AS/A-Level Chemistry - Activation energy

Recalling A-Level Chemistry, to kick start either exothermic and endothermic reaction, besides from entropy and Gibbs free energy, we still need activation energy to start a reaction.

Activation energy

We can calculate the activation energy using the Arrhenius equation: 

k = rate constant                    E_A= activation energy (J) 

T = temperature (K)               R= gas constant (8.31 JK^-1mol^-1) 

A = another constant 

Some relationships to note: 

1) As E_A increases, k will get smaller.Therefore large activation energy, means a slow rate – this makes sense! 

2) As T increases, k increases. Therefore at high temperatures, rate will be quicker – this makes sense too! 

If we “ln” both sides of Arrhenius’ equation, we get; 

ln k = – E_A/RT+ ln A 

(don’t forget, ln A is just a constant, a number)  

This looks a bit like: 

y = mx + c 

If we plot ln k (y) against 1/T (x), the gradient we produce will be –E_A/R (m). Then R is just a number that we know (8.31 JK^-1mol^-1) we can rearrange and find the activation energy. 

EXAMPLE: 

Iodine clock reaction 

S₂O₈^2- (aq) + 2I^-(aq) —> 2SO₄^2- _(aq) + I₂ (aq) 

Rate of reaction is inversely proportional to the time taken for the solution to change colour.  

i.e. increased rate = decreased time taken 

k α 1/t 

We can say that 1/t is the same as k (rate constant) and we can substitute 1/t instead of k in Arrhenius’ equation and find the gradient again to find a value for E_A. 

Catalysts 

Catalyst = increases rate of a reaction by providing an alternative reaction pathway with a LOWER activation energy (E_A). A catalyst will be chemically unchanged at the end of a reaction. 

Advantage: Small amount needed to catalyse a lot of reactions, also they are remade, thus reusable. 

Disadvanatge: High specificity to the reactions they catalyse. 

There are two types of catalysts: 

Homogenous Catalysts

These are catalysts in the same state as the reactants.   

E.G. when enzymes catalyse reactions in your body, all reactants are aqueous, this is a homogenous catalysis.

Heterogenous catalysts

These are catalysts in different physical states to the reactants. 

Advantage: 

• They are easily separated from products
• Solid catalysts provide a large surface area for the reaction to occur e.g. mesh/powder   (E.G. vanadium pentoxide in the contact process to make nsulphuric acid)

Disasvantage: Can be poisoned (i.e. a substance clings to a catalyst stronger than the reactant would,preventing reaction speeding up) example: sulphur in the Haber process is a “poison”

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Drafted by Eunice (Chemistry)

References: 

https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Kinetics/06%3A_Modeling_Reaction_Kinetics/6.02%3A_Temperature_Dependence_of_Reaction_Rates/6.2.03%3A_The_Arrhenius_Law/6.2.3.04%3A_The_Arrhenius_Law_-_Arrhenius_Plots

https://www.khanacademy.org/science/ap-chemistry-beta/x2eef969c74e0d802:kinetics/x2eef969c74e0d802:activation-energy-and-reaction-rate/v/arrhenius-equation#:~:text=The%20Arrhenius%20equation%20is%20k,energy%20Ea)%20at%20temperature%20T.