Energy Changes

• what is an exothermic reaction

  exothermic reactions are those which involve the transfer of energy from reacting chemicals -> surroundings

  • this shows an increase in temp as reactions happen
  • examples of exothermic reactions are respiration + combustion
  • examples of exothermic objects are hand warmers
• what is an endothermic reaction

  endothermic reactions are those which involve the transfer of energy from surroundings -> reacting chemicals

  • this shows an decrease in temp as reactions happen
  • examples of exothermic reactions is thermal decomposition
  • examples of exothermic objects are ice packs
• what is an energy level diagram


  shows what is happening in a particular chemical reaction + shows the difference in energy between reactants + products
• what is the reaction profile for exothermic reactions

  • the reactants are at a higher energy level than products
  • the difference in energy is released to the surrounding -> the temperature of the surroundings increases

  
• what is the reaction profile of an endothermic reaction

  • the reactants are at a lower energy level than products
  • the difference in energy is absorbed from the surrounding -> the temperature of the surroundings decreases

  
• what is activation energy

  the minimum amount of energy required for a chemical reaction to take place
• what do catalysts do

  increase the rate of reaction

  • provide an alternative pathway for a chemical reaction to take place by lowering the activation energy
• how do bonds change in an endothermic reaction

  • energy is needed to break chemical bonds
  • the energy change (Δ H) in an endothermic reaction is positive
• how do bonds change in an exothermic reaction

  • energy is needed to form chemical bonds
  • the energy change (Δ H) in an endothermic reaction is negative
• energy change (Δ H) =

  sum of bonds broken - sum of bonds made

  • if answer is negative -> exothermic reaction
  • if answer is positive -> endothermic reaction
• what is a chemical cell

  converts chemical energy -> electrical energy

  • more than one cell connected in a series is called a battery
• what are the two types of chemical cell

  • rechargeable
  • non rechargable
• how does a non rechargeable cell work

  • produces voltage until the chemicals inside are used up
  • after this the cell is no longer useful + can be recycled
• how does a rechargeable cell work

  • these can be recharged multiple times
  • an electrical current passes backwards through the cell
  • this works by reversing the chemical reaction + the cell can be used again to produce more electricity
  • mobile phones contain rechargeabe batteries
• how is voltage in a cell affected by the metals used inside it

  • the bigger the difference in the reactivity of the 2 metals, the bigger the voltage produced (e.g. magnesium + copper)
  • the smaller the difference in the reactivity of the 2 metals, the smaller the voltage produced (e.g. magnesium + zinc)
• how do fuel cells work

  • they require a supply of fuel + oxygen
  • a constant supply of these 2 ingredients will allow the fuel cell to continuously produce voltage
  • inside the fuel cell, hydrogen is oxidised electrochemically so fuel is not combusted
  • this allows the reaction to take place at lower temps
  • these hydrogen - oxygen fuel cells are an alternative to rechargeable cells as the only product produced is water
• how are electrons changed in a fuel cell

  • oxygen is being reduced (gaining electrons)
  • hydrogen is being oxidised (losing electrons)
  • oxidation + reduction happen simultaneously = redox reaction