Colliding particles need at least the activation energy
Definition
Activation energy
The minimum energy that colliding particles must have for a reaction to happen.
Definition
Successful collision
A collision between reactant particles that has at least the activation energy, so the particles react.
- Reactant particles must collide with each other before they can react.
- Most collisions do not lead to a reaction, because the particles have too little energy.
- Only collisions with energy equal to or greater than the activation energy are successful.
- The rate of reaction depends on the number of successful collisions per second.
- So anything that makes collisions more frequent or more energetic increases the rate.
Key Idea
Every rate explanation comes back to how often particles collide and what fraction of those collisions have enough energy to react.
Higher temperature gives more frequent, more energetic collisions
Definition
Kinetic energy
The energy an object or particle has because it is moving.
- Raising the temperature increases the rate of a reaction, and for many reactions a 10 ∘C10\ ^{\circ}\text{C}10 ∘C rise roughly doubles it.
- The particles gain kinetic energy and move faster.
- Faster particles collide more frequently, so there are more collisions per second.
- More of the collisions have energy equal to or greater than the activation energy.
- This effect is the larger of the two, so a much greater fraction of collisions is successful.
- Lowering the temperature has the opposite effect, which is why food keeps longer in a fridge.
Practical
The effect of temperature on rate
- Aim: find how temperature affects the rate of the reaction between magnesium and hydrochloric acid.
- Apparatus: a conical flask with a bung and delivery tube, a 50 cm350\,\text{cm}^350cm3 measuring cylinder, a 100 cm3100\,\text{cm}^3100cm3 measuring cylinder to collect the gas, a trough of water, a water bath, a thermometer, a stop clock, a ruler, scissors, magnesium ribbon and dilute hydrochloric acid.
Method
- Cut magnesium ribbon into 4 cm4\,\text{cm}4cm strips of equal length.
- Measure 50 cm350\,\text{cm}^350cm3 of dilute hydrochloric acid into the conical flask and warm it in the water bath until it reaches the chosen temperature.
- Fill the 100 cm3100\,\text{cm}^3100cm3 measuring cylinder with water and turn it upside down in the trough, with the delivery tube from the flask leading into it.
- Add a strip of magnesium to the acid, replace the bung at once and start the stop clock.
- Stop the clock when 30 cm330\,\text{cm}^330cm3 of hydrogen has collected.
- Repeat at four or five other temperatures, using the same volume and concentration of acid and the same length of ribbon.
- Work out the mean rate at each temperature by dividing the volume of gas by the time, then plot rate against temperature.
Accuracy and safety
- Measure the temperature of the acid immediately before adding the magnesium, because it starts to cool once out of the water bath.
- Repeat each temperature and find a mean time, so that any anomalous result can be spotted.
- Keep flames away, because hydrogen is flammable.
- Wear eye protection when handling the acid.
Higher concentration or pressure makes collisions more frequent
- Increasing the concentration of a solution puts more reactant particles in the same volume.
- The particles are closer together, so they collide more frequently and the rate increases.
- Increasing the pressure of a gas squeezes the same number of particles into a smaller volume.
- This also gives more frequent collisions, so pressure affects the rate only when a reactant is a gas.
- Neither change gives the particles more energy, so the fraction of collisions that succeed stays the same.
Example
Magnesium ribbon fizzes much more vigorously in concentrated hydrochloric acid than in dilute acid, because acid particles hit the surface of the metal more frequently.
Smaller pieces react faster through larger surface area
Definition
Surface area to volume ratio
The surface area of a particle divided by its volume.
- Only the particles on the surface of a solid can collide with particles in the solution or gas around it.
- Cutting a solid into smaller pieces keeps its volume the same but increases its surface area, so its surface area to volume ratio increases.
- A greater fraction of the solid's particles is then exposed to collisions.
- Collisions become more frequent, so the rate increases, although the energy of each collision is unchanged.
- Powdered calcium carbonate reacts with acid much faster than the same mass of marble chips.
- Very fine powders such as flour dust can burn so fast in air that they explode.
Example
- Compare the surface area to volume ratio of a 2 cm2\,\text{cm}2cm cube with that of the same cube cut into eight 1 cm1\,\text{cm}1cm cubes.
- For the large cube, surface area =6×22=24 cm2= 6 \times 2^2 = 24\,\text{cm}^2=6×22=24cm2 and volume =23=8 cm3= 2^3 = 8\,\text{cm}^3=23=8cm3, so the ratio is 24:8=3:124 : 8 = 3 : 124:8=3:1.
- For the eight small cubes, surface area =8×6×12=48 cm2= 8 \times 6 \times 1^2 = 48\,\text{cm}^2=8×6×12=48cm2 and volume =8 cm3= 8\,\text{cm}^3=8cm3, so the ratio is 48:8=6:148 : 8 = 6 : 148:8=6:1.
- Cutting the cube doubles the surface area to volume ratio, so more particles are exposed to collisions.
Explaining rate changes through collision frequency and energy
- For temperature, link it to faster particles, more frequent collisions and more collisions with at least the activation energy.
- For concentration or pressure, link it to more particles in the same volume and more frequent collisions.
- For size of pieces, link it to the surface area to volume ratio, more exposed particles and more frequent collisions.
- For a decrease in any factor, reverse the chain to give fewer successful collisions per second.
- Say that the reaction has a greater rate or takes less time, rather than calling it a faster reaction time.
Exam technique
When a question asks you to explain a change in rate, name the factor, say what happens to the particles and finish with the effect on successful collisions per second.
Self review
- What is a successful collision?
- Give the two reasons why a higher temperature increases the rate of reaction.
- Why does increasing the concentration of an acid increase the rate of its reaction with magnesium?
- Why does pressure affect only reactions involving gases?
- Why does powdered calcium carbonate react faster than marble chips of the same mass?
