Thermochemistry: Heat and Enthalpy

Introduction to Thermochemistry

In this section, Professor Dave introduces the concept of thermochemistry and explains the terms exothermic and endothermic reactions. He also discusses the change in energy for a reaction, represented by Delta H or the change in enthalpy.

Thermochemistry Basics

• Thermochemistry is a branch of thermodynamics that deals with the heat absorbed or produced by a chemical reaction.

• A reaction can be exothermic (releases energy) or endothermic (absorbs energy).

• The change in energy for a reaction is represented by Delta H, which is the change in enthalpy.

• Delta H can be thought of as the difference between the energy stored in all the bonds of the products and the energy stored in all the bonds of the reactants.

• If there is more energy in the products, Delta H will be positive, indicating an endothermic reaction. If there is less energy in the products, Delta H will be negative, indicating an exothermic reaction.

Energy and Heat

• Energy is defined as the capacity to do work and can exist in various forms such as mechanical, electrical, chemical, and heat.

• Energy is conserved and can change forms but cannot be created or destroyed.

• Heat (represented by q) is defined as the energy that flows between a system and its surroundings due to a temperature difference.

• Heat always flows spontaneously from an area of high temperature to an area of low temperature.

• For a system, q will be positive if heat is absorbed by it and negative if heat is released by it.

• When temperature and pressure are constant, q equals Delta H (the change in enthalpy).

Units of Energy

• Energies in thermochemistry are typically measured in joules (J) or kilojoules (kJ).

• Another common unit used by chemists is the calorie, which is equal to 4.184 joules.

• One calorie is the amount of energy needed to raise the temperature of one gram of water by one degree Celsius.

• The food calorie (Calorie with a capital C) is actually equal to a kilocalorie (kcal) or 1000 calories.

Thermochemical Equations

• A thermochemical equation reports the change in enthalpy for a given reaction in stoichiometric terms.

• The Delta H listed in the equation is specific to the molar quantities given by the balanced equation.

• To find the enthalpy change for a specific amount of material, stoichiometry can be used.

• If twice as many moles of material react compared to what is listed in the equation, the enthalpy change will also be twice as much.

Comprehension Check

In this section, Professor Dave checks comprehension on the topics discussed so far.

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