Enduring Understanding 5.B.3: Chemical Reactions and Calorimetry
- Chemical reactions result in the formation and breaking of chemical bonds, and therefore can absorb or release energy.
- When a chemical reaction takes place, the energy of the system can increase (endothermic reaction) or decrease (exothermic reaction).
- In an endothermic reaction, the energy of the system increases, so energy must be taken up from the surroundings by heat absorption or by having work done on the system. This can be observed, for example, as a decrease in the temperature of the reaction mixture.
- In an exothermic reaction, the energy of the system decreases, and energy is released to the surroundings as released heat or work. This might be observed as an increase in the temperature of the reaction mixture.
- The heat exchanged or transferred in a chemical system can be measured by calorimetry.
- Calorimetry involves putting a chemical system in contact with a heat bath, composed of a material with a known heat capacity (e.g. water). A chemical process occurs and the temperature change of the heat bath is determined.
- The amount of energy exchanged between the chemical system and the heat bath can be determined.
- If the temperature of the heat bath increased, it means the heat bath increased in energy content, and the same amount of energy was released by the chemical system (i.e. an exothermic reaction).
- If the temperature of the heat bath decreased, the heat bath lost energy to the system, and the same amount of energy was absorbed by the chemical system (i.e. an endothermic reaction).
- Calorimetry can be used to determine enthalpies of vaporization, fusion, and reaction, and heat capacities.
- Sample Question: An 8.0 g sample of ammonium nitrate is dissolved in 60.0 mL water in a calorimeter. The temperature changes from 25.0 °C to 16.0 °C. What is the molar enthalpy of solution of ammonium nitrate? Assume the specific heat of the ammonium nitrate solution is the same as water - 4.2 J/g.°C. Ammonium nitrate has a molar mass of 80 g/mol
- Temperature change is (25.0 - 16.0) = 9.0 °C.
- Mass of the final solution is 60.0 g (1 mL=1 g) + 8.0 g = 68 g.
- Total energy lost by system = 4.2 J/g•°C x 68 g x 9.0 °C = 2570 J.
- Moles of ammonium nitrate dissolved = 8.0 g/ 80 g/mol = 0.10 mol
- Molar enthalpy of solution: 2570 J/0.10 mol = 25700 J/mol = + 25.7 kJ/mol. Enthalpy is positive, because the reaction absorbs heat.
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