Energy changes

• State the law of conservation of energy.

  Energy cannot be created or destroyed; it can only transferred from one place to another.
• How does the law of conservation of energy apply to chemistry?

  In all chemical reactions, energy is either transferred to the surroundings or from the surroundings.
• What is an exothermic reaction?

  A reaction where energy is transferred to the surroundings from the reacting system
• Two examples of exothermic reactions

  Combustion

  Respiration
• What happens to the temperature of the surroundings during an exothermic reaction?

  They increase. The thermometer is included in "the surroundings" so shows the temperature increasing.
• What is an endothermic reaction?

  A reaction where energy is transferred from the surroundings into the reacting system
• Two examples of endothermic reactions.

  Photosynthesis

  Melting ice
• What happens to the temperature of the surroundings during an endothermic reaction?

  They decrease. The thermometer is included in "the surroundings" so shows the temperature decreasing.
• State two uses of exothermic reactions

  Self-heating cans

  hand warmers
• State two uses of endothermic reactions

  Some cooling sports injury packs
• What are reactants?


  The substances involved in a chemical reaction.
• What are products?


  The substances formed when reactants have a chemical reaction
• What is a reaction profile?


  A diagram which shows whether the reactants have more or less energy than the products.
• If the reactants have more energy than the products, what kind of a reaction must have taken place?

  An exothermic one. The missing energy has been transferred to the surroundings.
• If the reactants have less energy than the products, what kind of a reaction must have taken place?

  An endothermic one. The extra energy has been take in by the surroundings.
• Is breaking bonds endothermic or exothermic?

  Endothermic. Chemical bonds are strong so require energy to break (like when you have to put energy in to separate magnets from each other)
• Is making bonds endothermic or exothermic?

  Exothermic. Energy is released when chemical bonds are formed (like how two magnets move together when close and generate kinetic energy)
• How do we work out the overall energy change of a reaction?

  Calculate the difference between the energy needed to break all the bonds in the reactants and the energy released to form all the bonds in the products using bond energy data given.