical energy inside a substance is called the enthalpy (or heat content)
  • When chemical reactions take place, changes in chemical energy take place and therefore the enthalpy changes
  • An enthalpy change is represented by the symbol ΔH
    • Δ= change; H = enthalpy
  • An enthalpy change can be positive or negative

Exothermic reactions

  • A reaction is exothermic when the products have less energy than the reactants
  • Heat energy is given off by the reaction to the surroundings
    • The temperature of the environment increases - this can be measured with a thermometer
    • The energy of the system decreases

  • There is an enthalpy decrease during the reaction so ΔH is negative
  • Exothermic reactions are thermodynamically possible (because the enthalpy of the reactants is higher than that of the products)
  • However, if the rate is too slow, the reaction may not occur
    • In this case the reaction is kinetically controlled

5-1-2-exo-reaction-profileThe enthalpy changes during an exothermic reaction

Endothermic reactions

  • A reaction is endothermic when the products have more energy than the reactants
  • Heat energy is absorbed by the reaction from the surroundings
    • The temperature of the environment decreases - this can be measured with a thermometer
    • The energy of the system increases

  • There is an enthalpy increase during the reaction so ΔH is positive

5-1-2-endo-reaction-profile

The enthalpy changes during an endothermic reaction

Enthalpy Profile Diagrams

  • An energy level diagram is a diagram that shows:
    • The energy level of the reactants
    • The transition state(s) - an unstable intermediate in the reaction which cannot be isolated and is higher in energy than the reactants and products
    • The energy level of the products
    • The activation energy (Ea)
      • The minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction
    • The enthalpy change for the reaction (ΔH
      • This can be describes as the overall energy taken in from / given out to the surroundings OR the energy difference from reactants to products

1-6-23-2-1-energy-level-diagram-for-hclThe energy level diagram for the reaction of hydrogen with chlorine to form hydrogen chloride gas

Exam Tip

You do not need to know or label the specific term transition state on diagrams 

Worked example

Drawing energy level diagrams of the combustion of methane

The activation energy, Ea, and enthalpy change, ΔH, for the complete combustion of methane are +2653 kJ mol-1 and -890 kJ mol-1 respectively.

Draw the energy level diagram for this reaction.

Answer

Step 1: The chemical equation for the complete combustion of methane is:

CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)

Step 2: Combustion reactions are always exothermic (ΔH is negative) so the reactants should be drawn higher in energy than the productsChemical Energetics Step 2 - Drawing energy level diagrams of the combustion of methane, downloadable AS & A Level Chemistry revision notes

Step 3: Draw the curve in the energy level diagram Chemical Energetics Step 3 - Drawing energy level diagrams of the combustion of methane, downloadable AS & A Level Chemistry revision notes

Step 4: Draw arrows to show the Ea and ΔH  including their values1-6-2-we-1

Exam Tip

Remember to label the axes of the energy level diagrams!

Worked example

Determining the activation energy

ΔH for a reaction is +70 kJ mol-1 and Ea for the reverse reaction is +20 kJ mol-1.

Use the reaction pathway diagram below to determine the Ea for the forward reaction.

1-6-2-we-2

The reaction pathway diagram for a reversible reaction

Answer

    • The Ea  is the energy difference from the energy level of the reactants to the top of the ‘hump’
    • Ea (forward reaction) = (+70 kJ mol-1) + (+ 20 kJ mol-1 ) = +90 kJ mol-1

Standard Enthalpy Definitions

  • To be able to compare the changes in enthalpy between reactions, all thermodynamic measurements are carried out under standard conditions
  • These standard conditions are:
    • A pressure of 100 kPa (you may see some older exam questions that use a figure of 101 kPa; the exact figure is 101 325 Pa, but it has been simplified in the current syllabus for problem-solving purposes)
    • A temperature of 298 K (25 oC)
    • Each substance involved in the reaction is in its standard physical state (solid, liquid or gas)

  • To show that a reaction has been carried out under standard conditions, the symbol Ꝋ is used
    • ΔH = the standard enthalpy change

  • There are a number of key definitions relating to enthalpy changes that you need to know

Enthalpy Definitions Table

Chemical Energetics Enthalpy Definition Table, downloadable AS & A Level Chemistry revision notes

Worked example

Calculating the enthalpy change of reaction

One mole of water is formed from hydrogen and oxygen, releasing 286 kJ of energy

H2 (g) + ½O2 (g)  H2O (I)      ΔHr = -286 kJ mol-1

Calculate ΔHr for the reaction below:

2H2 (g) + O2 (g)  2H2O (I)

Answer

    • Since two moles of water molecules are formed in the question above, the energy released is simply:
      • ΔHr = 2 mol x (-286 kJ mol-1
      • ΔHr = -572 kJ mol-1 

Worked example

Calculating the enthalpy change

Calculate ΔHf for the reaction below, given that ΔHf [Fe2O3(s)] = -824.2 kJ mol-1

4Fe (s) + 3 O2 (g) → 2 Fe2O3 (s)

Answer

    • Since two moles of Fe2O3 (s) are formed the total change in enthalpy for the reaction above is:
      • ΔHf =  2 x ( -824.2 kJ mol-1)
      • ΔHf = - 1648 kJ mol-1

Worked example

Calculating enthalpy changes

Identify each of the following as ΔHr, ΔHf, ΔHc or ΔHneut

  1. MgCO3 (s)  MgO (s) + CO2 (g)
  2. C (graphite) + O2 (g) → CO2 (g)
  3. HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (I)

Answers

Answer 1: ΔHr

Answer 2: ΔHf as one mole of CO2 is formed from its elements in standard state and ΔHc as one mole of carbon is burnt in oxygen

Answer 3: ΔHneut as one mole of water is formed from the reaction between an acid and an alkali

Exam Tip

The ΔHf of an element in its standard state is zero.

For example, ΔHf of O2(g) is 0 kJ mol-1

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