Energetics

• Chemical reactions where heat energy is given out are exothermic
• Chemical reactions where heat energy is taken in are endothermic
• Calorimetry:

  • 50g copper(II) sulphate is measured and transferred to a polystyrene cup (to reduce heat loss to surroundings)
  • the initial temperature of the solution is recorded
  • magnesium is added and the maximum temperature is recorded also
  • the temperature difference is calculated
• Calorimetry:

  • initial mass of ethanol in a spirit burner is recorded
  • 100g water is put into a copper can and then the spirit burner is lit underneath
  • stir the water constantly with the thermometer until it reaches 50°
  • the flame is extinguished and the max temperature is recorded
  • the ethanol and burner are reweighed
• energy change (J)

  q=mcΔT
• Molar enthalpy change (KJmol-1) --> (Kilojoules/mole) = $\Delta$H
• Displacement, dissolving and neutralisation reactions

  Example: magnesium displacing copper from copper(II) sulfate

  magnesium + copper(II) sulphate --> magnesium sulphate + copper

  Method:

  1. 50 cm3 of copper(II) sulfate is measured and transferred into a polystyrene cup.
  2. The initial temperature of the copper sulfate solution is measured and recorded.
  3. Magnesium is added and the maximum temperature is measured and recorded.
  4. The temperature rise is then calculated. 

  mc$\Delta$T = 50 x 4.18 x 32.5 = 6792.5J

  Note:  mass of 50 cm3 of solution is 50 g

   
• What is the purpose of the combustion reaction experiment?

  To measure energy produced by burning fuel
• How is the energy produced measured in the experiment?

  By burning fuel to heat water in a container
• What fuel is used in the example of the combustion experiment?

  Ethanol
• What is the chemical reaction for burning ethanol?

  Ethanol + oxygen → carbon dioxide + water
• What are the steps in the combustion reaction experiment?

  1. Measure initial mass of ethanol and burner.
  2. Transfer 100cm³ of water into a copper container.
  3. Measure initial temperature of water.
  4. Place burner under container and light it.
  5. Stir water until temperature rises (e.g., 30°C).
  6. Extinguish flame and measure maximum water temperature.
  7. Reweigh burner and remaining ethanol.
• If the temperature of the water rises by 30°C, what is the energy produced?

  $12540J$
• How is the energy produced per gram of ethanol calculated?

  By dividing total energy by mass of ethanol burnt
• What is the total energy produced when 0.78 grams of ethanol is burnt?

  $16077J/g$
• What is the formula used to calculate the energy produced in the experiment?

  $mc\Delta T$
• If the mass of ethanol burnt is 0.78 grams, what is the energy produced per gram?

  $16077J/g$
• What does the variable $c$ represent in the energy calculation formula?


  Specific heat capacity of water
• What is the specific heat capacity of water used in the calculation?

  $4.18 \text{ J/g°C}$
• Why is the water stirred during the experiment?

  To ensure even temperature distribution
• What happens to the flame after the maximum temperature is reached?

  The flame is extinguished
• What is the significance of measuring the initial and final mass of the ethanol burner?

  To determine the mass of ethanol burnt