Energy Change

• What happens to energy in chemical reactions?

  Energy is conserved
• What is an exothermic reaction?

  1. One that transfers energy to the surroundings

  2. So the temperature of the surroundings increases
• Give 3 examples of exothermic reactions


  Combustion, many oxidation reactions, neutralisation
• What is an endothermic reaction?


  1. One that takes in energy from the surroundings

  2. So the temperature of the surroundings decreases
• Give 3 examples of endothermic reactions

  Thermal decompositions, the reaction of citric acid and sodium hydrogencarbonate, some sports injury packs
• Describe an experiment to investigate the temperature changes when an acid is neutralised by an alkali

  1. Measure hydrochloric acid into a polystyrene cup (with a lid to prevent heat loss) and place into a beaker for stability

  2. Measure the temperature of the acid with the thermometer through a hole in the lid

  3. Measure 5cm3 of sodium hydroxide and pour it into the cup with the acid

  4. Stir the solution with the thermometer

  5. When the thermometer STOPS changing, record the temperature

  6. Repeat steps 3 to 5 to add further 5 cm3 amounts of sodium hydroxide to the cup.

  The last few additions should produce a temperature fall.

  7. Repeat steps 1 - 6 and calculate a mean temperature for each of the sodium hydroxide volumes.
• Activation energy


  The energy needed for a reaction to occur
• Chemical reactions can only occur when...

  1. Reacting particles collide with each other

  2. With sufficient energy
• Exothermic reaction profile


  (you also need to put the overall energy change on the diagram)

  
• Endothermic reaction profile


  (you also need to put the overall energy change on the diagram)

  
• What happens to energy when bonds are broken?

  Energy must be supplied to break bonds

  
• What happens to energy when bonds are formed?

  Energy is released
• In an exothermic reaction, the energy ...

  In an exothermic reaction, the energy released from FORMING new bonds is greater than the energy needed to break existing bonds
• In an endothermic reaction, the energy ...


  In an exothermic reaction, the energy needed to BREAK existing bonds is greater than the energy released from forming new bonds