chemistry topic 5 - energy changes

• What happens to energy during chemical reactions?

  Energy is conserved
• What does it mean when a reaction transfers energy to the surroundings?

  The product molecules have less energy than the reactants
• What is an exothermic reaction?

  A reaction that transfers energy to the surroundings
• What happens to the temperature of the surroundings during an exothermic reaction?

  The temperature of the surroundings increases
• Give 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 the surroundings
• What happens to the temperature of the 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?

  The minimum amount of energy that particles must have to react
• What do reaction profiles show in chemical reactions?

  • Relative energies of reactants and products
  • Activation energy
  • Overall energy change of a reaction

  
• How can you determine if a reaction is exothermic or endothermic from a reaction profile?

  By comparing the energy levels of reactants and products

  
• What indicates an exothermic reaction in terms of energy levels?

  Products have less energy than reactants

  
• What indicates an endothermic reaction in terms of energy levels?

  Products have more energy than reactants

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

  Energy is supplied to break bonds and released when bonds are formed
• How can you calculate the overall energy change of a reaction?

  Sum of energy taken in to break bonds minus sum of energy released to form bonds
• What condition indicates an endothermic reaction based on energy calculations?

  Energy taken in to break bonds is greater than energy released when formed

  
• What condition indicates an exothermic reaction based on energy calculations?

  Energy taken in to break bonds is less than energy released when formed

  
• What do cells contain that allows them to produce electricity?

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

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

  By connecting two different metals in contact with an electrolyte
• What is a battery composed of?

  Two or more cells connected together in series
• What happens in non-rechargeable cells and batteries when one of the reactants is used up?

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

  Non-rechargeable
• Why can rechargeable cells and batteries be recharged?

  Because the chemical reactions are reversed when an external electrical current is supplied
• What is a fuel cell supplied by?

  An external source of fuel and oxygen or air
• What is produced when hydrogen is oxidized in a hydrogen fuel cell?

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

  Advantages:

  • Can be used constantly if fuel is supplied
  • Only produces water when burnt

  
  

  Disadvantages:

  • Hydrogen needs to be stored at high pressure
  • Hard to transport and dispose of
• What is the overall reaction in a hydrogen fuel cell?

  The oxidation of hydrogen to produce water