4.6 the rate and extent of chemical change

chemistry aqa gcse

How do you calculate mean rate of reaction?

Change in quantity (e.g. mass, volume)/change in time

What is catalyst?

A substance that increases rate of reaction without being used up

How does temperature affect the rate of reaction? (explain using collision theory)

If temperature increases particles have more energy so move quicker, collide more, collide with more energy, rate of reaction increases

How does concentration/pressure affect the rate of reaction?

higher concentration/pressure = more reactant particles per unit volume, more frequent collisions, rate of reaction increases

How does surface area affect the rate of reaction?

greater surface area = greater SA:V = more particles are exposed to other reactant(s), more frequent collisions, rate of reaction increases

How does a catalyst affect the rate of reaction?

A catalyst provides an alternative pathway for the reaction with a lower activation energy, which increases the rate of reaction as particles can react sooner/faster

how do you convert from cm3 to dm3

divide by 1000

how do you convert from g/dm3 to mol/dm3

divide by RFM

explain collision theory

chemical reactions can only occur when reacting particles collide with each other with sufficient energy

define activation energy

the minimum amount of energy that particles need to react

what is a reversible reaction?

a reaction where the products can react to produce the original reactants

what is an exothermic reaction?

a reaction that releases energy to its surroundings

what is an endothermic reaction?

a reaction that uses energy from its surroundings

if a reversible reaction is exothermic one way, will it be endo- or exothermic the other way?

endothermic

when is equilibrium reached?

when a reversible reaction occurs in apparatus which prevents the escape of reactants/products and the forward and reverse reactions occur at the same rate

what is Le Chatelier’s Principle?

if a system is at equilibrium and a change is made to any of the conditions, the system will respond to counteract that change

how does a change in concentration effect equilibrium?

if the concentration of a reactant is increased, more products will be formed until the concentration of the reactant is reduced but if the concentation of a product is decreased, more reactants will react until equilibrium is reached

how does a change in temperature effect equilibrium?

if the temperature is increased the equilibrium will move in the endothermic direction to reduce the increase in temperature, meaning the yield from the endothermic reaction increases and the yield from the exothermic reaction decreases

how does a change in pressure effect equilibrium?

in gaseous reactions, if the pressure is increased, the equilibrium will move towards the side with the least number of gaseous molecules so that the pressure is reduced

does pressure always affect equilibrium?

no, if the number of gaseous molecules of both sides of the equation is the same and the pressure is changed, there will be no impact on equilibrium

do catalysts affect equilibrium?

no

haber process equation (reversible and uses iron catalyst)

N₂ (g) + 3H₂ (g) → ← 2NH₃ (g)

what is the haber process used for

to produce pure ammonia

give sources of hydrogen

reacting methane with steam

what is ammonia used for

to make nitrogen based fertilisers for farming

what are the pure nitrogen and hydrogen passed over

an iron catalyst at 200 atmospheric pressure and 450 degrees C

how is the yield of the haber process increased

ammonia cooled to form liquid which is collected

how are the conditions of the haber process changed to produce the greatest yield of ammonia

temperature and pressure adjusted to shift the position of equilibrium to the right hand side (to produce more ammonia)

why is the temperature of 450C a compromised temperature for the haber process

it gives a relatively high rate of reaction and a relatively high yield of ammonia, rather than very high of either and low of the other