14 - rates and equilibrium Flashcards
Define rate of reaction
Change in concentration of a reactant or product in a given time
What is the equation for rate of reaction?
Rate = delta c / delta t
Where c is concentration and t is time
What is the unit of rate of reaction?
mol dm^-3 s^-1
What 4 things increase the rate of reaction?
If you increase the following, you increase rate of reaction:
Concentration (pressure for gases)
Surface area
Temperature
You can also increase rate of reaction by adding a catalyst
Define collision theory
Reactions occur when reactant particles collide provided that they collide with a minimum kinetic energy (the activation energy)
What statement do you always put at the end of an explanation to why rate of reaction is increased?
“There are more frequent successful collisions”
How does orientation come into collision theory?
Molecules must collide in the correct orientation in order for a reaction to happen
How does increasing concentration / pressure increase rate of reaction?
There are more molecules per unit volume so you get “more frequent successful collisions”
How does increasing temperature increase rate of reaction?
Increasing temperature increases the kinetic energy of molecules so you get “more frequent successful collisions”
How does increasing surface area increase rate of reaction?
More surface area that can collide so there are “more frequent successful collisions”
Define catalyst
A substance that increases the rate of reaction without being used in the process by providing an alternative route for the reaction to lower the activation energy
What happens to the catalyst throughout the reaction?
It initially reacts and forms different intermediate products but it then regenerates at the end so none has been used up
What is the difference between a homogeneous and a heterogeneous catalyst?
Heterogeneous catalyst is in different state of matter to the reactants
Homogeneous catalyst is the same state
What do “adsorbed” and “desorbed” mean?
Adsorbed means stuck to the surface
Desorbed means unstuck from the surface
Give 3 examples of heterogeneous catalysts
V~2 O~5 (vanadium pentoxide)
Iron
Nickel
How do heterogeneous catalysts work?
Reactants are ADSORBED onto surface of catalyst
Reaction takes place, old bonds weaken and new bonds form
Products are DESORBED from catalyst and diffuse away
Give an example of a homogeneous catalyst
The Cl• radical in ozone depletion
What are the 2 main environmental effects of using catalysts?
Less energy used so less finite fossil fuels need to be used
Also, less fossil fuels so less polluting emissions like CO~2
Give a well known reaction which uses a catalyst
Haber process
What is the Boltzmann distribution?
It shows energy on the x axis and shows how many molecules will have that energy on the y axis
It gives a smooth curve
Which molecules on the Boltzmann distribution can react?
The ones with higher energy than the activation energy
What is the area under the curve equal to in the Boltzmann distribution?
The total number of molecules
describe the Boltzmann graph
Starts at origin
-Shows that no molecules can have 0 energy
important to remember the line never touches the x axis, even at the end as it approaches 0, since no molecules can have 0 energy
What is a reversible reaction?
Products and reactants are in equilibrium and the reaction doesn’t go to completion
How do you show a reversible reaction in an equation?
Draw an equals sign
Put a forward facing half arrow on the top stick
Put a backwards facing half arrow on the bottom stick
Define dynamic equilibrium
Where the rate of the forwards reaction is equal to the rate of the backwards reaction
What is equilibrium position?
Extent of a reaction at equilibrium
symbol: A triple equals sign, then “m”
What is Chatelier’s principle?
When a system in dynamic equilibrium is subjected to change, the equilibrium position will shift to minimise the change
What happens to equilibrium position when concentration of reactants is increased?
Increasing conc. of reactants favours forwards reaction
Decrease this favours the backwards reaction
When is equilibrium position affected by pressure?
Only when gases are present
What happens to equilibrium position when pressure is increased?
The reaction where the product has less moles is favoured when pressure is increased
What happens to equilibrium position when temperature is increased?
What happens to equilibrium position when temperature is increased?
How can you see which reaction is endothermic in an exam question?
The enthalpy of the forwards reaction will be given
What are the pressure and temperature for the Haber process?
High pressure and low temperature to favour the forward reaction
What are the reasons for not making the temperature too low and not making the pressure too high in the Haber process?
Very high pressure is dangerous
Very low temperature would decrease rate of reaction so a compromise needs to happen
What catalyst is used in the Haber process?
Iron catalyst
What are the actual values for the temperature and pressure of the Haber process?
350 - 500 C temperature
100 - 200 atm (atmospheres) pressure
What percentage of reactants in the Haber process are turned into ammonia?
Only 15% of reactants
What do square brackets mean?
The concentration of the substance inside the square brackets
What is K~c and what does it tell us?
K~c is the equilibrium constant and it tells us if you favour reactants or products
How do you calculate K~c for a given reaction?
You write the reaction equation
You calculate K~c by multiplying the concentrations of the products on the numerator
Then you multiply all concentrations of reactants and put on the denominator
Any “big” numbers in the reaction equation are turned into powers
What is the value of K~c which tells you a reaction favours reactants or products?
If K~c < 1, reactants are favoured
If K~c > 1, products are favoured
If K~c = 1, products and reactants are favoured equally
Give 2 features of a reversible reaction when dynamic equilibrium is set up
Rate of forwards reactions equals rate of backwards reaction
Concentrations of all substances REMAIN CONSTANT
For a reaction in equilibrium between an acid and alkali
What effect does adding a DIFFERENT acid have and why?
Favours the reaction producing the alkali
Because concentration of H+ ions is greater
