Factors Change Reaction Rates example essay topic

Reaction, Rate of, amount of a substance that takes part in a chemical reaction in a given time. Chemical reactions happen at widely different rates. The weathering of buildings and statues caused by acid rain is very slow, but the reactions that take place when a firework explodes are very fast. Most reactions take place at a rate somewhere between that of a firework explosion, which is almost instantaneous, and that of weathering stone. Finding out how quickly reactions take place and understanding why they happen at the rate they do is very important to the chemist. Speeding up useful reactions and slowing down harmful ones can be important in industrial production processes, and in activities such as preserving buildings or foodstuffs.

The use of domestic refrigerators and freezers, as well as cold storage by wholesalers, in transit and in shops, makes modern methods of feeding large populations possible by slowing down the speed at which foods decay. The chemical reactions that take place in the human body happen at extremely slow rates when they are unaided. However, there are thousands of special proteins called enzymes in the body, which speed up these reactions millions of times. These enzymes are biological catalysts (substances that speed up reactions but are themselves left unchanged at the end). Without them, human beings and other living things could not survive.

Finding catalysts for industrial reactions is a vitally important field of research. For example, without a catalyst the conversion of nitrogen and hydrogen into the valuable compound ammonia in the Haber process would be too slow to be useful. An iron catalyst speeds up the reaction and makes the process fast enough to be economically worthwhile. Other factors change reaction rates. If reacting substances are heated, the rate of the reaction usually rises; conversely, if they are cooled, the reaction slows down. In order to react, the particles in the substances must collide with each other.

Heat gives them more energy to move around and so increases the chances of a collision. Also, when particles do collide, they are more likely to react, rather than just bounce off each other, if they are moving faster. Cooling has the opposite effects. For example, when sodium thiosulphate is mixed with dilute hydrochloric acid the mixture becomes cloudy as solid sulphur comes out of the solution (precipitates).

If the mixture is heated, it becomes cloudier more quickly. If it is cooled, it takes longer to become cloudy. Increasing the concentration of reactants (the amount dissolved in a given volume of solution) can have an effect similar to heating them, because the more particles present, the more likely a collision, and so the higher the reaction rate. Particle size can also affect reaction rate. Marble chips will dissolve in hydrochloric acid more slowly than an equal amount of ground marble, because less surface area is exposed for the acid to attack. The rate of a reaction can be obtained by following some measurable property that alters as the reaction occurs.

Examples of such measurable properties include gas volumes, concentration of reaction mixture, and electrical conductivity. Measuring gas volumes could be appropriate in a reaction such as the evolution of carbon dioxide gas when dilute hydrochloric acid is added to calcium carbonate, CaCO 3 + 2 HCl CaCl 2 + H 2 O + CO 2. The rate at which carbon dioxide is evolved can be followed by attaching a gas syringe to a test tube in which the reaction is occurring and noting the volumes at fixed time intervals. By sampling the reaction mixture and the concentration of one of the components of the reaction mixture (either reactants or products) at regular intervals, the concentration of the reaction mixture can be estimated by titration. An example is the acid-catalyzed hydrolysis of an ester such as ethyl ethanoate, CH 3 COOL 2 H 5 + H 2 O CH 3 COOH + C 2 H 5 OH. The reaction mixture is titrated at intervals against a standard solution of sodium hydroxide; there is a constant concentration of acid present as catalyst, so as the reaction progresses, more alkali is required due to the formation of ethan oic acid.

In some reactions, electrical conductivity can be measured, as this changes as the reaction proceeds. For example, during the alkaline hydrolysis of bromo ethane, C 2 H 5 Br + OH- C 2 H 5 OH + Br-, the conductivity decreases because the fast-moving hydroxyl (OH) ions are replaced by the slower-moving bromide ions. Other properties which can be followed in order to determine reaction rates include changes of pressure, for gaseous reactions; changes in optical rotation, where optically active materials are involved; and absorption of electromagnetic radiation, such as light, using a spectrophotometer. Rates of reaction can be changed not only by catalysts but also by changes in temperature and by changes in concentrations. Raising the temperature increases the rate by increasing the kinetic energy of the molecules of the reactants, and therefore the probability that any given molecule will have more than the activation energy.

Increasing the concentration or temperature can also increase the reaction rate by increasing the rate of molecular collisions. Rates of reaction of solid materials can also be increased by finely dividing the solid. This increases the surface area so that more molecules can collide. VI CHEMICAL EQUILIBRIUM As a reaction proceeds, the concentration of the reactants decreases as they are used up.

The rate of reaction will, therefore, decrease as well. Simultaneously, the concentrations of the products increase, so it becomes more likely that they will collide with one another to reform the initial reactants. Eventually, the decreasing rate of the forward reaction becomes equal to the increasing rate of the reverse reaction, and net change ceases. At this point the system is said to be at chemical equilibrium.

Forward and reverse reactions occur at equal rates. Changes in systems at chemical equilibrium are described by Le Ch^atelier's principle, named after the French scientist Henri Louis Le Ch^atelier: Any attempt to change a system at equilibrium causes it to react so as to minimize the change. Raising the temperature causes the equilibrium to shift in the endothermic direction; lowering the temperature causes the equilibrium to shift in the exothermic direction. Raising the pressure favours reactions that lower the volume, and vice versa.

Collision theory All substances are made up of particles. The particles might be atoms, molecules or ions. Before we can get a chemical reaction, particles must crash together. They must collide. This is called the collision theory.

The more collisions between particles in a given time the faster the reaction. Effect of concentration As we increase the concentration the faster the reaction. More concentrated solutions react more quickly. The acid particles can only react with the marble chips when they collide.

DRAW DIAGRAM p. 195 The acid particles move randomly through the water. As you increase the concentration of the acid, there are more acid particles in the same volume. Therefore there is a greater chance of acid particles colliding, and reacting, with particles on the surface of the marble. You increase the rate of reaction.