Enthalpy Changes (OCR A Level Chemistry A): Revision Note
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
The 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
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
The energy level diagram for the reaction of hydrogen with chlorine to form hydrogen chloride gas
Examiner Tips and Tricks
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 products
Step 3: Draw the curve in the energy level diagram
Step 4: Draw arrows to show the Ea and ΔH including their values
Examiner Tips and Tricks
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.
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
ΔHꝊ = the standard enthalpy change
There are a number of key definitions relating to enthalpy changes that you need to know
Enthalpy Definitions Table
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) ΔHrꝊ = -286 kJ mol-1
Calculate ΔHrꝊ 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:
ΔHrꝊ = 2 mol x (-286 kJ mol-1)
ΔHrꝊ = -572 kJ mol-1
Worked Example
Calculating the enthalpy change
Calculate ΔHfꝋ for the reaction below, given that ΔHfꝋ [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:
ΔHfꝊ = 2 x ( -824.2 kJ mol-1)
ΔHfꝊ = - 1648 kJ mol-1
Worked Example
Calculating enthalpy changes
Identify each of the following as ΔHrꝊ, ΔHfꝊ, ΔHcꝊ or ΔHneutꝊ
MgCO3 (s) → MgO (s) + CO2 (g)
C (graphite) + O2 (g) → CO2 (g)
HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (I)
Answers
Answer 1: ΔHrꝊ
Answer 2: ΔHfꝊ as one mole of CO2 is formed from its elements in standard state and ΔHcꝊ as one mole of carbon is burnt in oxygen
Answer 3: ΔHneutꝊ as one mole of water is formed from the reaction between an acid and an alkali
Examiner Tips and Tricks
The ΔHfꝊ of an element in its standard state is zero.
For example, ΔHfꝊ of O2(g) is 0 kJ mol-1
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