Chemical reactions

C2
1. How Much?
  1. Masses of atoms and moles
    1. One mole of any substance is its relative atomic mass in grams.
    2. we work out the relative formula mass of a compound by adding up the relative atomic masses of the elements in it.
    3. We use relative atomic masses to compare the masses of atoms.
    4. Relative formula mass (Mr).
  2. The yield of a chemical reaction
    1. The yield of a chemical reaction describes how much product is made.
    2. percentage yield= (amount of product collected/maximum amount of product possible) x100%
    3. The percentage yield of a chemical reaction tells us how much product is made compared with the maximum amount that could be made.
    4. It is important to maximise yield and minimise energy wasted to conserve the Earth's limited resources and reduce pollution.
    5. The products often don't go to completion, other reactions may happen and some product may be lost when it is separated or collected from the apparatus.
  3. Reversible reactions
    1. In a reversible reaction the products of the reaction can react to make the original reactants.
  4. Instrumental analysis
    1. Compounds in a mixture can be separated using a gas specrometry
      1. Once separated, compounds can be identified using a mass spectrometer
    2. Modern instrumental techniques provide fast, accurate and sensitive ways of analysing chemical substances.
    3. The mass spectrometer can be used to find the relative molecular mass of a compound from its molecular ion peak
  5. Analysing substances
    1. Chemical analysis is used to identify food additives
    2. Paper chromatography can detect and identify artificial colours.
2. Rates and Energy
  1. How fast?
    1. The rate of reaction found by measuring reactants used up over time or products made over time.
    2. The gradient or slope of the line on a graph of amount of reactant or product against time tells us the rate of reaction at that time. The steeper the gradient, the faster the reaction.
  2. Collision theory and surface area
    1. Particles must collide with a certain amount of energy before they can react.
    2. The minimum amount of energy that particles must have in order to react is called the activation energy.
    3. The rate of a chemical reaction increases if the surface area of any solid reactants is increased. This increases the frequency of collisions between reacting particles
  3. The effect of temperature
    1. Reactions happen more quickly as the temperature increases.
    2. Increasing the temperature increases the rate of reaction because particles collide more frequently and more energetically.
    3. At a higher temperature more of the collisions result in a reaction because a higher proportion of particles have energy greater than the activation energy.
  4. The effect of concentration or pressure
    1. Increasing the concentration of reactants in solutions increases the frequency of collisions between particles, and so increases the rate of reaction.
    2. Increasing the pressure of reacting gases also increases the frequency of collisions and so increases the rate of reaction.
  5. The effect of catalysts
    1. A catalyst speeds up the rate of a chemical reaction
    2. A catalyst is not used up during a chemical reaction.
    3. Different catalysts are needed for different reactions.
  6. Catalysts in action
    1. Catalysts are used in industry to increase the rate of reactions and reduce energy costs.
    2. Traditional catalysts are often transitional metals or their compounds.
    3. Enzymes are catalysts.
    4. Modern catalysts are being developed in industry which result in less waste and are safer for the environment.
  7. Exothermic and endothermic reactions
    1. Energy may be transferred to or from the reacting substances in a reaction.
    2. A reaction in which energy is transferred from the reacting substances to their surroundings is called an exothermic reaction.
    3. A reaction in which energy is transferred to the reacting substances from their surroundings is called an endothermic reaction.
  8. Energy and reversible reactions
    1. In reversible reactions, the reaction in one direction is exothermic and in the other direction it is endothermic.
    2. In any reversible reaction, the amount of energy released when the reaction goes in one direction is exactly equal to the energy absorbed when the reaction goes in the opposite direction.
  9. Using energy transfers from reactions.
    1. Exothermic changes can be used in hand warmers and self-heating cans
    2. Endothermic changes can be used in instant cold packs for sports injuries.

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