3.2.2 Reaction Rates Flashcards
What is collision theory?
Theory that states that particles must collide with the right orientation and energy higher than the activation energy for a reaction to occur
What factors affect the rate of reaction?
- Temperature
- Concentration/pressure
- Use of a catalyst
- Surface area of solid reactants
What effect does increasing concentration have on rate of reaction?
- When concentration of reactants increases, the rate of reaction increases
- Increasing concentration increases the number of particles in the same volume, so the particles are closer together and will collide more frequently
What effect does increasing pressure have on rate of reaction?
- When the pressure increases, the rate of reaction increases
- The same number of gas particles now occupy a smaller volume so the concentration of gas molecules increases
- Therefore, there are more frequent successful collisions
How can you measure the rate of reaction when a product is a gas?
- Monitor the volume of gas produced at regular time intervals using gas collection
- Monitor the loss in mass of reactants using a balance
What does a rate of reaction graph look like?
- The gradient is the rate
- The gradient of a tangent is the rate of reaction at a specific point
- Initial rate is highest as there is the highest concentration of reactants
- Rate slows as the concentration of reactants decreases
- Graph plateaus as the reaction stops when the reactants are used up
What is a catalyst?
A substance that increases the rate of reaction without being used up in the overall reaction
How does a catalyst work?
Provides an alternative reaction pathway with a lower activation energy
What is a homogenous catalyst?
A catalyst that has the same physical state as the reactants
e.g. Making esters with sulfuric acid or ozone depletion with chlorine radicals (2O3 <-> 3O2)
What is a heterogenous catalyst?
A catalyst that has a different physical state from the reactants
E.g. Haber process with iron (N2 + 3H2 <-> 2NH3) or contact process with V2O5 (2SO2 + O2 <-> 2SO3)
How does a heterogenous catalyst work?
- Acts as a surface for the reaction to take place
- Reactant molecules are adsorbed (weakly bonded) onto the surface of the catalyst
- After the reaction, product molecules are desorbed
How are catalysts increasing sustainability and economics?
- Catalysts lower activation energies, which reduces the temperatures needed for processes and the energy requirements
- Less energy means less electricity or fossil fuels used, which results in a reduction of CO2 emissions
- Cuts costs, increases profitability and increases sustainability
What is the Boltzmann distribution?
- Graph of energy and number of molecules
- Some molecules have low energy and some have high energy, but most molecules have energy close to the average energy
- No particles have no energy
- The peak is the most probable energy
- There is no maximum energy for a molecule
- The area under the curve represents the total number of particles (which is fixed)
- A small proportion of molecule have E>Ea
What happens to the Boltzmann distribution when temperature increases?
- The peak is lower and shifted to the right
- A greater proportion of molecules can overcome the activation energy
- As temperature increases, the average energy of the molecules increases
- A greater proportion of collisions will lead to a reaction so rate of reaction increases
- Collisions are also more frequent as the increased energy means that molecules are moving faster
What happens to the Boltzmann distribution when a catalyst is used?
- Activation energy is reduced
- A greater proportion of molecules exceeds the new lower activation energy
- A catalyst provides an alternative reaction route with a lower activation energy so more particles have E>Ec
- More molecules will react to form products on collision, which increases the rate of reaction
