The rate of reaction measures how much product is formed in a certain time.
The mass of a solid product is often measured in grams while the volume
of a gaseous product is often measured in cm3.
The time period chosen may depend upon the rate of the reaction. For example, it
may be a few seconds for a fast reaction or a few minutes for a slow reaction.
The units for rate of reaction are commonly written as:
g/s or g/min
cm3/s or cm3/min
Measuring the rate where a
gas is produced
The apparatus needed depends on the nature of the
product being measured:
The volume of a
gas is usually
measured with a
gas syringe, or
sometimes an
upside-down
measuring cylinder
or burette
The mass of a
substance - solid,
liquid or gas - is
measured with a
balance
Calculating the Rate Of Reaction
Gradient = y/x
Rate of reaction can be worked out from the
gradient of a graph
Limiting Reactants
A reaction stops when all the particles of
one of the reactants are used up.
For example, magnesium reacts with hydrochloric
acid. When the reaction is over: Magnesium is the
limiting reactant if it is all gone at the end Hydrochloric
acid is the limiting reactant if some magnesium is left at
the end
In a reaction involving
two reactants:
The reactant in excess is still there at the end of the
reaction (although in a smaller amount than at the start)
The limiting reactant is the one that is all
used up at the end of the reaction
Directly proportional
The amount of product formed in a reaction is directly
proportional to the amount of limiting reactant used.
This means that a graph showing
amount of product formed against
amount of limiting reactant will give a
line that:
Is straight
Has a
positive
gradient
Passes through
the origin (0,0)
Reactions happen when particles collide with enough energy. The more
reactant particles there are to begin with, the more product can be formed.
This is why the amount of product formed is directly proportional to the amount
of limiting reactant used.