Rates of Reaction

Rates of Reaction To revise the 4 factors that affect rates To 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 theory Collision theory Chemical 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 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 Temperature Why? At lower temperatures, particles will collide: a) less often b) with less energy How? Put more energy into reaction Increasing 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 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

lower pressure higher pressure • 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.

4.4 Pressure Why? Pressure is used to describe particles in gases The 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 Catalysts Why? Expensive to increase temperature or pressure Do not get used up in reaction and can be reused How? Catalysts are made from transition metals, e.g. iron, nickel, platinum Provide 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 reaction time e.g. Change in volume gas made time

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

Calculating from graphs rate of reaction = 45 cm3 20 s 70 60 x In this case 50 Rate of reaction graphs ARE always curves (we will be looking more at the reasons for this next lesson) 40 Change in y rate of reaction = hydrogen produced (cm3) 30 y Change in x 20 = 2.25 cm3/s 10 0 0 10 20 30 40 50 time (seconds) The gradient of the graph is equal to the rate of reaction at that time

LOW LEVEL (F)

STANDARD LEVEL (F&H)

HIGH LEVEL (H)

Rates of Reaction