Calculating Enthalpy Change

Introduction:

• Enthalpy change (ΔH) is a fundamental concept in chemistry used to quantify the energy changes in chemical reactions.
• ΔH can be calculated using the equation: ΔH = mcΔT, where ΔH is in kJ or kJ mol⁻¹, m is the mass of water (in kg), c is the specific heat capacity of water (a constant, 4.18 kJ kg⁻¹°C⁻¹), and ΔT is the change in temperature (°C).

Example Calculation

• Question: A solution was made by dissolving a spatula of potassium nitrate into 50 cm3 of water. The temperature changed from 20.4°C to 18.7°C. Calculate the enthalpy change for this reaction.
• To calculate ΔH, we need values for c, m, and ΔT.
  • Specific heat capacity (c) is a constant (4.18 kJ kg⁻¹°C⁻¹).
  • The mass of water (m) is 0.05 kg (since 50 cm3 = 0.05 kg).
  • ΔT is 1.7°C (temperature decrease, indicating an endothermic reaction).

Calculation

• ΔH = mcΔT
• ΔH = 4.18 x 0.05 x 1.7
• ΔH = 0.3553 kJ

Combustion Calculations

• Combustion calculations involve the enthalpy change associated with the burning of a substance.
• Enthalpy of combustion (ΔHc) measures the energy released when one mole of a substance burns completely in oxygen.
• The balanced equations for enthalpies of combustion should include one mole of the substance.

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Calculating Enthalpy Change

Worked Example 1

• Given: When 1g of ethanol is burned, 30 kJ of energy is released.
• Calculate the Enthalpy of Combustion for ethanol.
• Calculation: 1g → 30 kJ, GFM 46g → X kJ
  • X kJ x 1g = 30 kJ x 46g
  • X kJ = 30 kJ x 46g / 1g
  • X kJ = 1380 kJ
• Enthalpy of Combustion for ethanol = -1380 kJ mol⁻¹

Worked Example 2

• Given: 0.16g of methanol (CH₃OH) is burned, raising the temperature of 100g of water from 20°C to 27°C.
• Calculate the enthalpy of combustion of methanol.
• Calculation: ΔH = mcΔT
  • ΔH = 4.18 x 0.1 kg x 7°C
  • ΔH = 2.926 kJ
• 0.16g → 2.926 kJ, 32g → X kJ
  • X = 2.926 kJ x 32 / 0.16
  • X = 585.2 kJ
• Enthalpy of Combustion of methanol = -585.2 kJ mol⁻¹

Conclusion:

• Enthalpy change (ΔH) can be calculated using the formula ΔH = mcΔT.
• Enthalpy of combustion (ΔHc) measures the energy released when one mole of a substance burns completely in oxygen.
• These calculations help chemists understand the energy changes in chemical reactions.