Enthalpy and Calorimetry Enthalpy (H) is used to quantify the heat flow into or out of a system in a process that occurs at constant pressure. Enthalpy is defined as H = E + PV E = internal energy of the system P = pressure of the system V = volume of the system Enthalpy is a state function (independent of path).

∆H = heat given off or absorbed during a reaction at constant pressure For a chemical reaction, the enthalpy change is given by the following equation: ∆H = H (products) – H (reactants) ∆H = heat given off or absorbed during a reaction at constant pressure Hproducts < Hreactants Hproducts > Hreactants ∆H < 0 ∆H > 0

Thermochemical Equations Is ∆H negative or positive? System absorbs heat Endothermic ∆H > 0 6.01 kJ are absorbed for every 1 mole of ice that melts at 00C and 1 atm. H2O (s) H2O (l) ∆H = 6.01 kJ

Thermochemical Equations Is ∆H negative or positive? System gives off heat Exothermic ∆H < 0 890.4 kJ are released for every 1 mole of methane that is combusted at 250C and 1 atm. CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) ∆H = -890.4 kJ

Calorimetry The science of measuring heat. Device used experimentally to measure the heat associated with a chemical reaction is a calorimeter. Coffee Cup calorimeter Calorimetry is based on observing the temperature change when a body absorbs or discharges energy as heat.

When substances are heated they respond differently When substances are heated they respond differently. Some substances might require a great deal of energy to raise its temperature, whereas another may not. A measure of this property is heat capacity. Heat capacity (C) = heat absorbed/increase in temperature When an element or a compound is heated, the energy required will depend on the amount of the substance present. For example, it takes twice as much energy to raise the temperature of two grams of water by one degree than it does to raise the temperature of water by one degree.

The amount of a substance must be specified when defining its heat capacity. If heat capacity is given per gram, it is called specific heat capacity and has units of J/oC•g or J/K•g. If heat capacity is given per mole, it is called molar heat capacity, and it has the units J/oC•mol or J/K•mol. Note the heat capacities of metals are less than that of water. It takes less heat to change the temperature of a gram of a metal by 1 oC than for a gram of water.

Constant Pressure Calorimetry Pressure (atmospheric pressure) remains constant. Used to determine the changes in enthalpy (heats of reactions) for reactions occurring in solution. Change in enthalpy equals the heat.

Constant Volume Calorimetry Under constant volume, a bomb calorimeter is used. Weighed reactants are placed inside and ignited. Energy change is determined by measuring the increase in the temperature of the water and other calorimeter parts. Change in volume is zero so no work is done so change in energy is equal to the heat.