The Rate and Extent of Chemical Change
The Rate and Extent of Chemical Change
As the reactants react, their concentrations fall
- so the
forward reaction will slow down.
more and more products
are made and their
backwards reaction will speed up
After a while the
forward reaction will be going at the exact same rate as the backward one
system is in equilibrium.
both reactions are still happening, but there's no overall effect
. This means the
of reactants and products have
reached a balance and won't change.
Equilibrium is only reached if the reversible reaction takes place in a 'closed system'
. A closed system just means that
none of the reactants or products can escape and nothing else can get in.
Rates of Reaction
The rate of a chemical reaction is how fast the reactants are changed into products.
One of the slowest is the rusting of iron.
Other slow reaction include chemical weathering (like acid rain damage to limestone buildings).
An example of a moderate speed reaction would be the metal magnesium reacting with an acid to produce a gentle stream of bubbles.
Burning is a fast reaction, but explosions are even faster and release a lot of gas.
Explosive reactions are all over in a fraction of a second.
Understanding Rate of Reaction Graphs
Speed of a Reaction = Amount of product formed OR Amount of reactant used up / Time (s)
steeper the line
on the graph, the
faster the rate of reaction.
Over time the
line becomes less steep
as the reactants are
reactions have the
flat in the least time.
Factors affecting Rates of Reaction
Two things the rate of a chemical reaction depends on...
collision frequency of reacting particles
(how often they collide).
The more collisions there are the faster the reaction is.
For example, doubling the frequency of collisions doubles the rate.
energy transferred during a collision.
Particles have to collide with enough energy for the collision to be successful.
Activation energy is the minimum amount of energy that particles need to react.
Particles need this much energy to
break the bonds
in the reactants and
start the reaction.
What the Rate of Reaction depends on...
When temperature is increased, the
particles all move faster.
If they're moving faster,
they're going to collide more frequently.
faster they move the more energy they have
more of the collisions will have enough energy to make the reaction happen.
of a solution or the
If a solution is made
it means there are
more particles knocking about in the same volume of water
(or other solvent).
Similarly, when the
pressure of a gas is increased
, it means that
the same number of particles occupies a smaller space.
This makes collisions between the reactant particles more frequent.
- this changes depending on the size of the lumps of a solid.
If one of the reactants is a solid, then
breaking it up
into smaller pieces will
increase its surface area to volume ratio.
This means that for the
of the solid, the
particles around it will have more area to work on
- so there will be
collisions more frequently.
The presence of a
A catalyst is a substance that speeds up a reaction, without being used up in the reaction itself. This means it's not part of the overall reaction equation.
Different catalysts are needed for different reactions, but they
all work by decreasing the activation energy needed for the reaction to happen.
They do this by providing an
alternative reaction pathway
with a lower activation energy.
Finding Reaction Rates from Graphs
Mean Reaction Rate
A rate of reaction graph
amount of product formed or amount of reactant used up on the y-axis
time on the x-axis.
To find the mean rate of the whole reaction, just work out the
overall change in the y-value and then divide this by the total time taken for the reaction.
You can find the mean rate of reaction between
any two points
in time on a rate of reaction graph.
Tangents and Reaction Rates at a particular point
To find the rate of reaction at a particular point in time, you need to find the gradient of the curve at that point.
The easiest way to do this is to
draw a tangent to the curve
- a straight line that touches the curve at one point but doesn't cross it.
You then work out the gradient of the tangent.
SESSION 6 REVISION