Rates and Equilibria Flashcards
Effect of concentration on rate of reaction
As concentration increases, there are more molecules present in the same volume. Therefore, the molecules are closer together and there is an increased chance of collisions. This means more collisions will occur in a certain length of time, thus increasing the rate of reaction.
Effect on pressure on rate of reaction
As the same number of molecules now occupy a smaller volume (increasing concentration), the molecules are closer together and there is an increased chance of collisions. This means more collisions will occur in a certain length of time, thus increasing the rate of reaction.
Catalysts….
Speed up the rate of reactions, but are not consumed by the overall reaction
Catalysts and conditions
If a catalyst is used, often the conditions required for a reaction are less extreme - lower temperatures and pressures, thus reducing energy demand and CO2 emissions from burning fossil fuels
Catalysts and waste
Often, catalysts allow different reactions to be used with better atom economy and reduced waste
Biological catalysts
Catalysts are often enzymes, generating very specific products in conditions close to room temperature and pressure
Examples of catalysts in industry
Have great economic importance!
- Iron in ammonia production
- Zeigler-Natta catalyst in poly(ethene) production
- Platinum/palladium/rhodium in catalytic converters
Catalyst enthalpy profile diagram
- enthalpy on y axix, progress of reaction on x axis
- line at beginning for reactants, line at end for products
- gap between lines is dH (arrow from reactants to products)
- activation energy is a hump that goes from reactants, above products to land on products
- activation energy with catalyst is a smaller hump
- both these arrows point up
Point of lowering activation energy
If the activation energy is lowered (e.g. with a catalyst), more molecules will have the required energy to react, without a change in conditions. Therefore, more of the collisions occurring will be successful
Boltzmann distribution
- y axis is number of molecules with a given energy
- x axis is energy
- curve, starting at the origin, peaking, then leveling out, with an asymptote as the x axis
- mark on E(a), usually after peak
- area under curve is equal to total number of molecules
Boltzmann distribution and catalysts
Catalysts lower the activation energy
- Therefore, E(c) will be further left of E(a)
- Therefore more molecules will have the required activation energy
- Therefore more collisions will be successful
- Therefore the rate of reaction will increase
Boltzmann distribution and temperature
As temperature increases, curve gets squashed to the right. The peak will be further to the right, but it will be lower, as the number of molecules have changed. The high temperature line will be above the low temperature line past the peak. Therefore, more molecules will have the required activation energy, as the area under the curve is greater
Dynamic equilibrium
The equilibrium in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction
Le Chatelier’s principle
When a system in dynamic equilibrium is subjected to a change, the position of the equilibrium will shift to minimise the change
N(2)(g) + 3H(2)(g) 2NH(3)(g) if the pressure is increased, state and explain which way the equilibrium will shift
Equilibrium will shift to the side with the fewest gas molecules, thus decreasing the pressure. Therefore, the equilibrium will shift to the right
