05- 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?

  It remains the same before and after
• What is an endothermic reaction?

  Energy is taken in from surroundings
• What occurs to the temperature of surroundings during an endothermic reaction?

  The temperature decreases
• Give an example of an endothermic reaction.

  Thermal decomposition
• What is another example of an endothermic reaction?

  Reaction of citric acid and sodium hydrogencarbonate
• What is an exothermic reaction?

  Energy is transferred to the surroundings
• What happens to the temperature of surroundings during an exothermic reaction?

  The temperature increases
• Give an example of an exothermic reaction.

  Combustion
• What is another example of an exothermic reaction?

  Oxidation reactions
• What is a third example of an exothermic reaction?

  Neutralisation reactions
• What is a reaction profile?

  A graph showing relative energies of reactants and products
• What does a reaction profile indicate about activation energy?

  It shows the activation energy of the reaction
• What is activation energy?

  Minimum energy needed for particles to react
• What is the equation to find enthalpy change in terms of bond energies?

  Energy of reaction = sum of bonds broken – sum of bonds made
• What occurs in a chemical reaction in terms of bond energies?

  Energy is supplied to break bonds
• What happens when bonds are formed in a reaction?

  Energy is released
• How do exothermic reactions relate to bond energies?

  More energy is released from forming bonds
• How do endothermic reactions relate to bond energies?

  More energy is needed to break bonds
• What is a battery?

  Two or more cells connected in series
• What is a cell?

  Two electrodes in an electrolyte solution
• What does a cell produce?

  Electricity from a chemical reaction
• What are the advantages of using cells and batteries?

  They are cheap and convenient energy sources
• What are the disadvantages of using cells and batteries?

  They contain harmful chemicals
• What determines the voltage obtained from a cell?

  Identities of metals and electrolyte concentration
• What is a rechargeable cell?

  Reactions are reversed with external current
• What is a non-rechargeable cell?

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

  Supplied by fuel and oxygen to generate electricity
• What are the advantages of hydrogen fuel cells?

  No pollutants and no recharging needed
• What are the disadvantages of hydrogen fuel cells?

  Flammable and difficult to store
• What is the overall reaction in a hydrogen fuel cell?

  2 H2 + O2 → 2 H2O
• What are the half equations in a hydrogen fuel cell at the cathode?

  2 H2 → 4 H+ + 4 e−
• What are the half equations in a hydrogen fuel cell at the anode?

  O2 + 4 H+ + 4 e− → 2 H2O
• How do the half equations in a hydrogen fuel cell relate to the overall reaction?

  They show the process of generating water