Reaction rates

Rates and yields

In a chemical change, the rate of reaction is almost as important as the reactants and products. Reaction rates can be fast or slow. Reactions may be so slow that they take many years. On the other hand, reactions may be so fast they are instantaneous. These fast reactions, like an explosion, can be very dangerous.

Reaction rates are usually measured in terms of the amount of product made or the amount of reactant used up.

The reaction rate also affects the yield, which is the amount of product resulting from the reaction. When profitable substances undergo chemical reactions, industrial companies are interested in what might speed up the process and increase the yield.

Factors that affect reaction rates and yield:

• activation energy
• catalysts
• inhibitors
• product management
• reactant management

Activation energy

Heat is a common activation energy. Others include light and electricity. Heat affects atoms by giving them energy to move. Observe the effect of heat during a substance's change of state. As a solid, atoms sit tightly packed together, relatively still. When heated to melting point, the solid becomes liquid because the heat energy given to the substance excites the atoms, making it difficult to keep the tightly packed structure. Liquid is less dense than solid because of this expansion. When more energy is transferred to the substance, the liquid becomes gas, using the same principle - the atoms move even more and the space between them becomes even greater, making gas the most expansive state. See image 1.

The opposite happens when energy is removed from atoms. When the temperature drops, the atoms stop moving vigorously and start to find each other, becoming liquid. As the heat is removed, the atoms bond more tightly, becoming solid.

A common exception to this rule is water, which expands as it becomes ice because the hydrogen atoms bond in a particular structure that leaves bigger spaces between the molecules than when it is an unstructured liquid.

Adding heat to a reaction speeds up the process because heat excites the atoms, breaking the bonds and allowing the atoms to form new bonds with other substances more easily. The amount of heat may depend on the strength of the bonds holding the atoms in the substance together.

Light affects many silver compounds, which is why early photography methods involved silver nitrate to produce photographs, which depended on exposure to light for black and white contrast.

Occasionally, chemists use electricity to speed up reactions. An electrical current acts in the same way as heat, by exciting the atoms and causing them to break apart and react with other substances. Because electricity uses a charge to operate, it may have an unwanted effect on the reaction by attracting or repelling the ions involved.

Catalysts and inhibitors

Catalysts are substances used to hasten the rate of reaction, but which remain unaffected by the reaction. Usually the catalyst combines with a reactant to form an intermediate substance, which then reacts with another reactant to form the final product. The catalyst emerges as itself at the end of the reaction, in addition to the desired product.

Inhibitors are substances used to slow down the rate of reaction, but which remain unaffected by the reaction. They work the same way as a catalyst, but have the opposite effect. An inhibitor would be used to prevent dangerously quick reactions, such as an explosion.

Product and reactant management

Every reaction has an equilibrium, the point at which the energy fuelling the reaction, that is, the energy between the reactants, is used up. When activation energy is added, it is possible to keep the reaction going, or the reaction process happens more quickly.

Another way to manage a reaction is to remove the product as soon as it forms. This allows the reactants to continue reacting instead of reaching equilibrium.

Reactant management involves either adding more reactant as product forms, or ensuring that the reactants are used in a manner that optimises the energy contained within the reactants. For example, highly compressed gases will have a higher reactivity than unpressurised gases, a concentrated solution will be more effective in a reaction than one that has been heavily diluted. A solid with a greater surface area (e.g. powder), will react more readily than the same amount with a lower surface area (e.g. a lump). As discussed in the chapter on 'Acids & Bases', the reactants may need to match in strength for the best yield result. See image 2.

The manner in which chemists treat reactants and products therefore affects the speed and yield of a reaction.

Mass in chemistry

Atoms are extremely small particles, so chemists must use a measurement to indicate the relative mass of the atoms involved in a reaction. In the 19th century, a chemist named Lorenzo Avogadro developed a hypothesis that equal volumes of gas in the same conditions have the same number of molecules. Avogadro's Law led to the development of the mole, a chemical unit of measurement.

A mole refers to 6.023 x 1023 (602 300 000 000 000 000 000 000) atoms. With this number of atoms, a substance becomes physically manageable. Detailed periodic tables include the atomic mass of an element in addition to its atomic number. The atomic mass is the amount in grams of one mole of an element. Different elements have a different mass. One mole of magnesium, for example, is 24.3g. The atomic mass of gold is 196.9g.

In a chemical change, the reactants and products react according to their relative amounts in moles. For example, a mole of water (H2O) is two moles of hydrogen plus one mole of oxygen; 2 x 1g + 1 x 15.9g = 17.9g. Note that the ratio of atoms is still two hydrogen atoms to one oxygen atom, even though hydrogen only makes up 2g of the 17.9g.