Thermochemical Equations

Student Expectation

The student is expected to use thermochemical equations to calculate energy changes that occur in chemical reactions and classify reactions as exothermic or endothermic.

Key Concepts

    • 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).

    • 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, the change in enthalpy (ΔH) is positive and the energy absorbed during the reaction is represented as a reactant in the thermochemical equation.

THERMOCHEMICAL EQUATIONS

Heat Transformed or Transferred in Chemical Reactions

Thermochemistry is the study of energy changes that occur in chemical systems. All chemical reactions involve some change in energy as the atoms in reactants recombine to form new products. Chemical reactions not only generate new compounds, but can also transform or transfer energy. Chemical energy can be transformed into thermal energy, and vice versa. Furthermore, the produced thermal energy can transfer to its surroundings.

Enthalpy (H) is the Total Heat of a System

Many people like to go to parks for barbecues. As food is cooked on the grills, the glowing embers indicate a chemical reaction is occurring in the burning charcoal. The glow also indicates that heat is generated as a result of the reaction. The thermal energy generated by this reaction is included in a special type of chemical equation known as a thermochemical equation. 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 thermal energy of the system is known as the enthalpy (H) of the system.

Change in Enthalpy ΔH

The thermochemical equation specifically illustrates the change in enthalpy, or ΔH, that occurs during the reaction. As stated before, energy can either be gained or lost during chemical reactions. Therefore, *DeltaH values may either be positive or negative. In thermochemical equations, ΔH is often expressed in kilojoules (kJ).

    • Exothermic Reactions: In an exothermic reaction, heat flows from the system to the surroundings. Therefore, the change in enthalpy (ΔH) is negative. The energy released during the reaction is represented as a product in the thermochemical equation. It can be expressed as:

A + B → P + Heat, ΔH < 0

In this formula, A and B stand for chemical agents, P means product and change in enthalpy (ΔH) is negative.

    • Endothermic Reactions: In an endothermic reaction, heat flows into the system from the surroundings. Therefore, ΔH is positive. The energy absorbed during the reaction is represented as a reactant in the thermochemical equation. Thus we have:

A + B + Heat → P, ΔH > 0

The ΔH (change in enthalpy) of a system is equivalent to the heat of the system (q). The heat gained or lost may be calculated using the masses, specific heats, and temperature change of the system.

The following two equations are examples of thermochemical equations. The first equation is:

S + O2 → SO2 + 296 kJ

In this equation, thermal energy is a product, which means that 269 kJ of thermal energy are released. This makes the above equation a representation of an exothermic reaction. In this reaction, the ΔH value will be negative. Therefore, the thermochemical equation may also be written as:

S + O2 → SO2; ΔH = - 296 kJ

The second equation is:

CaCO3 + 178 kJ → CO2 + CaO

In this equation, thermal energy is a reactant, which means that 178 kJ are absorbed in (required for) this reaction. This make the above equation a representation of an endothermic reaction. In this reaction, the ΔH value will be positive. Therefore, the thermochemical equation may also be written as:

CaCO3 → CO2 + CaO; ΔH = + 178 kJ

Comparing Heat Flow

In an endothermic reaction, the enthalpy (H) of the products is greater than the enthalpy of the reactants, showing that energy was absorbed (gained) and ΔH is positive. In an exothermic reaction, the enthalpy of the products is less than the enthalpy of the reactants, meaning that energy was released (lost) during the reaction and ΔH is negative

A laboratory device called a calorimeter is used whenever ΔH needs to be measured for a particular reaction. The reaction is carried out inside an insulated chamber. The amount of heat produced or absorbed is determined based on changes in temperature that occur as the reaction proceeds.

Often in chemistry, a specific type of calorimeter, known as a bomb calorimeter, is used to determine the energy contained within compounds. Bomb calorimeters are used to measure the energy given off by the sample that is burned within the apparatus. The energy then released by the sample can be measured with great accuracy.

Bomb Calorimeter

In most high school laboratory settings, an apparatus called a coffee cup calorimeter is used instead of a bomb calorimeter. This setup consists of a lid placed on a Styrofoam cup that is placed inside a second Styrofoam cup. This set-up minimizes heat loss to the environment, and provides a way to accurately measure the heat flow into or out of a reaction inside the cup.

Coffee Cup Calorimeter