SAT Subject Test Chemistry
REVIEW OF MAJOR TOPICS
Chemical Reactions and Thermochemistry
CHANGES IN ENTHALPY
Changes in enthalpy for exothermic and endothermic reactions can be shown graphically, as in the examples below.
Notice that the ΔH for an endothermic reaction is positive, while that for an exothermic reaction is negative. It should be noted also that changes in enthalpy are always independent of the path taken to change a system from the initial state to the final state.
This paragraph gives the definition of the molar heat of formation.
Because the quantity of heat absorbed or liberated during a reaction varies with the temperature, scientists have adopted 25°C and 1 atmosphere pressure as the standard state condition for reporting heat data. A superscript zero on ΔH (i.e., ΔH0) indicates that the corresponding process was carried out under standard conditions. The standard enthalpy of formation (ΔH0f) of a compound is defined as the change in enthalpy that accompanies the formation of 1 mole of a compound from its elements with all substances in their standard states at 25°C. This value is called the molar heat of formation.
To calculate the enthalpy of a reaction, it is necessary to write an equation for the reaction. The standard enthalpy change, ΔH, for a given reaction is usually expressed in kilocalories and depends on how the equation is written. For example, the following equations express the reaction of hydrogen with oxygen in two ways:
Experimentally, ΔH0f for the formation of 1 mole of H2O(g) is −241.8 kJ. Since the second equation represents the formation of 2 moles of H2O(g), the quantity is twice −241.8, or −483.6 kJ. It is assumed that the initial and final states are measured at 25°C and 1 atmosphere, although the reaction occurs at a higher temperature.
PROBLEM: How much heat is liberated when 40.0 grams of H2(g) reacts with excess O(g)? The reaction equation is:
This represents 1 mol or 2 g of H(g) forming 1 mol of H2O(g):
Since each mole gives off −241.8 kJ, then
Notice that the physical state of each participant must be given since the phase changes involve energy changes. Combustion reactions produce a considerable amount of energy in the form of light and heat when a substance is combined with oxygen. The heat released by the complete combustion of 1 mole of a substance is called the heat of combustion of that substance. Heat of combustion is defined in terms of 1 mole of reactant, whereas the heat of formation is defined in terms of 1 mole of product. All substances are in their standard state. The general enthalpy notation, ΔH, applies to heats of reaction, but the addition of a subscript c, ΔHc, specifically indicates heat of combustion.