Heat energy change

• Exothermic reactions give out (lose) heat energy. The temperature of the surroundings rises.
• Endothermic reactions take in (gain) heat energy. The temperature of the surroundings decreases.
• Enthalpy change is the change in overall energy in a reaction. It is the amount of energy (kJ) per mole of reactant, measured in kJ/mol.
• Exothermic reactions have negative enthalpy change as is loses energy.
• Endothermic reactions have positive enthalpy change because takes in energy.
• What is the purpose of calorimetry in chemical reactions?

  To measure the heat changes during dissolving, displacement, and neutralization reactions
• Why is it important to ensure the temperature of the reactants is the same before mixing?

  (calorimetry for dissolving, displacement and neutralisation)

  
  

  To obtain accurate and reliable measurements of temperature change
• What is the first step in conducting a calorimetry experiment or dissolving, displacement and neutralisation?

  Mix the reactants in a polystyrene cup
• How can energy loss to the surroundings be minimized during a calorimetry experiment for displacement, dissolving and neutralisation?

  By insulating the polystyrene cup with cotton wool and reducing evaporation by using a lid
• What methods can be used to reduce energy loss in a calorimetry experiment?

  • Place the polystyrene cup in a beaker of cotton wool for insulation
  • Put a lid on the cup to reduce evaporation
• Calorimetry for dissolving, displacement and neutralisation

  1. Put the solutions in a 25 degree water bath and take them out when they are both 25 degrees
  2. mix the reactants in a polystyrene cup
  3. put the polystyrene cup into a beaker of cotton wool to increase insulation
  4. put a lid on the cup to reduce evaporation
  5. measure the temperature of the mixture every 30 seconds and record the highest
• heat energy transferred (joules) = mass of liquid heated (grams) x specific heat capacity x change in temperature of liquid (degrees celsius)
• Specific heat capacity is the amound of energy needed to raise the temperature of 1 gram of liquid by 1 degree. (given in the exam)

  e.g. water has a SHC of 4.2, so it takes 4.2 joules of energy to make 1 gram of water 1 degree hotter