Edexcel A Level Chemistry:复习笔记1.8.3 Calorimetry

Calculating Energy Transferred, Q

 

Measuring enthalpy changes

  • Calorimetry is the measurement enthalpy changes in chemical reactions
  • A simple calorimeter can be made from a polystyrene drinking cup, a vacuum flask or metal can1.5-Chemical-Energetics-Calorimeter

A polystyrene cup can act as a calorimeter to find enthalpy changes in a chemical reaction

 

  • The energy needed to increase the temperature of 1 g of a substance by 1 oC is called the specific heat capacity (c ) of the liquid
  • The specific heat capacity of water is 4.18 J g-1 K-1
  • The energy transferred as heat can be calculated by:1.5-Chemical-Energetics-Equation-for-Calculating-Energy-Transferred-in-Calorimeter

Equation for calculating energy transferred in a calorimeter

Worked Example

Specific heat capacity calculations

In a calorimetry experiment 2.50 g of methane is burnt in excess oxygen.

30% of the energy released during the combustion is absorbed by 500 g of water, the temperature of which rises from 25 °C to 68 °C.

The specific heat capacity of water is 4.18 J g-1 K−1

What is the total energy released per gram of methane burnt?

Answer

Step 1

    • q = m x c x ΔT
    • m (of water) = 500 g
    • c (of water) = 4.18 J g-1 °C-1
    • ΔT (of water) = 68 oC - 25 oC = 43 oC

Step 2:

    • q = 500 x 4.18 x 43 = 89 870 J

Step 3:

    • This is only 30% of the total energy released by methane
    • Total energy x 0.3 = 89 870 J
    • Total energy = 299 567 J

Step 4:

    • This is released by 2.50 g of methane
    • Energy released by 1.00 g of methane = 299 567 ÷ 2.50 = 120 000 J g-1 (to 3 s.f.) or 120 kJ g-1

Calculating Enthalpy Changes

  • Aqueous solutions of acid, alkalis and salts are assumed to be largely water so you can just use the m and c values of water when calculating the energy transferred.
  • To calculate any changes in enthalpy per mole of a reactant or product the following relationship can be used:

ΔH =

  • When there is a rise in temperature, the value for ΔH becomes negative suggesting that the reaction is exothermic
    • This means that your value should be negative for an exothermic reaction, e.g. combustion
  • When the temperature falls, the value for ΔH becomes positive suggesting that the reaction is endothermic
    • This means that your value should be positive for an endothermic reaction, e.g. combustion

Worked Example

1.50 g of an organic liquid (Mr = 58.0) underwent complete combustion. The heat formed raised the temperature of 100 g of water from 20 oC to 75 oC.

Calculate the enthalpy of combustion for the organic liquid

Answer

Step 1: Calculate the energy released by the organic liquid

 

    • Q = mcΔT
    • Q = 100 x 4.18 x (75 - 20)
    • Q = 22990 J
    • Q = 22.99 kJ

Step 2: Calculate the number of moles of the organic liquid

    • Number of moles = 0.0259 moles (to 3s.f.)

Step 3: Calculate the enthalpy change of combustion

    • ΔcHθ -887 kJ mol-1 (to 3s.f.)
      • Remember, combustion is an exothermic process and will, therefore, be a negative enthalpy change value

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