rates of reaction to revise the 4 factors that affect rates to revise methods of measuring rates
TRANSCRIPT
Rates of Reaction
To revise the 4 factors that affect ratesTo revise methods of measuring rates
• Reactions occur because there are successful collisions between reactant particles, i.e. they collide in the correct direction and with enough energy to overcome the minimum amount of energy needed to start a reaction, the activation energy Ea.
• Anything that increases the frequency of these collisions will increase the rate.
4.1 Collision theory4.1 Collision theory
Collision theoryChemical reactions only occur when reacting particles collide with each other with sufficient energy. The minimum amount of energy is called the activation energy
Rate of reaction increases if:• temperature increases• concentration or pressure increases• surface area increases• catalyst used
Surface area• The more particles that are exposed,
the higher the frequency of successful collisions therefore faster the rate
4.2 Surface area4.2 Surface area
Why?The inside of a large piece of solid is not in contact with the solution it is reacting with, so it cannot react
How?Chop up solid reactant intosmaller pieces or crush intoa powder
Temperature• This gives particles more energy so they
collide with more energy and more frequently. More of the particles have energy ≥ EA . This increases rate as there are more successful collisions
4.3 Temperature4.3 Temperature
Why?At lower temperatures, particles will collide:a) less oftenb) with less energy
How?Put more energy into reactionIncreasing the temperature by 10oC will double the rate of reaction
Concentration• Doubling the concentration will double the
number of particles and hence double the chance of successful collisions, therefore the reaction should take half the time and the rate should be twice as fast
4.4 Concentration4.4 Concentration
Why?Concentration is a measure of how many particles are in a solution. Units = mol/dm3
The lower the concentration, the fewer reacting particles, the fewer successful collisions
How?Add more reactant to the same volume of solution
Pressure•As the pressure increases, the space in which the gas particles are moving becomes smaller. The gas particles become closer together, increasing the frequency of collisions. This means that the particles are more likely to react.
lower pressure higher pressure
4.4 Pressure4.4 Pressure
Why?Pressure is used to describe particles in gasesThe lower the pressure, the fewer successful collisions
How?Decrease the volume or Increase the temperature
Catalyst• This provides a surface for the reaction to
take place on and hence increases the frequency of collisions. It also provides a different route for the reaction with a lower activation energy. This means that more of the collisions will result in a reaction
4.5 Catalysts4.5 Catalysts
Why?Expensive to increase temperature or pressureDo not get used up in reaction and can be reused
How?Catalysts are made from transition metals, e.g. iron, nickel, platinumProvide surface area for reacting particles to come together and lower activation energy
Rates of Reaction• The rate is a measure of how fast the
products are made or reactants are used up
Rates of Reaction• A demonstration of the change in
rate of reaction is the “disappearing cross” where a constant amount of precipitate is made
Shows effect of concentration or temperature
Rates of Reaction• This could be measured by collecting
a volume of gas Shows effect of surface area
Rates of Reaction
• This could be measured using the loss in mass of a gas
Shows effect of surface area
Rates of Reaction
Rate = change in reactiontime
e.g. Change in volume gas made time
30s 60s 90s 120s 160s
Amount of product made (cm3)
25 45 60 70 75
To work out the rate of reaction between 30-60s:
45 cm3 - 25 cm3 = 20 cm3 = 0.666667 cm3/s 60s-30s 30s DON’T
FORGET THE
UNITS!!!
Now work out the rates between:A) 90 to 120s
B) 30 to 160s
10 cm3/30s = 0.33cm3/s
50 cm3/ 130s =0.38cm3/s
hyd
rog
en p
rodu
ced
(cm
3)
time (seconds)10 20 30 40 50
10
20
30
40
50
60
70
00
x
y
Calculating from graphs
The gradient of the graph is equal to the rate of reaction at that time
rate of reaction =
Change in x
Change in y
rate of reaction =20 s
45 cm3
= 2.25 cm3/s
In this case
Rate of reaction graphs ARE always curves (we will be looking more at the reasons for this next
lesson)
LOW
LEVEL
(F)
STANDARD
LEVEL
(F&H)
HIGH
LEVEL
(H)
