The rate and extent of chemical change

  • Created by: holly6901
  • Created on: 10-06-19 17:25

Rates of chemical reactions

Rates of reaction

  • In industry, making the required amount of product as cheaply as possible maximises all-important profit.
  • Faster reactions are often better as they yield more product in a given amount of time.
  • It may be expensive to generate the conditions for very fast reactions.
  • There can be safety concerns associated with very fast reactions.
  • Therefore, reaction rates used in industry are the result of a trade-off between speed, cost and safety.

Plotting rates of reaction

Rates of reactions at specific times can be calculated using graphs:

  • Draw a tangent to the curve (straight line that touches the curve at just 1 point).
  • Work out the gradient of the tangent:
    • Rate of reaction = change in y / change in x
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Collision theory

  • Particles must collide for chemical reactions to happen.
  • Importantly, these collisions must happen with enough energy. This amount of energy is called the activation energy.
  • The activation energy is the minimum energy with which particles must collide in order to cause a chemical reaction.
  • We can increase the rate of a reaction by increasing the frequency (number) of collisions and/or the energy of reactant particles.
  • This can be achieved by changing lots of different factors.
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The rate of chemical reactions

  • Increasing the concentration increases the frequency (number) of collisions. This increases the rate of reaction.
  • Increasing pressure is like increasing the concentration.
  • It increases the frequency (number) of collisions. This increases the rate of reaction (makes the reaction faster).
  • Increasing the temperature increases the frequency (number) of collisions.
  • Increasing the temperature increases the energy of reactant particles. This means that a greater proportion (more) of the particles will have more energy than the activation energy that is needed.
  • Together, these lead to more successful collisions. This increases the rate of reaction.
  • Increasing the surface area increases the frequency (number) of collisions. This increases the rate of reaction.
  • This is often done by breaking up solids into smaller lumps (e.g. powders).
  • This increase in surface area to volume ratio makes sure that more particles are exposed to attack.
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Catalysts

  • Catalysts are substances that increase the rate of a chemical reaction without being used up in the process.
  • Catalysts are not used up during chemical reactions. This means that:
    • They can be reused indefinitely.
    • They are not found in chemical equations.
  • However, catalysts will often need cleaning or regenerating, which has knock-on effects for energy expenditure and environmental impact.
  • Different reactions need different catalysts:
    • Iron is used in the process that makes ammonia.
    • Platinum and palladium are used in catalytic converters in cars.
    • Enzymes catalyse reactions in biological systems.
  • Catalysts often come as powders, pellets or fine gauzes because these types of substance have particularly high surface areas.
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Catalysts in industry

  • Despite the fact that some catalysts are expensive precious metals (e.g. platinum and palladium), they are often cost-effective.
    • This is because a small quantity can speed up a reaction by a lot. It may be cheaper to pay for the catalyst at the start and increase the rate of reaction after that.
  • Paying for a catalyst may be cheaper than paying for the energy needed to increase either temperature or pressure.
  • By reducing the necessary temperatures and/or pressures, fewer fossil fuels need to be burned, thus reducing negative environmental impact.
  • However, many catalysts are toxic (e.g. transition metals), and these can escape into the environment and contaminate ecosystems.
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Reversible reactions

  • A + B ⇌ C + D
    • The reactants (A + B) can combine to give the products (C + D).
    • The products (C + D) can combine to give the reactants (A + B).
  • If the forwards reaction is exothermic (transfers energy from reactants to the environment), the backwards reaction will be endothermic (transfers energy from environment to the reactants).
  • Exothermic and endothermic reactions can be monitored by measuring the temperature change of the surroundings.
  • Energy is conserved during chemical reactions.
  • The energy released/absorbed by the forward reaction will be exactly equal to the energy absorbed/released by the backward reaction.

Dynamic equilibrium

  • A closed system is a system where reactants and products can't be added or removed
  • At dynamic equilibrium, the rate of the forwards reaction equals the rate of the backward one
  • The equilibrium is dynamic because both the forward and backward reactions are still taking place, but the concentrations of reactants and products remain constant.
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