Rate Of Reaction

PLANNING

I am going to investigate how the concentration of an acid affects the rate of reaction. The rate or speed of a chemical reaction is a measure of how fast the reaction takes place. There are a few factors that affect the rate of reaction: concentration, surface area, temperature and using a catalyst.

Concentration - as you know, particles in a solution move about, they bump into one another, this is the collision theory. If the concentration of the acid is increased, the number of ions moving about in the solution is increased, and therefore there will be an increase in the number of collisions. However not all of these collisions will have enough energy to react because they do not have enough energy to cross the activation energy barrier. Nonetheless, the more often that particles with enough energy to collide, the more chemical reaction will take place. I think that the rate of chemical reaction will increase if the concentration of reactants increased. I f the concentration of reactants is decreased; the rate of reaction will decrease.

When pieces of magnesium are placed in a boiling tube containing hydrochloric acid, bubbles of hydrogen are seen as the magnesium dissolves and magnesium chloride is formed. If the acid is very dilute then only a few bubbles are seen. If more concentrated acid is poured onto a similar quantity of magnesium two things happen; more bubbles of hydrogen is seen and the reaction stops after a shorter period of time. From these observations, it can be said that increasing the concentration of a substance will increase the rate at which it reacts.

Surface Area - in a solid a reaction can only take place on the surface of the metal. If in this reaction we use smaller pieces the reaction will happen faster. The diagram shows a piece of magnesium, as well as the same piece cut up into smaller pieces. Eventually a powder is produced. By cutting it up the amount of surface area has been increased that is available for the acid to react with.

Temperature - We can alter the rate of reaction by increasing or decreasing the temperature at which the reaction takes place, e.g. If dilute hydrochloric acid was heated from room temperature to 60 degrees Celsius before it was poured onto magnesium ribbon then a faster rate of reaction would take place. This occurs because when we increase the temperature we increase the energy of the particles. These particles will now move a lot faster. So the particles of hydrochloric acid will collide more often with the magnesium. Also because they have more energy these particles will be more likely to produce a chemical reaction because they will be able to cross the activation energy barrier. Therefore it is usually found that if the temperature at which a reaction takes place is increased then the rate of reaction will increase.

Using a catalyst - a catalyst is a substance, which can alter the rate of reaction without being chemically, changed at the end of the reaction. Catalysts will usually speed up a chemical reaction although there are catalysts, called inhibitors, which slow down chemical reactions. Catalysts work by lowering the activation energy barrier, which makes it possible for more collisions of particles to take place and so more chemical reactions. Hydrogen peroxide decomposes very slowly to produce water and oxygen. In the presence of manganese (IV) oxide, the reaction is much faster. It is a catalyst for the decomposition of hydrogen peroxide. At the beginning of the reaction the manganese (IV) oxide is a black powder; at the end of the reaction it is still a black powder.

In my investigation I want to investigate the effect of concentration on the rate of reaction. I predict that if I decrease the concentration of the acid, the time taken for a reaction to occur will increase. I think this because if the solution is more dilute then there will be less chance of collisions between the particles.

I am going to change the concentration of sodium thiosulphate. I will measure the time taken for the reaction to occur. I will keep the volume of the overall solution the same by adding water. I will also keep the temperature the same (room temperature) so that I know that temperature will not affect my results

Method -

1) Put on safety goggles and protective gloves.

2) Set up apparatus as shown

3) Draw an X on a piece of paper and place a conical flask on top of it.

4) Measure out 50cm3 of hydrochloric acid and pour it into the conical flask, pour out 50cm3 of sodium thiosulphate. As soon as the sodium thiosulphate is added start the stop clock. Look at the solution from a birds eye view, when the X disappears stop the stop clock and record the time.

5) Repeat this four times, only decreasing the volume of the sodium thiosulphate by ten each time, and replace the difference with water so that the volume of the overall solution is always 100cm3

6) Repeat each concentration