Rates Of Reaction…

A chemical reaction involves reactants turning into products. You can tell when a reaction has happened because there will be production (or sometimes absorption) of energy; usually in the form of heat but sometimes light & sound too. There may also be a change of colour but this doesn’t always happen. The end products will be new substances that can’t be turned back into the reactants without a further chemical reaction taking place. The products will contain exactly the same number and type of atoms as the reactants did in the first place.

The speed at which reactants turn into products is known as the rate of reaction. Some reactions, like burning hydrogen, are very quick, whilst others, such as rusting, are very slow. There are various factors that affect the rate of a reaction, although the primary one is the basic nature of the type of reaction itself. Combustion is fundamentally fast and energetic; the oxidation of copper coins is not. Also solids, because of how their particles are arranged, tend to react much more slowly than liquids or gases.

There are some aspects of a reaction that you can alter to either speed up or slow down the reaction. The reaction between the particles will only happen when the particles meet, so anything you can do to make those meetings more probable will speed up the reaction. Anything you do to reduce the chance of the reactants encountering one another will slow the reaction down.

Temperature

The higher the temperature, the more kinetic energy the particles have. They are whizzing around with greater speed so they are more likely to bump into each other. Also, the particles will be closer to the activation energy necessary for the reaction to occur when they do collide. A general rule of thumb is that for most common chemical reactions at room temperature, the reaction rate doubles for every 10° Celsius increase in temperature.

Concentration

The more concentrated the solutions you are reacting, the more particles of reactant there are per cm³. This increases the probability that reactant particles will collide, increasing the chance that they will react together. For gases, increasing the pressure effectively increases the concentration, so higher pressure results in higher rates of reaction.

Particle Size

For solids in reactions, the smaller their particle size, the greater the surface area that is available for collisions to occur. Powdered metals react much more quickly with acids than do large lumps of metal. Stirring increases the rate of a reaction because it forces more interactions between molecules to occur. It may also break up lumps of solid, increasing the surface area.

Catalysts

A catalyst speeds up a chemical reaction but is not consumed by the reaction itself. That means that the catalyst never runs out. They often work for reactions that would happen anyway (but very slowly), by providing an alternative, lower energy pathway for the molecules to take. In effect, they lower the activation energy of the reaction.

The Elephant’s Toothpaste demonstration is a reaction that is speeded up by a catalyst. It is a mixture of hydrogen peroxide and washing-up liquid to which is added the catalyst potassium iodide. That causes the hydrogen peroxide to very rapidly decompose releasing oxygen which makes the foam. It is quite an easy experiment to perform but this is one of the more dramatic demonstrations I have seen. They used yeast as the catalyst in the first example (it produces an enzyme called catalase that breaks the peroxide down) and sodium iodide in the second case; as well as concentrated hydrogen peroxide which you would not be able to buy on the high street.

You may be asked to plot a graph showing the change in mass, or temperature, over time for a particular reaction. The rate of the reaction is represented by the gradient of the line. A steep line means a fast reaction and a horizontal line means that the reaction is complete. Remember to plot the independent variable on the x axis and the dependent variable on the y axis.

Questions…

What is/are the product(s) of the following reactions?

(a) copper + oxygen
(b) copper oxide + hydrochloric acid
(c) copper carbonate + sulphuric acid
(d) copper + sulphur
(e) copper nitrate_(aq) + zinc_(s)

1. What colour change would you see if you added water to anhydrous cobalt chloride?
2. What two substances are needed to make iron rust?
3. How would you describe the arrangement of particles in a solid?
4. Name an organic catalyst and its function in the human body.