1.5 Energy changes

• What is the conservation of energy principle?

  Energy is conserved in chemical reactions.
• What happens to the amount of energy in the universe during a chemical reaction?

  The amount of energy remains the same before and after the reaction.
• What is an exothermic reaction?

  A reaction where energy is transferred to the surroundings, increasing their temperature.
• Give examples of exothermic reactions.

  Combustion, oxidation reactions, and neutralisation reactions.
• What is the sign of energy change in an exothermic reaction?

  Negative sign of energy change.
• What is an endothermic reaction?

  A reaction where energy is taken in from the surroundings, decreasing their temperature.
• Give examples of endothermic reactions.

  Thermal decomposition and the reaction of citric acid with sodium hydrogencarbonate.
• What is the sign of energy change in an endothermic reaction?

  Positive sign of energy change.
• What is activation energy?

  The minimum amount of energy that particles need to react.
• What is a reaction profile?

  A graph showing the relative energies of reactants and products, as well as activation energy.
• How do bond energies relate to exothermic and endothermic reactions?

  Exothermic reactions release more energy from forming bonds than is needed to break them, while endothermic reactions require more energy to break bonds than is released when forming them.
• What is the equation to find energy change in terms of bond energies?

  Energy of reaction = sum of bonds broken – sum of bonds made.
• What is a cell?

  A cell is made of two different electrodes dipped in an electrolyte solution that produces electricity from a chemical reaction.
• What is a battery?

  A battery consists of two or more cells connected in series.
• What determines the voltage obtained from a cell?

  The identities of metals used as electrodes and the identity and concentration of the electrolyte.
• What are the advantages and disadvantages of using cells and batteries?

  Advantages:

  • More or less cheap
  • Some are rechargeable
  • Convenient source of electrical energy

  
  

  Disadvantages:

  • Harmful chemicals
• Describe rechargeable and non-rechargeable cells.

  Rechargeable:

  • Chemical reactions are reversed when an external current is supplied.

  
  

  Non-rechargeable:

  • Reactants are used up and cannot be recharged.
• What is a fuel cell?

  Fuel cells are supplied by fuel and oxygen to oxidise the fuel to generate electricity.
• What are the advantages and disadvantages of hydrogen fuel cell?


  advantages: no pollutants, no recharging

  disadvantages: flammable, hydrogen difficult to store, toxic chemicals
• Overall reaction in hydrogen fuel cell?


  $2 H2O +$$O2 ——> 2 H2O$
• Half equations for hydrogen fuel cell?


  Oxidation: 2H2 -> 4H+ + 4e- Reduction: O2 + 4e- + 4H+ -> 2H2O
• Reaction profile for endothermic reaction?


  energy is absorbed

  
• Reaction profile for exothermic reaction?

  energy is released

  
• What is needed when breaking bonds?

  • energy
  • It’s an endothermic reaction as it takes in energy from surrounding to break bonds
• what is released when making bonds?

  • energy
  • It’s an exothermic reaction
• What does electrolysis allow us to do with ionic compounds?

  Separate the elements in insoluble ionic compounds
• What is an electrolyte?

  A liquid or solution containing ionic compounds
• Why must the ions in an electrolyte be free to move?

  To conduct electricity during electrolysis
• How would you prepare an electrolyte from copper sulfate?

  Dissolve it in water
• What must be done to lead bromide to use it as an electrolyte?

  It must be melted
• What ions are present in molten lead bromide?

  Positive lead ions and negative bromide ions
• What are the electrodes made of?

  Metal or carbon
• What is the positive electrode called?

  Anode
• What is the negative electrode called?

  Cathode
• Why do we need a power supply in electrolysis?

  To drive the flow of electrons
• What does the term electrolysis literally mean?

  Splitting up with electricity
• What happens to bromide ions at the anode?

  They are oxidized to form bromine gas
• What do the positive lead ions form at the cathode?

  Pure lead
• What is the process of bromide ions being oxidized at the anode represented as?

  A half equation
• How do lead ions become pure lead at the cathode?

  By combining with electrons