# How to calculate the heat of reaction

Every chemical reaction is associated with an energy expenditure. Examples: When butane gas is burned in the cartridge burner or when "drain free" is dissolved with sodium hydroxide in water, thermal energy is released. When dissolving ammonium chloride in water or when producing cold mixtures, the solution cools down because thermal energy is required.

With exothermic reactions, energy is released, with endothermic reactions, energy is required. In chemistry, the heat energy released or required is referred to as the heat of reaction (or enthalpy of reaction). The total amount of heat Q (unit of measure Joule) released during a chemical reaction can be measured with a calorimeter (see description of the experiment and calculation). Usually, however, the heat of reaction is related to the amount of substance n of the substances involved in joules per mole:

Formula symbol: ΔHR. (delta HR.)
Unit of measurement: joules per mole

Since chemical reactions are associated with the absorption or release of energy, a certain amount of energy must be assigned to each substance. This energy is called enthalpy (symbol H). The enthalpy of every substance depends on the temperature and the external pressure. Therefore, the information for the enthalpy is based on the standard state:

Standard state in thermochemistry: 298.15 Kelvin and 1013 millibars
(298.15 Kelvins are about 25 ° C)

Attention: The standard state in thermochemistry does not correspond to the normal state at 0 ° C! Since absolute enthalpies cannot be measured, the number 0 was specified for the enthalpies of the elements in the standard state. Example: When iron and sulfur react, iron sulfide is formed:

Fe + S FeS

In this case, the elements Fe and S are given the value 0, while FeS is given the heat of formation ΔH released during the exothermic reactionB. is assigned. The heat of formation is the amount of heat that is released or consumed when a certain compound is formed from the elements under standard conditions. It receives the following formula symbol:

Heat of formation: ΔHB.

With the help of the enthalpies, the heat of reaction can be redefined. The heat of reaction is the difference between the sums of all heats of formation of the starting and end materials:

ΔHR. = ∑ΔHB. (Final substances) - ∑ΔHB (raw materials)

In the case of exothermic reactions, the heat of reaction is ΔHR. due to the formula a value less than 0, for endothermic reactions greater than 0. The heat of formation is an approximate measure of the stability of a chemical compound. The more heat energy is released in the formation of a joint, the more stable it is.

Exercise example 1

Calculate the molar heat of reaction for the reaction between iron and sulfur:

Fe + S FeS

The heat of formation of iron sulfide can be read from tables. The elements Fe and S have the values ​​0. Thus, after inserting them into the above equation:

ΔHR. = −100kJ - (0 + 0) = −100 kJ / mol

Exercise example 2

Calculate the heat of reaction for the reaction in the blast furnace:

Fe2O3 + 3 CO 2 Fe + 3 CO2

ΔHR. = [2 × 0 + 3 × (−393kJ)] - [−824 kJ + 3 × (−111 kJ)] = −22 kJ / mol

Both reactions are exothermic due to the negative sign. In order to clearly identify the substances involved, the physical state of the substances is added to the formulas by a symbol with brackets. The labeling is necessary because the heat of formation of water and water vapor, for example, are different. It means:

(s) solid, (l) liquid and (g) gaseous

The heat of reaction of the chemical reactions is measured with a calorimeter. Sometimes, however, the heats of reaction cannot be measured directly with the calorimeter, for example when determining the heat of formation of carbon monoxide. When carbon is burned, carbon monoxide and carbon dioxide are always produced. However, the following heats of reaction can be measured:

C (s) + O2 (G) CO2 (G)ΔHR. = −393 kJ / mol

CO (g) + ½ O2 (G) CO2 (G) ΔHR. = −282 kJ / mol

The law of constant heat sums helps here. The law is often described using Hess's sentence:

According to Hess's theorem, the same amount of heat is obtained from the combustion of carbon to carbon dioxide as the combustion via the intermediate stage via the carbon monoxide. The graph shows that the enthalpy of formation of carbon monoxide is −111 kJ / mol.

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