Rate of reactions
● Rate of reaction = Quantity of products formed/Time taken(s)
Quantity of reactants used / Time taken(s)
● Rate is a measure of the change that happens in a single unit of time
● Unit
○ Mass → gram
○ Time → seconds
○ Volume → cm2
● Mass of reactant will have negative slope as it is
being used up
● Volume of products formed will be a positive slope
as more and more is being produced
● Faster the reaction, the steeper the curve
● When the reaction is over, the curve goes flat
Graph questions
● Why is the slope of the graph steeper at the beginning of the reaction?
○ This is because there are more particles available in the reaction leading to more particles
passing the activation barrier
● What happens to the graph as the reaction progresses?
○ The graph gets less steep because there are less particles available leading to less
effective collisions leading to less particles passing the activation energy
● Why is the graph constant after a while?
○ This is because all the reactants are used up therefore no collisions can take place so no
reaction takes place
● Minimum amount of energy required for particles to react is called activation energy
● Rate of reaction depends on-
○ Frequency of collisions between particles
○ The energy by which they collide
● Collisions are measured by → per unit time/ per unit volume / per unit area
● The rate of a reaction depends on how many successful collisions there are per unit time
● If particles collide with less energy than the activation energy, they will not react and just
bounce off each other(ineffective collisions)
● Collision theory
○ Particles must collide with each other
○ The collisions must have enough energy to be successful. In other words, enough energy to
break bonds to allow reaction to occur
Changing the rate of reaction
● Concentration
○ In dilute acid, there are not so many particles per unit volume, so there is a less chance of
an effective collision
○ When acid is more concentrated, there are more particles per unit volume, so there is a
higher chance of successful collisions per unit time therefore the reaction becomes
faster.
● Temperature
○ A higher temperature means more energy which causes the particles to collide more often
which leads to more effective collisions which leads to more particles passing the
activation barrier which means a faster rate of reaction
● Pressure
○ It will not affect the reaction unless all the products are in gaseous state as pressure only
affects gases
○ When you increase pressure on 2 reacting gases, it means gas molecules per unit volume
therefore there is greater chance of effective collisions therefore the rate of reaction
increases
● Surface area
○ If a solid is split into multiple pieces, the surface area increases, this means an increased
area for the reactant particles to collide with each other
○ The smaller the pieces, the larger the surface area. This means more collisions and a
greater chance of reaction
● A catalyst alters the rate of reaction by providing an alternative pathway and lowering the
activation energy
● This allows more effective collisions and therefore rate of reaction increases
