chemistry- reactivity

rate of reaction meaning

how quickly reactants are used up or products are formed

3 conditions for a reaction to occur

1) the reactant particles must collide

2) the collision must meet a minimum energy (this is the activation energy)

3) they must collide with the correct orientation

what happens if a particle collision is not successful

the particles bounce apart unchanged

what does the rate of reaction depend on?

the frequency of successful collisions

how to increase the frequency of successful collisions?

1) increase how often particles collide

2) increase the fraction of collisions that have enough energy (a higher proportion succeeds)

activation energy

the minimum energy colliding particles must have in order to react

high vs low activation energy

high- few collisions have enough energy, so reaction is slow

low- many collisions succeed, reaction is fast

why do graphs plateau?

the reactants have been used up

exothermic graphs

go down, because they have negative enthalpy, so the products are lower than the reactants

what are the four factors affecting the rate of reaction?

1) higher concentration- more particles in the same volume, so particles are closer together and collide more often - due to frequency not energy

2) larger surface area- only the particles on the surface can be collided with, so increasing surface area by breaking or powdering exposes more particles

3) higher temperature- particles move faster and have more kinetic energy so a greater proportion of collisions have activation energy

4) catalysts- increases rate of reaction without being used up, provides alternate pathway with a lower activation energy, so a higher proportion of particles have enough energy

catalyst def

A catalyst provides an alternative reaction pathway with a lower activation energy, so a greater proportion of collisions have enough energy to react

when explaining a rate increase, what two points to use?

1) more frequent collisions,

2) greater proportion with enough activation energy

rate of reaction chain answer

what has changed→what it does to particles→what it does to collisions→the effect on the rate

activation energy acts as a

energy barrier

what is a reversible reaction?

a reaction that can go both ways- reactants become the products, and products reform reactants (Written with a double arrow, with forward reaction going to the right and reverse reaction to the left)

why do the two rates of the reverse and forward reactions reach an equilbrium?

forward reaction is fast at the start as only reactants available, begins to decrease as reverse reaction begins to use products for reactants- both eventually reach same rate which is called the equilibrium

what does ‘at equilibrium’ mean?

the forward and reverse reactions are still happening at the same rate. Product is being formed as fast as it is being used up- concentrations of everything remain constant

what are the 3 requirements for an equilibrium?

1) system must be closed (nothing entering or leaving)

2) forward and reverse reactions rates are equal

3) concentration of reactants and particles stay constant (not equal, equilibrium may lie to one side)

what happens if a system is open?

a product such as a gas can leave, altering the concentrations available for use so reverse reaction doesn’t build up, meaning the rates of forward and reverse reactions differ and equilibrium is not reached

when does a equilibrium lie to the right?

when there is more product than reactants, the forward reaction is favoured

when does an equilibrium lie to the left?

when there is more reactants than products, the reverse reaction is favoured

Le Chatelier’s principle

If a change is made to an system at equilibrium, the position of the equalibrium shifts to oppose that change.

what happens to equilibrium when you add a substance

position of equilibrium shifts away from it, to use it up

what happens to equilibrium when you remove a substance

system shifts towards it to replace it

what happens if you raise the temperature of a equilibrium?

shifts in the endothermic direction to absorb the heat and oppose the rise

what happens if you lower the temperature of a equilibrium?

shifts in the exothermic direction release heat and oppose the change

what is the only factor that can change Kc?

temperature

pressure in an equilibrium

only matters if gases are present- raise the pressure it shifts to the side with fewer gas molecules

catalyst in an equilibrium

a catalyst doesn’t change the position of the equilibrium at all, both reactions speed up equally and the equilibrium is simply reached sooner

answering equilibrium questions chain

the direction of the shift→the reason it opposes the change→the effect on the species amounts

amount of Kc (products over reactants) meaning

large- shifted to the right as more products, 1- roughly central, small- shifted to left as more reactants