Thermochemistry

• What is an exothermic reaction?

  A reaction in which heat energy is transferred to the surroundings

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

  The temperature of the surroundings increases

  
• Can you name some examples of exothermic reactions?

  Combustion, most displacement reactions, and neutralisation reactions

  
• What is an endothermic reaction?

  A reaction in which heat energy is taken in from the surroundings

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

  The temperature of the surroundings decreases

  
• Can you name some examples of endothermic reactions?

  Electrolysis and thermal decomposition reactions
• What is calorimetry used for?

  • To calculate heat energy changes in a reaction
  • By measuring temperature changes of the surroundings

  
• What are the steps to measure temperature changes in a calorimetry experiment?

  1. Measure the starting temperature of the solution

  2. Mix the two chemicals in a polystyrene cup

  3. Immediately close the lid

  4. Measure the end temperature

  
• Why is a polystyrene cup used in calorimetry experiments?

  It minimizes heat loss, making the results more accurate

  
• What is the method to calculate heat energy released from burning a fuel?

  1. Record the starting mass of the spirit burner

  2. Measure a known volume of water in a copper calorimeter

  3. Record the starting temperature of the water

  4. Heat the water with the flame from the burning fuel

  5. Record the final temperature of the water

  6. Record the final mass of the spirit burner

  7. Calculate the mass of fuel burnt

  
• Why is it important to control variables in the fuel burning experiment?

  To ensure a fair test
• What is the equation used to calculate heat energy changes in reactions?


  q = m x c x ΔT

  
• What does each variable in the equation q = m x c x ΔT represent?

  q = energy change, m = mass of the surroundings, c = specific heat capacity, ΔT = temperature change

  
• If the mass of the surroundings is 100 g, the specific heat capacity is 4.2 J/g/℃, and the temperature change is 7℃, what is the energy change?

  q = 100 g x 4.2 J/g/℃ x 7℃ = 2940 J
• How do you calculate the heat energy released when magnesium reacts with hydrochloric acid?


  Use the formula q = m x c x ΔT with m = 200 g, c = 4.2 J/g/℃, and ΔT = 30℃
• What is the heat energy released when 1 g of hexane is burned in 200 g of water, raising the temperature by 51.6℃?

  q = 200 g x 4.2 J/g/℃ x 51.6℃ = 43,344 J

  
• What is the molar enthalpy change (ΔH)?

  It is the energy change for 1 mole of a substance
• What does a negative ΔH indicate about a reaction?

  The reaction is exothermic, meaning heat has been lost to the surroundings

  
• What does a positive ΔH indicate about a reaction?

  The reaction is endothermic, meaning heat has been taken in from the surroundings

  
• How do you calculate the molar enthalpy change using the equation ΔH = -q/n?

  ΔH is calculated by dividing the energy change by the amount of substance in moles

  
• If 2.5 g of NaOH is dissolved in 100 g of water and the temperature rises by 6℃, what is the molar enthalpy change?

  ΔH = -40.32 kJ/mol
• How do you calculate the energy released when 0.5 g of ethanol burns?

  q = 200 g x 4.2 J/g/℃ x 14℃ = 11,760 J
• What is the molar enthalpy change for the combustion of ethanol if 11.76 kJ is released for 0.0109 mol?

  ΔH = -1,079 kJ/mol
• How do you calculate the molar enthalpy change for the neutralisation reaction of HCl and NaOH?

  ΔH = -672 kJ/mol

  
• What do energy profile diagrams show?

  • Changes in energy of a reaction system as the reaction proceeds
  • Energy difference between reactants and products
  • Activation energy required for a reaction to occur

  
• What happens to the enthalpy of a reaction in exothermic reactions?

  It decreases, meaning energy is lost to the surroundings
• What happens to the enthalpy of a reaction in endothermic reactions?

  It increases, meaning energy is gained from the surroundings
• What occurs during bond breaking and bond forming in chemical reactions?

  • Bonds in reactants are broken, requiring energy
  • New bonds are formed in products, releasing energy
• Why is a reaction exothermic when more energy is released from forming new bonds than is taken in to break bonds?


  Because the net energy change is negative
• Why is a reaction endothermic when more energy is taken in to break bonds than is released by forming new bonds?

  Because the net energy change is positive
• How is the enthalpy change of a reaction calculated from bond enthalpies?

  • Calculate total energies of bonds broken
  • Calculate total energies of bonds formed
  • ΔH = total energies of bonds broken - total energies of bonds formed
• What is the enthalpy change for the reaction between methane and chlorine?

  ΔH = -122 kJ/mol