Chemistry 4.13-4.16 - Equilibrium and the Haber process

Reactions that can go both ways

Many equations are first written with a single arrow:

reactants → products

This shows the forward reaction. In a reversible reaction, the products can react to form the reactants again when the conditions are suitable. The reverse reaction can be written as:

products → reactants

The two directions are combined using the reversible-reaction symbol:

reactants ⇌ products

The symbol ⇌ does not mean "equals", and it does not by itself mean that the reaction has reached equilibrium. It says that both reaction directions are possible. The two reactions do not need to take turns, and the forward reaction does not need to finish before the reverse reaction begins.

One example is the thermal decomposition of ammonium chloride:

ammonium chloride ⇌ ammonia + hydrogen chloride

$\ce{NH4Cl(s) <=> NH3(g) + HCl(g)}$

Heating ammonium chloride makes the decomposition direction observable. In a cooler region, ammonia and hydrogen chloride can react to form ammonium chloride again. Changing the temperature has therefore altered which reaction direction is favoured in this example.

Teacher demonstration: heating ammonium chloride

Purpose. To obtain visible evidence that changing the conditions can reverse a chemical reaction. This is a suggested demonstration, not a mandatory core practical.

Apparatus and arrangement. Use a very small spatula of powdered ammonium chloride in the closed end of a hard-glass test tube. Clamp the tube near its open end, point the mouth away from people, and place a loose mineral-wool plug near the mouth. The plug reduces the release of fumes but must not seal the tube.

Method. Work in a fume cupboard. Heat the solid gently and observe the hot and cooler regions of the tube. Stop heating and allow the tube to cool before recording where white solid is present. Do not handle the hot glass.

The changed condition is temperature. The qualitative response is the position and amount of visible white deposit. For a fair repeat, keep the starting mass, tube dimensions, heating position and heating time the same. Repeating the demonstration can show whether the observation is repeatable, but it does not turn the observation into a measurement of reaction rate.

Safety. Ammonium chloride is harmful, and the gases formed can irritate the respiratory system and eyes. A trained adult should use eye protection, a fume cupboard and the smallest practicable quantity. Never heat a sealed tube: gas formation could increase the pressure and cause the glass to fail.

What the evidence can show. Re-formation of a white solid in the cooler region is consistent with the reverse reaction. However, the white deposit alone does not identify the two colourless gases, and this open apparatus allows matter to escape. The demonstration supports reversibility, but it cannot show that a dynamic equilibrium has formed.

Dynamic equilibrium in a closed system

Consider a reversible reaction in a closed container that begins with reactants but no products, at constant temperature and volume.

1. At first, the forward rate is at its greatest for these starting conditions because the reactant concentration is at its highest. The reverse reaction cannot yet occur because there are no product particles.
2. As products form, reactant concentration falls, so the forward reaction slows. Product concentration rises, so the reverse reaction speeds up.
3. Eventually, the forward and reverse reactions occur at the same rate. For every set of particles changed in the forward direction, another set changes in the reverse direction.

At that point there is no overall change in the concentrations of reactants and products. The reaction has reached dynamic equilibrium.

A closed system does not allow matter to enter or leave. It may still exchange energy with the surroundings, for example by being heated or cooled. Preventing substances from escaping matters because losing a reactant or product would keep changing its concentration.