KEY CONCEPTS Texas Essential Knowledge and Skills Statement C.11: �The student understands the energy changes that occur in chemical reactions. The student is expected to:

(C) use thermochemical equations to calculate energy changes that occur in chemical reactions and classify reactions as exothermic or endothermic; (i) use thermochemical equations to calculate energy changes that occur in chemical

reactions (ii) classify reactions as exothermic or endothermic

(D) perform calculations involving heat, mass, temperature change, and specific heat; and (i) perform calculations involving heat (ii) perform calculations involving mass (iii) perform calculations involving temperature change (iv) perform calculations involving specific heat�

Key Concept #1: �A thermochemical equation is a type of balanced chemical equation that includes the amount of energy absorbed or released during a chemical reaction. The total heat energy of the system is known as the enthalpy (H) of the system. The thermochemical equation specifically illustrates the change in enthalpy, or ΔH, that occurs during the reaction. In thermochemical equations, ΔH is often expressed in kilojoules (kJ).

Key Concept #2: �In an exothermic reaction, heat flows from the system to the surroundings. Therefore, the change in enthalpy (ΔH) is negative and the energy released during the reaction is represented as a product in the thermochemical equation. In an endothermic reaction, heat flows into the system from the surroundings. Therefore, ΔH is positive and the energy absorbed during the reaction is represented as a reactant in the thermochemical equation.

Key Concept #3: The ΔH (change in enthalpy) of a system is equivalent to the heat of the system (q), and the heat gained or lost may be calculated using the masses, specific heats, and temperature change of the system. The equation used to calculate these variables is q = m (mass in grams) x c (specific heat) x ΔT (change in temperature). The specific heat (c) of a substance is defined as the amount of heat required to raise the temperature of 1 gram of the substance by 1 degree Celsius.

©2012RiceUniversity‐AllRightsReserved

C.11CD: Thermochemical Equations Gases and Thermochemistry

FUNDAMENTAL QUESTIONS

•  How is a thermochemical equation different from a chemical equation?

•  How are reactions classified as exothermic or endothermic?

•  What is the equation used to relate mass, specific heat, and temperature to the amount of heat produced or absorbed in a chemical reaction?

©2012RiceUniversity‐AllRightsReserved

C.11CD: Thermochemical Equations Gases and Thermochemistry