Temperature
From this graph between sodium thiosulphate and hydrochloric acid you can see that the rate of reaction is fastest at highest temperatures
C5.01 Fast and slow reactions
Some chemical reactions are slow like the rusting of iron.
Some chemical reactions are faster like the burning of wood.
Some chemical reactions are very fast like the explosion of gunpowder.
Task C5.01 State the rate for following reactions;
TNT exploding, petrol burning, rock reacting with water, copper roof turning
green, dynamite being used in a quarry, bread baking.
C5.02 Experiments to investigate rates
Concentration- Carry out
reactions between sodium thiosulphate and hydrochloric acid in a conical
flask using identical volumes and identical temperatures but at different concentrations.
Record the concentrations and the times needed for a cross
under the flask to disappear. Practical C5.02a
Temperature- Carry out the reaction between magnesium and hydrochloric acid in a conical flask for different acid temperatures but for identical volumes of acid, at identical concentrations and for identical masses of magnesium. Record the time for the magnesium to completely react. Practical C5.02b
Particle size- Carry out
the reaction between calcium carbonate and hydrochloric acid in a conical
flask fitted with a stopper and a delivery tube to a measuring cylinder
inverted in water. Use the same masses of powder, small chips and large
lumps of calcium carbonate with the same volumes, concentrations and temperatures
of acid. Record the volume of gas formed every 30 seconds for 10 minutes. Practical
C5.02c
Task C5.02 science investigation
plan
C5.03 Interpreting experimental results
Temperature
From this graph between sodium thiosulphate and
hydrochloric acid you can see that the rate of reaction is fastest at highest
temperatures
Concentration
The rate increases as the concentration increases.
model
showing conc against time graphParticle size
The rate gets bigger as the pieces get smaller.
The rate gets bigger as the surface area gets bigger.
C5.04 Explaining the effect of changing temperature,
concentration and surface area
(i)Concentration
The concentration of a substance,
normally a solution, is the amount in a given volume.
concentration = amount {units
= mol/dm3 or M}
volume
In a higher concentration solution
there are more particles to react therefore there are more collisions.
As a reaction depends on collisions happening, more collisions lead to
a faster reaction rate. If we were doing a reaction with acid and we double
the number of acid particles, we double the number of collisions and therefore
are likely to double the reaction rate.
Task C5.04.1 Draw diagrams to show particles in a
low concentration HCl solution and particles in a high concentration HCl
solution. Show water molecules and HCl and magnesium particles in the
diagrams. Show different numbers of collisions in each diagram.
(ii)Temperature
Increasing the temperature increases
the speed of the reacting particles and faster particles collide more often
than slow ones. The increase in the number of collisions leads to an increase
in the rate of reaction. Increasing the temperature also gives the particles
more energy so that they collide with more violence. Energetic particles
have a better chance of their collisions leading to a reaction.
Task C5.04.2 Draw diagrams to show water molecules
and HCl and magnesium particles in two diagrams at two different
temperatures. Show different numbers of collisions and different energies
for the collisions.
(iii) Particle size
Particle size is all to do with
surface area. Powder has a higher surface area than lumps and therefore
powder makes more collisions possible than lumps. This simple diagram explains
the idea of surface area clearly:
C5.05 Collision theory
Reactions can only occur when particles
collide and if you increase the frequency and/or energy of the collisions,
you increase the rate of reaction.
few collisions in 1 second (small surface area, low concentration, low
temperature) = slow reaction
many collisions in 1 second (large surface area, high concentration, high
temperature) = fast reaction
low energy collisions (low temperature) = slow reaction
high energy collisions (high temperature) = fast reaction
Task C5.05 Describe what would happen on a motorway
if cars were like particles in a chemical reaction and if (a) there were more
cars on the road, (b) the cars travelled faster. Link the description to
collision theory by using the words concentration and temperature in your
answer.
C5.06 The effects of catalysts
A catalyst is a substance that speeds up the rate of a reaction without being used
up. Catalysts are usually transition metals or transition metal compounds.
An example of a catalyst is iron which catalyses the reaction of nitrogen
and hydrogen to produce ammonia. A catalyst usually works either by providing
a surface for the reaction to take place or by forming intermediate compounds.
Catalyst practical
Task C5.06 Match the (catalysts) with the reactions
that they affect.
(iron, manganese dioxide, nickel, platinum, vanadium pentoxide)
hydrogen peroxide ---> water + oxygen
Carbon monoxide + oxygen ----> carbon dioxide
sulfur dioxide + oxygen ---> sulfur trioxide
nitrogen + hydrogen ----> ammonia
vegetable oil + hydrogen ---> margarine
C5.07 Enzymes
Enzymes are biological catalysts. They speed up reactions. They work
best around body heat (37oC). They break up (denature) and stop
working if the temperature rises much above 40oC.
Enzymes
are found in biological washing powder. Enzymes in yeast are used to help
fermentation in making bread and beer.
C5.08 Temperature, pH and enzyme reactions
| pH | rate of enzyme reaction | comments |
| low 1-5 acidic | very slow | |
| neutral about 7 | good rate | ideal pH |
| high 8-14 alkaline | very slow |
| temperature | rate of enzyme reaction | comments |
| cold | slow | |
| 40oC | fast | body temperature ideal |
| 60oC + | slow | enzyme breaks down |
Sketch a graph of rate of an enzyme reaction
against pH.
Sketch a graph of rate of an enzyme reaction against temperature.