energy changes

• What is conserved during chemical reactions?

  Energy
• What does it mean that energy is conserved in reactions?

  The total energy remains constant before and after
• What happens to product molecules in an exothermic reaction?

  They have less energy than the reactants
• What is an exothermic reaction?

  A reaction that transfers energy to surroundings
• What is the effect of an exothermic reaction on the surroundings' temperature?

  The temperature of the surroundings increases
• Name an example of an exothermic reaction.

  Combustion
• What are everyday examples of exothermic reactions?

  Self-heating cans and hand warmers
• What is an endothermic reaction?

  A reaction that takes in energy from surroundings
• What happens to the temperature of surroundings during an endothermic reaction?

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

  Thermal decomposition
• What are some applications of endothermic reactions?

  Sports injury packs
• What is required for chemical reactions to occur?

  Reacting particles must collide with sufficient energy
• What is activation energy?

  Minimum energy needed for particles to react
• How can reaction profiles be used in chemistry?

  To show relative energies of reactants and products
• How can you identify if a reaction is exothermic or endothermic using reaction profiles?

  By comparing the energy levels of reactants and products

  
• What indicates an exothermic reaction in a reaction profile?

  Products have less energy than reactants
• What indicates an endothermic reaction in a reaction profile?

  Products have more energy than reactants

  
• What happens to energy during a chemical reaction when bonds are broken?

  Energy must be supplied to break bonds
• What happens to energy when bonds are formed in a chemical reaction?

  Energy is released when bonds are formed
• How can overall energy change of a reaction be calculated?

  Sum of energy to break bonds minus energy to form bonds
• What characterizes an endothermic reaction in terms of energy change?

  More energy is taken in to break bonds

  
• What characterizes an exothermic reaction in terms of energy change?

  Less energy is taken in to break bonds
• What do cells contain that produces electricity?

  Chemicals that react
• What factors affect the voltage produced by a cell?

  Type of electrode and electrolyte
• How can a simple cell be made?

  By connecting two different metals with an electrolyte
• What are batteries composed of?

  Two or more cells connected in series
• What happens to non-rechargeable cells when reactants are used up?

  Chemical reactions stop
• What type of batteries are alkaline batteries?

  Non-rechargeable
• Why can rechargeable cells be recharged?

  The chemical reactions are reversed with current
• What do fuel cells require as inputs?

  Fuel and oxygen or air
• What is produced when hydrogen is oxidized in a fuel cell?

  Water
• What are the advantages and disadvantages of hydrogen fuel cells compared to rechargeable cells and batteries?

  Advantages:

  • Constant use as long as fuel is supplied
  • Only produces water when burnt

  
  

  Disadvantages:

  • Hydrogen needs high-pressure storage
  • Harder to transport
  • Non-biodegradable waste
• What is the overall reaction in a hydrogen fuel cell?

  Oxidation of hydrogen to produce water
• What happens at the anode of a hydrogen fuel cell?

  Hydrogen gas loses electrons
• What happens at the cathode of a hydrogen fuel cell?

  Hydrogen ions and oxygen combine to form water
• What is the half-cell reaction at the anode?

  H₂(g) → 2e^– + 2H⁺(aq)
• What is the half-cell reaction at the cathode?

  4H⁺(aq) + O₂(g) + 4e^– → 2H₂O(g)