Topic 6: Chemical reactions

Chemical reactions can be fast or slow, and the conditions we choose decide how quickly products form. Some reactions can also run both forwards and backwards, settling into a balance called equilibrium. This topic explains rate, collision theory, equilibrium, and redox.

Rate of reaction and how it is measured

The rate of reaction tells you how quickly reactants are used up or products are made. A fast reaction, such as a metal fizzing in acid, finishes in seconds, while rusting can take years. To measure rate you track a quantity that changes during the reaction and divide the change by the time taken. Common methods include collecting a gas in a syringe and recording its volume each minute, placing the flask on a balance and measuring the mass lost as gas escapes, or timing how long a cross drawn under a flask takes to disappear as a precipitate forms. A graph of product made against time is steepest at the start, when reactant concentration is highest, and gradually levels off to a flat line when the reaction is complete. The gradient of this curve at any point equals the rate at that moment.

Collision theory

Collision theory explains rate at the level of particles. For a reaction to happen, reacting particles must collide with each other, and they must collide with enough energy and the correct orientation. The minimum energy a collision needs to lead to a reaction is called the activation energy. Many collisions are too gentle and the particles simply bounce apart unchanged. Anything that increases the frequency of successful collisions, either by making collisions more often or by giving more particles the required energy, speeds the reaction up. This single idea is the key to understanding why concentration, pressure, surface area, temperature, and catalysts all affect rate.

Effect of concentration, pressure and surface area

Increasing the concentration of a dissolved reactant packs more particles into the same volume, so collisions happen more often and the rate rises. For reactions involving gases, increasing the pressure squeezes the gas particles closer together, which has the same crowding effect as raising concentration and again increases collision frequency. Surface area matters for solids. A large lump of a solid reacts only at its outer surface, but breaking it into a fine powder exposes far more surface, giving liquid or gas particles many more places to collide. This is why powdered marble reacts with acid much faster than a single marble chip of the same mass.

Effect of temperature and catalysts

Raising the temperature gives every particle more kinetic energy, so they move faster and collide more frequently. More importantly, a greater proportion of particles now have energy equal to or above the activation energy, so a much larger fraction of collisions are successful. As a rough guide a 10 degree Celsius rise can roughly double the rate of many reactions. A catalyst is a substance that speeds up a reaction without being used up itself; it can be recovered chemically unchanged at the end. It works by providing an alternative reaction pathway with a lower activation energy, so more collisions have enough energy to react. Enzymes are biological catalysts. Catalysts are valuable in industry because they lower energy costs.

Reversible reactions and dynamic equilibrium

Many reactions go to completion, but some are reversible, meaning the products can react together to re-form the original reactants. We write these with a double arrow, for example A + B <=> C + D. A simple example is heating blue hydrated copper(II) sulfate to drive off water, leaving white anhydrous copper(II) sulfate; adding water reverses the change. In a sealed container a reversible reaction reaches dynamic equilibrium. At equilibrium the forward and backward reactions are still happening, but at exactly the same rate, so the amounts of reactants and products stay constant. Equilibrium can only be reached in a closed system, where nothing enters or leaves.

Effect of conditions on equilibrium position

Changing the conditions can shift where the balance lies, changing the yield of products. Increasing the concentration of a reactant shifts the position of equilibrium toward the products to use up the added substance, making more product. For gas reactions, increasing the pressure shifts the equilibrium toward the side with fewer gas molecules. Temperature depends on whether the forward reaction releases heat (exothermic) or absorbs it (endothermic); raising the temperature shifts the equilibrium in the endothermic direction. A catalyst speeds up the forward and backward reactions equally, so it helps equilibrium be reached faster but does not change the final position or the yield. These ideas explain the conditions chosen in the Haber process for making ammonia.

Redox as oxygen transfer and as electron transfer

Redox stands for reduction and oxidation, which always happen together. The simplest definition uses oxygen: oxidation is the gain of oxygen and reduction is the loss of oxygen. For example, when copper(II) oxide reacts with hydrogen, the copper oxide loses oxygen so it is reduced, while the hydrogen gains oxygen so it is oxidised. A more powerful definition uses electrons: oxidation is the loss of electrons and reduction is the gain of electrons. A useful memory aid is OIL RIG, meaning Oxidation Is Loss, Reduction Is Gain of electrons. When magnesium reacts with oxygen, each magnesium atom loses two electrons (oxidised) and oxygen atoms gain electrons (reduced).

Oxidising agents, reducing agents and oxidation numbers

In a redox reaction the substance that is reduced is the oxidising agent, because it takes electrons from, and so oxidises, the other reactant. The substance that is oxidised is the reducing agent, because it gives electrons to the other reactant. Oxidation numbers are a bookkeeping tool that tracks electrons. Uncombined elements have an oxidation number of zero, simple ions equal their charge, and in compounds rules give oxygen as -2 and hydrogen as +1 in most cases. If an element's oxidation number increases during a reaction it has been oxidised; if it decreases it has been reduced. Acidified potassium manganate(VII) is a common oxidising agent that changes from purple to colourless, a useful test for a reducing agent.