GCSE OCR Gateway Chemistry
C5.2a-d
C5.2 Controlling reactions
Summary
Common misconceptions
Learners often misinterpret
rate graphs and think that catalysts take part in reactions and run out/get
used up.
Underlying knowledge and
understanding
C5.2a To be able to suggest
practical methods for determining the rate of a given reaction
C5.2b To be able to interpret
rate of reaction graphs
C5.2c To be able to describe
the effect of changes in temperature, concentration, pressure, and surface area
on rate of reaction
C5.2d To be able to explain
the effects on rates of reaction of changes in temperature, concentration and
pressure in terms of frequency and energy of collision between particles
Rate vs temperature
In this
experiment, we will find out how temperature affects the rate of reaction
between sodium thiosulphate Na2S2O3 with
hydrochloric acid.
The thing is
both sodium thiosulphate and the acid are colourless and no appreciable amount
of gas is evolved so how do we measure the reaction rate? However, when the two solutions are added to
each other they turn cloudy and then produce a yellow precipitate of sulphur.
Measuring
the time it takes for the sulphur precipitate to cover a mark on paper—it’s
sometimes called the “disappearing cross” experiment— is inversely proportional
to the reaction rate. In other words the
longer it takes for the cloudiness to obscure the mark on paper the slower the
reaction rate.
Results will
be consistent if we use the same mark in each experiment and the same apparatus
and solutions. All we will change is the
temperature of the solutions. It also
helps if the same person looks to see when the cross is covered.
The conventional
set up looks like this:
Here is a
set of typical results the like of which you can find any number on the
internet.
These are
reliable results since we have three results per temperature and these are then
averaged to give the final value that can then be plotted.
As you can
see the greater the temperature, the shorter the time it takes for the
precipitate of sulphur to cover the cross.
Increasing the reaction temperature increases the rate of formation of
the sulphur.
Explanation
We can use
collision theory to explain the effect of temperature on reaction rate. Increasing temperature we can argue increases
the frequency of motion and therefore the frequency of collision between particles
of acid and thiosulphate.
However,
reactions require a certain level of energy before the molecules collide
successfully. So increasing the
temperature must push more molecules over the energy limit for the reaction and
so ensure the reaction rate increases. In
other words, increasing the temperature of reaction increases the frequency of
successful collisions.
No comments:
Post a Comment