D17.3 Heat Capacity and Calorimetry
The specific heat capacity (c) of a substance is the heating required to raise the temperature of 1 g of a substance by 1 °C; that is, to change the object’s temperature by ΔT = 1 K. (Because 1 K is the same size as 1 °C, ΔT has the same numeric value whether expressed in K or °C.)
If we know the mass, m, of a sample and its specific heat capacity, c, we can calculate the heat transfer of energy (q) to or from the sample by measuring the temperature change during heating or cooling:
The sign of ΔT tells us whether the substance is being heated (positive value for q) or cooled (negative q ).
Exercise: Heat Transfer of Energy
Exercise: Heating a Hot Tub
Calorimetry is an experimental technique used to quantitatively measure heat transfer of energy. In calorimetry it is useful to define a system, the substance(s) undergoing the chemical or physical change, and the surroundings, everything else that can exchange energy with the system.
When we apply calorimetry to determine the heat transfer of energy involved in chemical reactions,
Here, qreaction is defined as the change in energy of all atoms present in reactants and products.
A reaction in which there is heat transfer from the reacting substances to their surroundings (a reaction that heats the surroundings) is an exothermic reaction. For example, the combustion reaction that occurs in the flame of a lighted match is exothermic. A reaction in which there is heat transfer from the surroundings to the reacting substances (a reaction that cools the surroundings) is an endothermic reaction. For example, when the substances in a cold pack (water and a salt such as ammonium nitrate) are mixed, the resulting process transfers energy from the surroundings, making the surroundings colder.
Activity: Energy Transfer to a Cold Pack
For additional review materials pertaining to heat capacity, calorimetry and heat measurements, see here.
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