rate of reaction

• reactions can go at all sorts of different...

  rates
• the rate of a chemical reaction is how fast...

  the reactants are changed into products
• slow reaction examples


  - rusting of iron

  - chemical weathering
• moderate speed reaction example

  metal magnesium reacting with an acid to produce a gentle stream of bubbles
• fast reaction examples


  - burning

  - explosions (which are over in a fraction of a second)
• rate of reaction graph

  - steeper line = faster rate

  - flat = reaction finished

  - higher line = more reactants and products

  
• particles must ... in order to react

  collide with enough energy
• collision theory


  states that atoms, ions, and molecules must collide in order to react
• the rate of chemical reaction depends on...

  - collision frequency

  - energy transferred during a collision
• the more collisions there are...

  the faster the rate of reaction

  (e.g. doubling the frequency of collisions doubles the rates)
• minimum amount of energy that particles need to react is called the...

  activation energy - particles need this much energy to break the bonds in the reactants and start the reaction
• factors affecting rate of reaction

  - temperature

  - concentration of solution

  - pressure of gas

  - surface area

  - presence of a catalyst
• how does temperature affect the rate of reaction?

  - when the temperature is increased, the particles all move faster

  - if they're moving faster, they're going to collide more frequently

  - the faster they move, the more energy they have, so more of the collisions will have enough energy to make the reaction happen
• how does concentration of solution affect rate of reaction?

  - if a solution is made more concentrated, it means there are more particles colliding in the same same volume of water

  - this makes the collisions between the reactant particles more frequent
• how does pressure of solution affect rate of reaction?

  - when the pressure of a gas is increased, it means that the same number of particles occupies a smaller space

  - this makes the collisions between the reactant particles more frequent
• how does surface area affect the rate of reaction?

  - if one of the reactants is a solid, then breaking it up into smaller pieces will increase its surface area to volume ratio

  - this means that for the same volume of the solid, the particles around it will have more area to work on, so there will be collisions more frequently
• how does a catalyst affect the rate of reaction?

  - a catalyst is a substance that speeds up a reaction, without being used up in the reaction itself. this means its not part of the overall reaction equation

  - different catalysts are needed for different reactions, but they all work work decreasing the activation energy needed for the reaction to occur. they do this by providing an alternative reaction pathway with a lower activation energy

  - enzymes are biological catalysts, they catalyse reactions in living things
• the rate of reaction can be used to see how quickly...

  - reactants are used up

  - products are formed
• rate of reaction =

  amount of reactant used or product formed / time
• how do you measure the amount of reactant/product if it's a gas?

  in cm³
• how do you measure the amount of reactant/product if it's a solid?

  in grams
• units for rate of reaction

  - cm³/s (if a gas)

  - g/s (if a solid)
• ways to measure rate of reaction

  -precipitation and colour change

  - change in mass

  - volume of gas given off
• precipitation


  - if the initial solution is transparent and the product is a precipitate which clouds the solution, you can record the visual change in a reaction

  - by observing a mark through the solution and measuring how long it takes for it to disappear, you can measure the rate of reaction by seeing how long it takes to disappear

  - the faster the mark disappears, the faster the reaction
• colour change


  - if the reactants are coloured and the products are colourless (or vice versa), you can time how long it takes for the solution to lose or gain its colour

  - the faster the process of losing/gaining colour, the faster the reaction
• change in mass

  - measuring the speed of a reaction that produces a gas can be carried out with a mass balance

  - as gas is released, the mass disappearing is measured on the balance

  - the quicker the reading on the balance drops, the faster the reaction
• volume of gas given off

  - involves the use of a gas syringe to measure the volume of gas given off

  - the more gas given off during a given time interval, the faster the reaction
• rate of reaction experiments