C5 - energy changes

• when is energy transferred?

  when chemical bonds are broken and when new bonds are made
• what is an exothermic reaction?

  those which involve the transfer of energy from the reacting chemicals to the surroundings
• during an investigation, what would an exothermic reaction show?

  an increase in temperature as the reaction takes place
• what are examples of exothermic reactions?

  • combustion
  • respiration
  • neutralisation
• what are endothermic reactions?

  those which involve the transfer of energy from the surroundings to the reacting chemicals
• during an investigation, what would an endothermic reaction show?

  a decrease in temperature as the reaction takes place
• what are examples of endothermic reactions?

  thermal decomposition of calcium carbonate
• what do reaction profiles show us?

  the difference in energy between the reactants and products
• in an exothermic reaction, is the energy level of the reactants or products highest?

  reactants
• in an endothermic reaction, is the energy level of the reactants or products highest?

  products
• what is activation energy?

  the minimum amount of energy required for a chemical reaction to take place
• what is a catalyst?

  a substance that increases the rate of a reaction without being used up.
• what happens to the chemical bonds in an endothermic reaction?

  they need to break, which means that the energy change becomes positive
• what happens to the chemical bonds in a exothermic reaction?

  they need to form bonds, which means that the energy change becomes negative
• what are bond energies used to calculate?

  the change in energy of a chemical reaction
• calculate the energy change for the reaction: $2H_2O_2\rightarrow\ 2H_2O+$$O_2$

  1. 2H-O-O-H $\rightarrow$ 2H-O-H + O=O
  2. 464 + 146 + 146 = 1074
  3. 1074 x 2 = 2148 , this is energy for the reactants
  4. 464 + 464 = 928
  5. 928 x 2 = 1856
  6. 1856 + 498 = 2354, this is energy for the products
  7. 2148 - 2354 = -206 KJ/mol, this shows the reaction is exothermic
• what is the aim of the required practical?

  the investigate the variable that affect temperature change in reaction solutions e.g. acid plus metals, acid plus carbonate, neutralisations and displacement of metals
• what equipment is needed for the required practical?

  • polystyrene cup
  • measuring cylinder
  • thermometer
  • 250cm3 glass beaker
  • top pan balance
• What is the first step in the reaction method between a metal and an acid?

  Gather the equipment
• Why is the polystyrene cup placed inside the beaker?

  To prevent the cup from falling over
• How much acid should be measured for the reaction?

  30 cm³
• Which acids can be used in the reaction?

  Hydrochloric acid or sulfuric acid
• What should be done after pouring the acid into the polystyrene cup?

  Record the temperature of the acid
• What equipment is used to measure the solid for the reaction?

  Top pan balance
• What type of solid can be used in the reaction?

  One strip of metal such as magnesium
• What should be done after adding the solid to the acid?

  Record the temperature
• How often should the temperature be recorded after adding the solid?

  Every minute for 10 minutes
• what does a chemical cell do?

  converts chemical energy into electrical energy
• what are the two types of chemical cells?

  rechargeable and non-rechargeable
• what do non-rechargeable cells do?

  they will produce a voltage until the chemicals inside are used up

  once this occurs the cell is no longer useful and can be recycled
• what do rechargeable cells do?

  an electrical current is passed backward through the cell. This works by reversing the chemical reactions and the cell or battery can then be used again to produce more electricity.
• how are fuel cells different to chemical cells?

  they need to be supplied with fuel and oxygen which allows the fuel cell to produce a voltage continuously
• what happens inside the fuel cell?

  hydrogen is oxidised electrochemically and the fuel is not combusted which allows the reaction to take place at a lower temperature
• what are hydrogen-oxygen fuel cells alternative to?

  rechargeable batteries and the only product that is produced is water
• what is the voltage of a cell affected by?

  the combination of metals used inside it

  the bigger the difference in reactivity, the bigger the voltage produced
• what will happen us magnesium and copper are used inside a cell?

  then the voltage produced will be larger as the metals are further apart in the reacivity series
• what happens to oxygen in a fuel cell?

  it is being reduced (reduction is the gaining of electrons)
• what happens to hydrogen in a fuel cell?

  it is being oxidised (oxidation is the loss of electrons)
• what is the reaction called when oxidation and reduction happen simultaniously?

  redox reaction
• what is an example of a redox reaction?

  $2H_2\ +$$\ 4OH^-\rightarrow\ 4H_2O\ +$$4e^-$