- Rate of reaction is the change in the amount of reactants or products over time
Rate = Amount of Reactants used or Amount of product formed
- To find the rate of reaction several tests must be carried out throughout the experiment using either:
Time taken for precipitate to form
When the product formed causes a precipitate to form as a cloud in a solution
Method: 1) stand a conical flask over a white tile with a cross on it
2) add a fixed amount of reactants to the conical flask and start a stopwatch
3) observe the solution to until you are unable to see the cross
Change in Mass:
When the product formed is a gas, then the rate of reaction can be measured using the change in mass
Method: 1) place a conical flask on a weighing scales
2) Reactants are mixed together in a conical flask and a stopwatch is started
3) at regular time intervals, read the new mass of the reactants
If a gas is given off in a reaction, then the rate of reaction can be calculated based on the volume of gas collected in a gas syringe and recording the volume over different time periods
Method: 1) Reactants are mixed together in a conical flask and a gas syringe is attached and sealed onto it
2) start a stopwatch as soon as the reaction starts
3) at regular time intervals, read how much volume of gas has been produced
- A catalyst increases the rate of reaction by providing an alternative route with a lower activation energy
- The catalyst is chemically unchanged by the end of the reaction. This result of this is that a small quantity of the catalyst can be used many times over many reactions.
- Note that a catalyst does get chemically changed throughout the reaction process however by the end of the reaction its original state reforms again.
- Catalysts usually are specific to only one type of reaction
How do Catalysts work?
- A catalyst lowers the required activation energy resulting in there being more particles with enough energy to react upon collision.
- Catalysts do not affect the enthalpy change of a reaction
- Catalysts do not affect the position of equilibrium in a reversible reaction
- When a homogeneous catalyst is present, one of the reactants reacts with the catalyst forming an intermediate product. The intermediate product then reacts with the other reactant to form the final product.
- The activation energies of both these steps is lower than the activation energy if a catalyst was not used, resulting in more molecules with the energy to react using the catalyst; hence the rate of reaction is increased.
Types of catalysts
- Heterogenous Catalyst: The catalyst is in a different state compared to the reactants e.g. a solid catalyst with a liquid reactant
- Homogenous Catalyst: The catalyst is in the same state as the reactants
- No particle has zero energy
- Most particles have intermediate energies (displayed around the peak of the curve)
- Few particles have very high energies (displayed on the right-hand side of the curve)
- The average energy is not the same as the most probable energy
Maxwell Boltzmann graph
- The number of molecules in a gas with different kinetic energies is displayed on a Maxwell Boltzmann graph
- To the left of the graph is all of the molecules that are slow moving
- At the peak of the graph, is the most common amount of molecules which are at an average speed of movement
- To the right are the molecules that are fast moving
- Past the activation energy line is all of the molecules which are moving fast enough for them to react
Effect of Temperature
- Increasing the temperature of a gas will thus give each molecule more kinetic energy
- The result of this is more particles will have the minimum activation energy and be able to react successfully
- The Maxwell Boltzmann graph moves to the right as there are more molecules with more kinetic energy
- As more molecules would have successfully collided, more molecules after the activation energy (Ea) mark will be present in the high temperature compared to the lower temperature
Effect of Concentration
- Increasing the concentration of reactants in a given solution will result in there being more particles in a closer proximity to one another
- As the particles are closer together there is an increased chance of collisions, and therefore more particles can react
- If the reaction is a gas, increasing the pressure of the gas results in the same effect. Raising the pressure causes the particles to be closer together, increasing the number in a given volume.
- Particles in a fluid are always moving and colliding with one another. These particles do not react unless the conditions are right for a successful collision
- Collision theory states that a reaction will not occur between two particles unless both particles collide at each other with at least a certain minimum amount of kinetic energy
- The collision must take place between the parts of the molecule which will react with one another
- Most collisions do not react either of the result of insufficient amount of energy or the molecule is in the wrong orientation for the molecules to react
Activation Energy (Ea)
- The minimum amount of kinetic energy required for a particle to react is the Activation Energy
- This energy is required to break the bonds within the reactant particles, which thus starts the reaction
- Reactions with low activation energies are easier to start compared to ones that have high activation energies.
- Activation Energy also explains why not all exothermic reactions can occur instantly at room temperature
Factors which affect Rate of Reaction
- Increasing the concentration: more particles in a given volume will lead to more collisions as there is a greater chance of hitting another particle successfully. However, as the reaction continues more reactants will be used up and therefore there will be fewer reactants for the reaction to continue. The result being the rate of reaction decreases towards the end
- Increasing the temperature: Each particle has more kinetic energy which increases the speed the particles move in. The result is that there are more collisions with more energy
- Increasing the surface area of a solid Reactant: a greater surface area of a solid results in there being more of the solid’s particles available for collision with molecules of a gas/liquid.
- Increasing the pressure of a gas reaction: Similar to increasing the concentration, either decreasing the volume of the reaction vessel or add more particles in a given volume will result in more successful collisions
- Catalyst: a substance that offers an alternative route for the reaction with a lower activation energy