General Chemistry/Thermodynamics/Enthalpy

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The absolute enthalpy of a system cannot be measured directly. However, it is possible to measure changes in enthalpy (ΔH) by measuring temperature changes, which represent heat being lost or gained.

The enthalpy of a chemical system is the "heat content" of the system—the energy it contains. Enthalpy is represented by the symbol H.

Enthalpy of Reaction[edit]

All reactions have some amount of enthalpy. It is simply the amount of heat absorbed or released by the reaction. Combustion reactions obviously release large amounts of heat. They have negative enthalpy. A negative enthalpy represents an exothermic reaction, releasing heat. A reaction that absorbs heat is endothermic. Its enthalpy will be positive, and it will cool down its surroundings.

Consider these two reactions:

3\hbox{C} + 4 \hbox{H}_2 \to \hbox{C}_3\hbox{H}_8 \quad(\Delta H = -104.67\ kJ/mol) This reaction is exothermic (negative enthalpy, release of heat). When the reaction occurs, the surroundings will increase in temperature due to the gain of heat the system releases. For every mole of propane (C3H8) formed, 104.67 kilojoules of energy are released.
\hbox{N}_2 + \hbox{O}_2 \to 2 \hbox{NO} \quad (\Delta H = +756.05\ kJ/mol) This reaction is endothermic (positive enthalpy, absorption of heat). When the reaction occurs, the surroundings will decrease in temperature due to the loss of heat the system absorbed. For every mole of NO formed, 756.05 kilojoules of heat are absorbed. (Note that the reaction shows two moles being formed, so the enthalpy is double that of one mole being formed.)

Although every reaction has a change in enthalpy, the amount depends on the temperature. The same reaction could be endothermic at one temperature and exothermic at another. Enthalpies must be given at a specific temperature. The reactions above show the enthalpies at 25 °C.

Temperature and Heat[edit]

There is a relationship between temperature and heat. If an object gains or loses heat without any phase changes, this reaction relates temperature to heat:

Q = mc\Delta T

Q is the amount of heat (in joules or kilojoules) and \Delta T is the change in temperature (Celsius degrees). m is the mass of the object (in grams, usually) and c is the object's specific heat.

Specific heat is a constant that depends on the particular material of the object. It is measured in joules per gram-degree. It is determined experimentally. You can look it up in a chart, or you can calculate it if you measure the amount of heat added and the change in temperature. The toe in here heat of water is 4.186 J/g-degree. For one gram of water, the temperature rises/falls by one degree Celsius for every 4.186 joules of heat added/removed.