Thermochemical equations are just like other balanced equations except they also specify the heat flow for the reaction. The heat flow is listed to the right of the equation using the symbol ΔH. The most common units are kilojoules, kJ. Here are two thermochemical equations:

H₂ (g) + ½ O₂ (g) → H₂O (l); ΔH = -285.8 kJ

HgO (s) → Hg (l) + ½ O₂ (g); ΔH = +90.7 kJ

When you write thermochemical equations, be sure to keep the following points in mind:

1. Coefficients refer to the number of moles. Thus, for the first equation, -282.8 kJ is the ΔH when 1 mol of H₂O (l) is formed from 1 mol H₂ (g) and ½ mol O₂.
2. Enthalpy changes for a phase change, so the enthalpy of a substance depends on whether is it is a solid, liquid, or gas. Be sure to specify the phase of the reactants and products using (s), (l), or (g) and be sure to look up the correct ΔH from heat of formation tables. The symbol (aq) is used for species in water (aqueous) solution.
3. The enthalpy of a substance depends upon temperature. Ideally, you should specify the temperature at which a reaction is carried out. When you look at a table of heats of formation, notice that the temperature of the ΔH is given. For homework problems, and unless otherwise specified, temperature is assumed to be 25°C. In the real world, temperature may different and thermochemical calculations can be more difficult.

Certain laws or rules apply when using thermochemical equations:

1. ΔH is directly proportional to the quantity of a substance that reacts or is produced by a reaction.Enthalpy is directly proportional to mass. Therefore, if you double the coefficients in an equation, then the value of ΔH is multiplied by two. For example:

   H₂ (g) + ½ O₂ (g) → H₂O (l); ΔH = -285.8 kJ

   2 H₂ (g) + O₂ (g) → 2 H₂O (l); ΔH = -571.6 kJ

2. ΔH for a reaction is equal in magnitude but opposite in sign to ΔH for the reverse reaction.For example:

   HgO (s) → Hg (l) + ½ O₂ (g); ΔH = +90.7 kJ

   Hg (l) + ½ O₂ (l) → HgO (s); ΔH = -90.7 kJ

   This law is commonly applied to phase changes, although it is true when you reverse any thermochemical reaction.

3. ΔH is independent of the number of steps involved.This rule is called Hess’s Law. It states that ΔH for a reaction is the same whether it occurs in one step or in a series of steps. Another way to look at it is to remember that ΔH is a state property, so it must be independent of the path of a reaction.

   If Reaction (1) + Reaction (2) = Reaction (3), then ΔH₃ = ΔH₁ + ΔH₂

Learn what catalysts are and how they affect the activation energy and reaction rate of a chemical reaction.

Catalysts and Catalysis

A catalyst is a chemical substance that affects the rate of a chemical reaction by altering the activation energy required for the reaction to proceed. This is called catalysis. A catalyst is not consumed by the reaction and it may participate in multiple reactions at a time. The only difference between a catalyzed reaction and an uncatalyzed reaction is that the activation energy is different. There is no effect on the energy of the reactants or the products. The ΔH for the reactions is the same.

Positive and Negative Catalysts

Usually when someone refers to a catalyst, they mean a positive catalyst, which is a catalyst which speeds up the rate of a chemical reaction by lowering its activation energy. There are also negative catalysts or inhibitors, which slow the rate of a chemical reaction or make it less likely to occur.

Promoters and Catalytic Poisons

A promoter is a substance that increases the activity of catalyst. A catalytic poison is a substance that inactivates a catalyst.

How Catalysts Work

Catalysts permit an alternate mechanism for the reactants to become products, with a lower activation energy and different transition state. A catalyst may allow a reaction to proceed at a lower temperature or increase the reaction rate or selectivity. Catalysts often react with reactants to form intermediates that eventually yield the same reaction products and regenerate the catalyst. Note that the catalyst may be consumed during one of the intermediate steps, but it will be created again before the reaction is completed.

Learn what Le Chatelier’s Principle is and how it can be applied to predict the effect of a change in conditions on chemical equilibrium.

What Is Le Chatelier’s Principle?

Le Chatelier’s Principle is the name given to the principle in which a change in a chemical system prompts an opposing reaction. In chemistry, this principle was discovered independently by Henry Louis Le Chatelier and Karl Ferdinand Braun, so it is sometimes called the Le Chatelier-Braun Principle. The principle can be stated as follows:

If the temperature, concentration, volume, or partial pressure of a chemical system at equilibrium changes, then the equilibrium shifts to compensate for the change.

The more general form of the principle applies to other disciplines. Homeostasis and Lenz’s law are examples. Le Chatelier’s Principle is known by the same name when applied to economic equilibrium.

How Is Le Chatelier’s Principle Used?

Le Chatelier’s Principle is used to predict how a change in pressure, volume, concentration, or temperature will affect chemical equilibrium. Knowing the impact on equilibrium allows chemists to manipulate the chemical reaction. For example, a chemist might apply Le Chatelier’s Principle to maximize yield from a reaction.