Enthalpy Level Diagrams (Edexcel International AS Chemistry): Revision Note
Enthalpy Level Diagrams
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
Examiner Tips and Tricks
Activation energy is not shown in enthalpy level diagrams
Activation is shown in reaction profile diagrams
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 change 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 change during an endothermic reaction
Examiner Tips and Tricks
It is important to specify the physical states of each species in an equation when dealing with enthalpy changes as any changes in state can cause very large changes of enthalpy. For example:
NaCl (s) → Na+ (aq) + Cl- (aq) ΔH = +4 kJ mol-1
NaCl (g) → Na+ (g) + Cl- (g) ΔH = +500 kJ mol-1
Also, remember that the system is the substances that are reacting (i.e. the reaction itself) and the surroundings is everything else (e.g. the flask the reaction is taking place in).
Reaction profile diagrams
Reaction profile diagrams are similar to enthalpy level diagrams
The difference between the energy level of the reactants and the energy level of the products is still the overall enthalpy change of the reaction, ΔH
The main differences are that they include information about:
The activation energy, Ea, of the reaction
Activation energy can be defined as ‘the minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction’
This is the difference between the energy level of the reactants and the peak of the curve
Possible transition states
The peak of the curve indicates a transition state
The transition state is a stage during the reaction at which chemical bonds are partially broken and formed
The transition state is very unstable – a molecule in the transition state cannot be isolated and is higher in energy than the reactants and products
Exothermic reaction profiles
In an exothermic reaction, the reactants are higher in energy than the products
The reactants are therefore closer in energy to the transition state
This means that exothermic reactions have a lower activation energy compared to endothermic reactions
The reaction profile diagram for a general exothermic reaction
Endothermic reaction profiles
In an endothermic reaction, the reactants are lower in energy than the products
The reactants are therefore further away in energy from the transition state
This means that endothermic reactions have a higher activation energy compared to exothermic reactions
The reaction profile diagram for a general endothermic reaction
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