# Enthalpy & Calorimetry

Most chemistry is done under conditions of constant pressure or constant volume (e.g. all of your body chemistry occurs at about atmospheric pressure - no pressure changes occur within single cells doing chemistry). Thus it is convenient to define a term for the heat involved in processes occurring with no change in pressure:

Enthalpy = H = E - w = E - PV = q @ constant P

where PV is the pressure-volume work

## Calorimetry

Calorimetry is the science of measuring heat. It is particularly useful because under two readily achievable laboratory conditions heat = E.

• For solution and solid state reactions at constant pressure there is no significant change in volume, so we can assume no work: therefore heat = E.
• If a reaction takes place in a rigid container, there can be no volume change, and no work can have occurred: therefore heat = E.

### Heat

Heat is a measure of energy transferred between objects of different temperatures. We are already familiar with the units of temperature, what are the units of heat?

• joule (SI unit): It takes 4.184 J to raise 1 g of water at 20 °C 1 °C.
• calorie (metric, non-SI unit): It takes 1 cal to raise 1 g of water at 20 °C 1°C. Thus 1 cal = 4.184 J (defined).
• Calorie (large Calorie = nutritional Calorie): 1 Calorie = kilocalorie. [2000 Calorie burger is 8.4 million juoles!]

### Specific Heat

Specific Heat is the amount of heat it takes to raise 1 g of a specific substance 1 °C. Specific heats for other substances are relative to water, so no units (comparing results in canceling out units).

The heat transferred in a process (q) is summarized in the equation:

Heat = q = mCspT

where m is the mass of substance and Csp is the specific heat of the substance.

Example: 750 calories of heat is transferred to 100.0 g of water at 20.00 °C. What will the new temperature of the water be assuming no heat is lost to the container or the surroundings?

Known: heat capacity of water = 1 cal / (g°C) [assume exact for problem]; q = mCspT

Rearranging equations gives: T = q/ (mCsp)

Substituting values into the equations get: T = 750 cal / {(100.0 g)(1 cal / (g°C)} = 7.50 °C

Adding the difference to the original temperature gives: 20.00 °C + 7.50 °C = 27.50 °C = 27.5 °C

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