Chapter 20- Collision Theory Flashcards
What is the collision theory?
- reactions can only happen when particles collide, but not all collisions cause a reaction
- In order for a reaction to happen, particles have to collide with enough energy
- the minimum amount of energy particles have to collide with is called the activation energy
- collisions with more than or equal to the activation energy are called successful collisions
- the more successful collisions per second, the faster the rate of reaction
Why do reactions slow down?
- the particles are being used up as the reaction happens
- the collision rate decreases (there are fewer collisions per second) and the reaction slows down
How does collision theory explain why small solids react faster than large ones?
- the more finely divided a solid, the faster the reaction
- this is because the surface area is larger, so there are more particles exposed on the surface of the solid
- the particles collide more often and so there are more successful collisions per second
What is meant by rate of reaction?
- rate of reaction is the speed at which the amount of reactants decreases or the amount of products increases
- it is measured as a change in the concentration, volume or mass per unit time
How is rate of reaction calculated?
Rate of reaction= concentration or volume or mass/unit time
how are the rates of different states measured ? (Liquid, gas)
1) GAS - mass loss: -the mass goes down on a scale because the reaction produces a gas which escapes from the flask
- cotton wool is put in the top of the flask as this allows gas to escape but stops any acid spitting out of the flask
2) GAS- volume of gas: -the gas can be collected in a gas syringe or a measuring cylinder filled with water (this cannot be used if the gas dissolves in water)
3) FOR LIQUIDS -time taken for cross to disappear-if a solid is produced, the reaction turns cloudy and you can no longer see through it, down onto a picture of a cross
How does collision theory explain how the concentration of a solution changes rate of reaction
- increasing the concentration of a reactant increases the rate of reaction
- there are more particles in a given volume so the particles are closer together and so collide more often
- so there are more successful collisions every second
Explain a graph of two experiments, one with half the concentration of reactant used
- the one with half the concentration will have a slower rate of reaction
- the line on the graph will be less steep
- it will finish at half the height, because half the concentration means half the number of particles so half of the volume of gas will be produced
How does collision theory explain why increasing temperature increases rate of reaction?
Two factors:
1) -the particles mover faster/ have more kinetic energy and so collide more frequently
2) -there is a large increase in the number of particles that have more than or equal to the activation energy
- this means that it is much more likely two particles will react when they collide, so there are more successful collisions per second
Explain collision theory on rate of reaction when changing the pressure of a reactant (gas)
- increasing the pressure of a reaction when the reactants are gases speeds up the rate of reaction.
- the particles are forced closer together so they collide more often and there are more successful collisions per second
What are catalysts?
- substances which speed up a chemical reaction without being used up in the process
- they are chemically unchanged at the end of the reaction
What is an example of a reaction using a catalyst?
-the decomposition of hydrogen peroxide can be catalysed by manganese oxide (Mn02):
Hydrogen peroxide -> water + oxygen
2H202 -> 2H20 +O2
Explain the practical of the decomposition of hydrogen peroxide with a catalyst
- to show that manganese oxide is a catalyst the reaction can be done first with a catalyst, then without
- more gas will be produced in a given time when the catalyst is used
- if the catalyst is weighed at the beginning of the experiment, then washed, filtered and dried, the reweighed mass should be the same, showing that the catalyst is not used up.
Use collision theory to explain how a catalyst works
- a catalyst works by providing an alternative route for reaction, involving a lower activation energy
- this means more of the particles have energy higher than or equal to the activation energy, so there are more successful collisions every second, making the rate of reaction faster
What is the reaction formula for the reaction of solid calcium carbonate (marble chips) and hydrochloric acid? and what is this reaction used for?
CaCO3 + 2HCl -> CaCl2 + CO2 + H2O
-used to investigate the effect of particle size on rate by measuring volume of gas
