Rates of Reaction and Collision Theory

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• The speed at which a reaction happens
• Measured in an arbitrary value called “quantity per second”

$Rate=\frac{\Delta [Reactants]}{\Delta t}mol{L}^{-1}{s}^{-1}$

• [Reactants] is concentration
• $\Delta t$ is change in time in seconds
• Can also be measured in change in mass, in seconds
• For a reaction to happen, particles must collide
• The reaction is complete when sin(θ)=1, where θ is equal to the angle between the perpendicular line to the curve when the volume of gas produced does not change further

What affects rate of reaction

1. Catalysts
2. Temperature
3. Pressure
4. Concentration
5. Surface area

Finding the Rate of Reaction using a graph

• Here, we graph the amount of substance used/produced (x-axis) against time (y-axis)
• The instantaneous gradient of the curve shows us the rate of reaction at a given point in time
• Similarly, an average rate of reaction for a range in time (Time 1 to time 2) can be calculated by the following method given below: where y is the amount of substance and t is the time $\frac{y_2-y_1}{t_2-t_1}$
• Using a table of values showing the amount of substance vs. time we can calculate the average rate of reaction between given time intervals using the RoR formula given above
• Graphing this data provides a curve for which we can differentiate to find the gradient at different points.

Collision Theory

• Reactions take place when particles collide with a certain amount of energy
• The minimum amount of energy needed for the particles to react is called the activation energy and is different for each reaction
• The rate of a reaction depends on 2 things:
  • The frequency of collisions between particles
  • The energy with which particles collide
• If particles collide with less energy than the activation energy, they will not react
• The particles will just bounce off each other

The principles of collision theory

A reaction consists of the following requirements

1. Reacting particles must collide
2. Collision energy must be equal to or greater than the activation energy
3. Reacting particles must collide with a suitable orientation

Changing the rate of reaction

• Anything that increases the number of successful collisions between reactant particles will speed up a reaction
• What factors affect the rate of reactions?
  • Increased temperature
  • Increased concentration of dissolved reactants, and increased pressure of gaseous reactants
  • increased surface area of solid reactants
  • Use of a catalyst
• The number of successful collisions per unit of time depends on the following factors:
  • Total number of collisions
  • Percentage of collisions that were successful

Slower and slower!

• Reactions do not proceed at a steady rate
• They start off at a certain speed, then get slower and slower until they stop
• As the reaction progresses, the concentration of reactants decreases
• This reduces the frequency of collisions between particles and so the reaction slows down

How can rate of reaction be measured?

• Measuring the rate of reaction means measuring the change in the amount of a reactant or the amount of a product, over time

Effect of Temperature on Rate of reaction

• Temperature is the average kinetic energy of a substance’s particles

1. Increasing temperature leads to an increased average kinetic energy of particles
2. Also, there is an increased velocity/speed of particles
3. Thus, there is an increased frequency of collisions, and an increased proportion of collisions with sufficient kinetic energy to overcome activation energy
4. This leads to an increase in percentage of successful collisions
5. Thus, there is an increased frequency of successful collisions
6. Thus, the amount of successful collisions increases, as at higher temperature, more particles have the required Ek to react and the frequency of collisions has increased.
7. Amount of product produced/amount of reactant consumed, per unit of time, increases, reaction rate increases

Effect of State of Subdivision on Rate of reaction

• The state of subdivision refers to the amount of surface area exposed to particles

1. Increasing the S.o.s causes a greater exposed SA. This increases the probability and frequency of collisions
2. Thus, the frequency of successful collisions increases
3. Thus as the percentage of successful collisions (temperature dependent) is constant, the rate of reactant consumption/product formation increases
4. This causes the rate of reaction to increase
5. Amount of product produced/amount of reactant consumed, per unit of time, increases, reaction rate increases

Effect of Concentration on Rate of reaction

• Concentration refers to how much particles is present per unit volume

1. Increasing concentration leads to more particles per unit volume
2. Thus, there is a higher likelihood of a collision occurring, as more particles are present per unit volume
3. Frequency of collisions increases
4. Percentage of successful collisions remains constant (relies on temperature)
5. Thus, frequency of successful collisions increases
6. Amount of product produced/amount of reactant consumed, per unit of time, increases, reaction rate increases

Effect of Pressure on Rate of reaction

• Pressure refers to a force per unit area

1. Increased pressure means that there are more particles per unit volume
2. Thus, there is an increased likelihood of collisions occuring, as more particles are present per unit volume
3. Thus, the frequency of collisions increases
4. The percentage of successful collisions remains constant
5. Thus, frequency of successful collisions increases, as frequency of collisions increases
6. Amount of product produced/amount of reactant consumed, per unit of time, increases, reaction rate increases

Effect of a Catalyst on rate of reaction

• A catalyst is a substance that increases the rate of reaction, by providing an alternative pathway with a lower activation energy, and is not consumed in the reaction

1. There is a provision of a lower energy pathway
2. Thus, more particles have greater energy than activation energy
3. Percentage of successful collisions increases, probability of collisions remains constant
4. Thus, the amount of successful collisions increases
5. Amount of product produced/amount of reactant consumed, per unit of time, increases, reaction rate increases

General Model

These questions will most likely use the word ‘EXPLAIN’ For these question types the generalised wording will be along the lines of:  “Use the Collision Theory to EXPLAIN the effect of …… on the rate of reaction (5 Marks)

Step 1: State the imposed change in terms of molecular processes: (1)

1. Addition/removal of species: change in concentration. You must define this: i.e. “MORE PARTICLE PRESENT PER UNIT VOLUME”

2. Increase/Decrease of temperature: You must define this as the change in average Ek of particles

3. Increase/Decrease of pressure: change in frequency of collisions

4. Addition of Catalyst: provision of an alternative lower Ea pathway

Step 2: Refer to Relevant Chemical Principle : Here think if any of these two are affected: % of SUCCESSFUL or FREQUENCY of collisions (1 – 2)

Step 3: State the effect on Frequency of Collisions + % of Successful Collisions (1)

Step 4: State the effect on FREQUENCY OF SUCCESSFUL COLLISIONS. (These words MUST MUST be included.) (1)

Step 5: State the effect on the amount of product produced per unit time/the amount of reactants consumed per unit time and THEN state the net effect on the rate of reaction.  (1)