Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2015

Lecture Notes 19: 6 March

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Hydrogen/Oxygen gas stoichiometry demonstration

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 = DeltaH = DeltaE - w = DeltaE - PDeltaV = q @ constant P

where PDeltaV is the pressure-volume work

Enthalpy is often approximately = DeltaE for chemical processes, since little or no work is usually done in solution chemistry (both P and V are constant).

Calorimetry

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

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?

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 = mCspDeltaT

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 = mCspDeltaT

Rearranging equations gives: DeltaT = q/ (mCsp)

Substituting values into the equations get: DeltaT = 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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© R A Paselk

Last modified 6 March 2015