Chemistry - C2.4 - Rates and Energy

HideShow resource information

C2.4.1 - How Fast?

  • Rate of reaction - measures speed or how fast reaction is - can be found by measuring reactant used/product formed and time taken or time taken or certain amount of reactant to be used or product to form
  • Rate of reaction = amount of reactant used/time OR amount of product formed/time
  • Average rate - found by measuring time for certain amount of solid to appear in solution or time for certain amount of gas to be given off
  • Rate of reaction shown by gradient of line on graph of amount of reactant/product against time - steeper gradient = faster reaction
  • Graph produced by measuringmass of gas released or volume of gas produced at intervals of time - or measuring changes in colour, concentration or pH of reaction mixture over time
1 of 9

C2.4.2 - Collision Theory and Surface Area

  • Collision theory - reactions can only happen if particles collide - but they must collide with enough energy to change into new substances - minimum energy needed is caled activation energy
  • Factors that increase chance of collisions or particles energy increase rate of reaction:
    • temperature
    • concentration of solutions
    • pressure of gases
    • surface area of solids
    • using a catalyst
  • all increase the rate of reaction
  • Breaking solid into smaller pieces = larger surface area = more collisions in the same time: powder reacts faster than larger lumps
2 of 9

C2.4.3 - The Effect of Temperature

  • Increased temperature = increased speed of particles in reaction - collide more often with more energy = increased rate of reaction
  • Small changes in temperature have large effects on reaction rates - at ordinary temperatures 10C increase ~doubles rate of reaction; 10C decrease ~doubles time taken for reaction - why we refridgerate/freeze food to keep it fresh for longer
3 of 9

C2.4.4 - The Effect of Concentration or Pressure

  • Particles in solution move randomly - concentration of solution increased: more particles in the same volume - particles closer together: collide more often
  • Therefore: increased concentration = increased rate of reaction
  • Increasing pressure of gas increases molecules in volume so has the same effect and also increases rate of reaction
4 of 9

C2.4.5 - The Effect of Catalysts

  • Catalysts - change rates of chemical reactions - used to speed up reactions by lowering activation energy so more collisions result in a reaction
  • Catalyst change the rate of reaction but aren't used up - left at the end and can be used again
  • Solid catalysts are used so that they have large surface areas to be as effective as possible
  • Usually need different catalysts for different reactions
5 of 9

C2.4.6 - Catalysts In Action

  • Some catalysts are expensive - but economical as don't need replacing often - used in many industrial processes: reduce energy and time needed for reaction - helps reduce costs and reduces environmental impact - if fossil fuels are used for energy, catalysts help conserve resources and reduce pollution
  • Some catalysts are transition metals and their compounds - some are toxic and could cause harm to environment
  • Scientist trying to find new catalysts to use, eg. nanoparticles - new, highly efficient, enzymes - biological catalysts - replace more tradition catalysts to reduce energy costs
6 of 9

C2.4.7 - Exothermic and Endothermic Reactions

  • Chemical reactions - energy is transferred as bonds break and are made
  • Exothermic - transfers energy to surroundings - heats up surroundings: temperature increases
  • Exothermic reactions include:
    • combustion, eg. burning fuels
    • oxidation reactions, eg. respiration
    • neutralisation reactions involving acids and bases
  • Endothermic - takes in energy from surroundings, cause decrease in temperature or require supply of energy, if solid compound mixed with wter makes temperature decrease: endothermic reaction is happening
  • Thermal decomposition - need to be heated continuously to work
7 of 9

C2.4.8 - Energy and Reversible Reactions

  • Reversible reactions - forward and reverse reactions involve equal but opposite energy transfers - exothermic one way and endothermic the other - amount of energy released in exothermic reaction = amount taken in by endothermic reaction
  • When blue copper culfate crystals are heated: endothermic
  • When water is added to anhydrous copper sulfate: exothermic
8 of 9

C2.4.9 - Using Energy Transfers from Reactions

  • Exothermic reactions - used to heat things, eg. hand warmers and self heating cans - in some, reactants are used up and can't be used again (oxidation of iron or calcium oxide and water), others use reversible reactions (crystallisation of salt) - pack is heate to re-dissolve the salt and can be used again
  • Endothermic reactions - used to cool things, eg. cool packs for sports injuries or drinks - some use ammonium nitrate and water, kept separate - when mixed the ammonium nitrate dissolves and takes in energy from the surroundings - reversible, but not in the pack: only can be used once
9 of 9


No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »See all Rate of reaction resources »