Calorimetry Chapter 5

Calorimetry Chapter 5

Calorimetry Since we cannot know the exact enthalpy of the reactants and products, we measure H through calorimetry, the measurement of heat flow.

Calorimetry Functions Can be used to determine: – Heat capacities – Enthalpies of vaporization (amount of heat required to change from a liquid to a gas) – Enthalpies of fusion (amount of heat lost to change from a liquid to a solid) – Enthalpies of chemical reactions (how much heat is gained or lost from a reaction)

Heat Capacity and Specific Heat The amount of energy required to raise the temperature of a substance by 1 K (1 C) is its heat capacity. We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 C (1 K).

Heat q m*c*ΔT q heat m mass c specific heat ΔT final – initial temperature

Heat Capacity and Specific Heat Specific heat, then, is heat transferred Specific heat mass temperature change q c m T

Constant Pressure Calorimetry By carrying out a reaction in aqueous solution in a simple calorimeter such as this one, one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter.

Constant Pressure Calorimetry Because the specific heat for water is well known (4.184 J/mol-K), we can measure H for the reaction with this equation: q m c T

Calorimetry Concepts Heat is always transferred from warmer object to cooler object If two object do not have the same specific heat, they will not have the same changes in temperature Law of Conservation of Energy applies to heat transfer. – Heat lost by object 1 is the amount of heat gained by object 2. Heat is not the same thing as temperature.

Practice Problem If the temperature of a 34.4 g of ethanol increases from 25.0 C to 78.8 C, how much heat has been absorbed by the ethanol (specific heat of ethanol 2.46 J/g* C)?

Practice Question If a 50.0 g unknown metal with a temperature of 115.0 C, is placed in 125 g of 25.6 C water in a foam cup calorimeter and the metal causes the water to raise it’s temperature to 29.3 C, what is the specific heat of the metal? Part 1: solve the heat the water gained Part 2: heat gained by water heat lost by metal Part 3: Using the change in temperature of the metal final T of water – initial T of metal