A LevelChemistryCambridge (CIE)Revision Notes8. Reaction KineticsRate of ReactionRate of Reaction & Collision Theory

Rate of Reaction & Collision Theory (Cambridge (CIE) A Level Chemistry): Revision Note

Exam code: 9701

Last updated

Rate of Reaction & Collision Theory

Reaction rate

  • The rate of a reaction is the speed at which a chemical reaction takes place and has units mol dm-3 s-1

  • The rate of a reaction can be calculated by:

Rate of reaction = fraction numerator bold change bold space bold in bold space bold the bold space bold amount bold space bold of bold space bold reactants bold space bold or bold space bold products bold space stretchy left parenthesis mol space dm to the power of negative 3 end exponent stretchy right parenthesis over denominator bold time bold space stretchy left parenthesis s stretchy right parenthesis end fraction

Worked Example

Calculating the rate of reaction

Calculate the rate of reaction when 0.0440 g of ethyl ethanoate, CH3COOC2H5, is formed in 1.0 minute from 400 cm3 of a reaction mixture

Answer

  • Step 1: Calculate the amount of ethyl ethanoate formed in mol:

    • Moles = fraction numerator mass space open parentheses straight g close parentheses over denominator molar space mass space open parentheses straight g space mol to the power of negative 1 end exponent close parentheses end fraction

    • Moles = fraction numerator 0.0440 space straight g over denominator 88.0 space straight g space mol to the power of negative 1 end exponent end fraction

    • Moles = 0.0005 mol

  • Step 2: Calculate the volume of the reaction mixture in dm3:

    • 400 cm3 = 0.400 dm3

  • Step 3: Calculate the concentration change of product formed:

    • Concentration = fraction numerator amount space open parentheses mol close parentheses over denominator volume space open parentheses dm cubed close parentheses end fraction

    • Concentration = fraction numerator 0.0005 space mol over denominator 0.400 space dm cubed end fraction

    • Concentration = 0.00125 mol dm-3

  • Step 4: Calculate the time in seconds:

    • 1.0 min = 60.0 s

  • Step 5: Use the equation to calculate the rate:

    • Rate of reaction = fraction numerator bold change bold space bold in bold space bold the bold space bold amount bold space bold of bold space bold reactants bold space bold or bold space bold products bold space stretchy left parenthesis mol space dm to the power of negative 3 end exponent stretchy right parenthesis over denominator bold time bold space stretchy left parenthesis s stretchy right parenthesis end fraction

    • Rate of reaction = fraction numerator 0.00125 space mol space dm to the power of negative 3 end exponent over denominator 60 space straight s end fraction

    • Rate of reaction = 2.08 x 10-5 mol dm-3 s-1 

Collision theory

  • The collision theory states that for a chemical reaction to take place, the particles need to collide with each other in the correct orientation and with enough energy

  • The minimum energy that colliding particles must have for a collision to be successful and a reaction to take place is called the activation energy (Ea)

Collision theory table

 

Effective collision

Ineffective collision

Orientation

Correct 

Incorrect

Energy

Sufficient energy (Ea)

Not enough energy

Chemical reaction

Yes

No

  • An ineffective collision is when particles collide in the wrong orientation or when they don’t have enough energy and bounce off each other without causing a chemical reaction

 Effective and ineffective collisions

Chemical reaction diagram showing hydrogen and chlorine molecules. Series A shows no reaction; Series B shows a successful reaction with energy.
(A) shows an ineffective collision due to the particles not having enough energy whereas (B) shows an effective collision where the particles have the correct orientation and enough energy for a chemical reaction to take place

Increase in reaction rate

  • The collision frequency is the number of collisions per unit time

  • When there are more collisions per unit time, the number of particles with energy greater than the Ea increases

  • This causes an increase in the rate of reaction

  • A catalyst is a substance that increases the rate of reaction without taking part in the chemical reaction by providing the particles with an alternative mechanism with a lower activation energy

How catalysts affect reaction pathways

Graph showing enthalpy change for catalysed and uncatalysed reactions. Labels: reactants, products, activation energy, and enthalpy change.
A catalyst increases the rate of a reaction by providing an alternative pathway which has a lower activation energy

Concentration

  • The more concentrated a solution is, the greater the number of particles in a given volume of solvent

  • An increase in concentration causes an increased collision frequency and therefore an increased rate of reaction

How increasing concentration affects collisions

Diagram showing reactants 1 and 2 colliding effectively. Increasing concentration from A to B increases collisions, enhancing reaction rate.
The higher concentration of particles in (B) means that there are more particles present in the same volume than (A) so the chance and frequency of collisions between reacting particles increase causing an increased rate of reaction

Pressure

  • An increase in pressure in reactions that involve gases has the same effect as an increased concentration of solutions

  • When the pressure is increased, the molecules have less space in which they can move

  • This means that the number of effective collisions increases due to an increased collision frequency

  • An increase in pressure therefore increases the rate of reaction

How increasing pressure affects collisions

Diagram showing reactants colliding in box A. Increasing pressure shifts reactants for more collisions in box B, indicating effective collisions.
The higher pressure in (B) means that the same number of particles occupy a smaller volume, resulting in an increased collision frequency and therefore increased rate of reaction

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