Chemical reactions and energy

• What happens when a chemical reaction occurs?

  Energy is transferred to or from the surroundings
• What form is the energy transfer usually?

  A temperature change
• What is an example of a temperature energy transfer?

  When a bonfire burns, it releases heat energy to the surroundings
• What can a temperature rise be measured with?

  Thermometer
• What is an exothermic reaction?

  Reactions that transfer heat energy to the surroundings
• Whats a good way to remember the process of an exoteric reaction?

  The energy exits a reaction- exothermic
• What is the energy in an exothermic reaction normally transferred as?

  Heat energy
• What are some examples of exothermic reactions?

  • Combustion
  • neutralisation reactions
• Does the temperature rise or decrease in an exothermic reaction?

  Rise
• What is an endothermic reaction?


  Reactions that take in energy from the surroundings
• Whats a good way to remember the process of an endothermic reaction?

  Energy enters the reaction- endothermic
• What is the energy transferred usually in an endothermic reaction?

  Heat
• What does an endothermic reaction cause?

  The reaction mixture and its surroundings to become colder
• What are some examples of an endothermic reaction?

  • Electrolysis
  • Thermal decomposition of calcium carbonate
• Does the temperature rise or decrease in an endothermic reaction?

  Decrease
• What happens in a chemical reaction?

  Bonds in the reactants are broken while new bonds are made in the products
• What is a reactant?


  A substance that reacts together with another substance to form products during a chemical reaction
• What are products?


  Substances formed in a chemical reaction
• What is absorbed to break bonds?

  Energy
• Is bond-breaking endothermic or exothermic?

  Endothermic
• What is released to make new bonds?

  Energy
• Is bond-making endothermic or exothermic?

  Exothermic
• What depends on the difference between the energy needed to break bonds and the energy released when forming new ones?

  Whether a reaction is exothermic or endothermic
• How is a reaction exothermic?

  If more heat energy is released when making bonds than taken when breaking bonds
• How is a reaction endothermic?

  If more heat energy was taken when breaking bonds than making them
• What do energy diagrams show?

  The level of energy of the reactant and of the products
• The bigger the difference between the energy of the reactants and the energy of the products, the more energy is given out or taken in
• How can we tell a reaction is exothermic on an energy level diagram?

  The reactants are higher than the products
• How can we tell a reaction is endothermic on an energy level diagram?

  The reactants are lower than the products
• What is an enthalpy change?


  The difference between the energy of the reactants and the energy of the products
• What is the enthalpy change always in an exothermic reaction?

  Negative
• What is the enthalpy change always in an endothermic reaction?

  Positive
• What is activation energy?

  The minimum amount of energy required for a reaction to take place
• What is the activation enrgu represented by on an energy profile?

  A ‘hump’
• What does the hump on an energy profile show?

  How much energy is needed for the reactants to react and become products
• What is bond energy?

  The amount of energy needed to break one mole of a particular substance
• How do you calculate bond energy?

  • Add the bonds broken energy
  • Add the bonds made energy
  • Take them away from each other
• What does a negative energy change show?

  Exothermic reaction
• What does a positive energy change show?

  Endothermic reaction