# Rates and size of particle slides

```Why study rates of reaction?
The rate of a reaction is the speed at which it happens.
Some reactions happen very quickly such as sodium reacting with water, whilst
other reactions are slower, such as iron rusting.
Some reactions take years to occur, such as the chemical weathering of limestone.
Some reactions take fractions of a second to happen. Explosions, such as hydrogen
burning are very fast reactions.
By studying rates we can learn how to control reactions.
● The chemical industry generally requires reactions to happen quickly - more
chemicals produced in a quicker time equals more profit!
● Some reactions need slowing down if they are too fast - explosions can be
avoided by applying your knowledge of rates of reaction.
● Chemical reactions in food can be controlled to slow spoilage.
Various factors can be investigated that affect the rate of a reaction
such as:
• size of solid particle reacting (surface area)
• amount of chemicals reacting (concentration / pressure)
• temperature
• catalysts
• radiation such as light or uv
Particles can only react once they have collided. A successful
collision will result in reactants changing into products. This idea
is known as collision theory.
Investigating the effect of particle size on the
rate of reaction
Marble chips (calcium carbonate) react with hydrochloric acid to produce carbon
dioxide gas.
calcium
carbonate
+ hydrochloric acid
As the gas given off leaves the flask the
total mass of the flask and its contents
decreases slightly.
Readings of the mass loss can be taken.
Typically at 30 second to 1 minute intervals.
The experiment can be carried out twice,
once with large marble chips and again with
small marble chips.
Plotting a graph of mass loss against time
will enable the rate of the reaction to be
deduced.
calcium chloride + water + carbon
dioxide
1.
2.
3.
4.
5.
6.
Measure the 50cm3 dilute acid into the
conical flask (1 to 2 mol/dm3).
Have a loose plug of cotton wool to
prevent “spitting” of droplets of liquid.
Have the large marble chips of known
mass ready. (20g is a suitable amount).
Add the marble chips, place the cotton
wool in the neck and start the stopwatch.
balance stops changing.
Repeat the experiment with small chips.
or draw out a table like the one on the right.
Record the mass loss every 30 s for 2.5
minutes and then every minute until the
mass stops changing (usually between 10 12 minutes)
Time/min
Mass loss/g
0
0.00
0.5
1
1.5
Time/m Mass loss/g Mass loss/g
in
(large chips) (small chips)
0
0.00
0.00
0.5
1
1.5
2
2.5
3
4
5
6
7
8
9
10
If you are not using the
instructions.
Plot a line graph with mass loss
on the vertical axis and time on
the horizontal axis.
Plot both sets of result on the
same axes.
• label axes inc units
• title
Explaining the effect of increasing particle size
• If we grind up a solid to a powder we massively increase
the surface area.
• We therefore massively increase the rate of any reaction
Slow
Very fast
Explaining the effect of increasing particle size
Look at the diagram below. Assume the large cube is a large marble
chip with dimensions 6mm x 6mm x 6mm. The second set of cubes
are medium sized marble chips, whilst the third set are small marble
chips. The volume of each set of cubes is the same (same amount
of calcium carbonate)
Work out the surface area of each set of cubes.
What happens to the surface area as the cubes get smaller?
Large marble chip
Small marble chips
Acid molecule
Look at the effect of splitting one large chip into 4 smaller chips.
• How does the number of acid molecules in contact with the marble chips
change?
• How will this affect the number of collisions between the acid and the marble?
• How will this affect the rate of reaction?
Slower and slower
Reactions do not proceed at a steady rate.
They start fast and get slower and slower.
This is not surprising because the reactant concentration (and
the chance of collision) gets lower and lower as time progresses.
slower
fast
0
25
very slow
75
Percentage completion of reaction
stopped
100
Rates and Graphs
Slow reaction
Fast reaction
Amount of reactant
Amount of product
• These show the increasing amount of product
or the decreasing amount of reactant.
Fast reaction
Slow reaction
Time
Time
Rate graphs and reactant concentrations
Amount of product
All product
Mix of reactant
And product
product
Reactant Concentration falls
Rate of Reaction falls
All reactant
Time
1. Why are the quantities of acid and marble kept the same in
both experiments?
2. Explain why the mass decreases as the experiment proceeds.
3. What is the purpose of the cotton wool?
4. How do the results compare for the 2 sizes of marble chips? In