Flashcards in C10 - Reaction Rates and Equilibrium Deck (18):
How does rate of reaction vary over time?
Initial rate is high due to high concentration of reactants.
Rate decreases due to a fall in concentrations of reactants.
Rate becomes zero when all reactants have been used.
What can change the rate of a reaction?
What is required for a successful collision (collision theory)?
Particles to collide with the correct orientation and sufficient energy to overcome the activation energy barrier of the reaction.
How can the progress of a chemical reaction be followed?
For reactions producing gas?
Monitoring removal of reactant (decrease in conc)
Following the formation of product (increase in conc)
For reactions producing gas, gas can be collected and measured or mass lost can be measured.
How does a catalyst work?
It increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy.
What's a homogenous catalyst?
One which has the same state as the reactants.
It reacts with reactants to form an intermediate. This intermediate then breaks down to give the product and regenerate the catalyst.
What's a heterogenous catalyst?
One which has a different state to the reactants.
They're usually solids in contact with gaseous reactants or reactants in solution.
Reactant molecules are absorbed/weakly bonded onto the catalyst surface where the reaction occurs.
The products then leave the surface by desorption.
What is the Boltzmann distribution?
The spread of molecular energies in gases.
No molecules has zero energy so the curve starts at the origin.
The area under the curve is equal to the number of molecules.
There is no max energy for a molecule - the curve doesn't meet the x axis at high energy.
What does the Boltzmann distribution look like if temperature is increased?
The peak is lower and shifted to the right and a greater proportion of molecules can reach the activation energy.
More molecules have an energy greater than or equal to the activation energy. Therefore a greater proportion of collisions will be successful and cause reaction. Collisions will also become more frequent.
What does the position of equilibrium of a reversible reaction indicate?
The extent of the reaction.
If temperature, concentration or pressure of reactants or products is changed, the position of equilibrium may change.
Reversible reactions are dynamic - forward and reverse reactions are taking place.
What is le Chatelier's principle?
When a system in equilibrium is subjected to external change, the system readjusts itself to minimise the effect of that change.
How does concentration affect a reaction in equilibrium?
If more products were being formed, the position of equilibrium shifts to the right. (Decrease products)
If more reactants were being formed, the position of equilibrium shifts to the left. (Increase products)
How does temperature affect a reaction in equilibrium?
The shift in equilibrium depends on the enthalpy change of the reaction.
An increase in temperature shifts the equilibrium position in the direction of the endothermic reaction. (ΔH positive).
A decrease in temperature shifts the equilibrium position in the direction of the exothermic reaction. (ΔH negative).
How does a catalyst affect a reaction in equilibrium?
It doesn't change the position of equilibrium but increases the rate at which equilibrium is reached.
What's the equilibrium law?
For any reaction at equilibrium, it is possible to write an expression for the equilibrium constant 'Kc' in terms of equilibrium concentrations, where:
Kc = [products] / [reactants]
= conc of products / conc of reactants
What do square brackets [ A ] represent?
They're shorthand for 'the concentration of' e.g. the concentration of A.
How is rate of reaction calculated and what are the units?
Rate = change in concentration / time