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Factors affecting the rate of a chemical reaction.

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INTRODUCTION A chemical reaction is a process by which one or more substances change into one or more other substances. Reactants are the substance(s) you start with and products are what you take out. A chemical reaction between two substances takes place when particles of these substances collide with each other. This either uses or releases energy. A reaction that releases energy is known as exothermic, where as a reaction that uses it up is called endothermic. This investigation looks at what affects the rate of a chemical reaction. FACTORS AFFECTING THE RATE OF A CHEMICAL REACTION Temperature: Kinetic theory says that the more energy a particle has, the more it will vibrate. In a solid, particles vibrate but do not move. They are fixed in position and cannot move past each other. They are close together and the forces between them are strong. But, if you heat them then they become a liquid. In a liquid, particles can move past each other, and are not held in fixed positions. They are instead joined in small groups and the forces between particles are not so strong. Heat up these particles some more and they become a gas. In a gas, the particles have hardly any forces between them, and are a very long way apart. They are moving quickly and so spread out. By heating substances, you are giving the particles more energy and move around more at a faster speed. If you heat the reactants in a chemical reaction then the particles will move around more and more collisions will occur. This means that the reaction will happen faster. Concentration: If one of the reactants in a reaction were dilute, for instance hydrochloric acid in water, then the concentration of the acid would have a bearing on the rate of the reaction. If the acid were at a higher concentration, there would therefore be more particles per cubic centimetre of water. ...read more.


When I am drawing graphs of the results, collecting gas will give me a graph like this graph A and temperature will give me a graph like graph B. Graph A is easier to read from, as the rate of reaction at any given point can be found just by measuring the gradient. I would not know how to measure the rate on Graph B, and after the peak, the heat would take a long time to fall anyway. Also, it is much easier to read off the measuring cylinder than the thermometer, which is another reason for choosing the gas collecting method. RANGE The greatest concentration of acid available to me is 2 molar, and from this I can make any lower concentration simply by diluting the acid with water. I will measure as many different concentrations as possible within the time given to give me a good graph. These concentrations will be from 0.25 molar acid all the way up to 2 molar acid with intervals of 0.25 moles. FAIRNESS To get meaningful results, the other factors that could affect the rate of this reaction must be controlled. This should not be too difficult. The experiments will all be conducted at room temperature, so the only influence on temperature will be that produced by the reaction itself. The pressure will remain the same, as there is no real way that I can change it anyway. The surface area of the magnesium filing may change marginally with each experiment, but the change will be so minimal that it will not affect our results. The affect of catalysis is easy to control, since I will just not put a catalyst into the boiling tube! RELIABILITY Reliability can be achieved in this experiment quite easily just by being careful when taking results and by checking all of my equipment and materials before use. I will: Make sure that my boiling tube is dry so as not to dilute the acid. ...read more.


There are two big possible sources of error. One is human error in reading the level of gas in the measuring cylinder. This is a minimal source of error, as we made sure that we were very careful in reading the cylinder. The possible error is about 1cm� either way, which would not affect my results or conclusions at all. The other possible source is in the actual concentrations of the acids. They may not have been exactly diluted as we expected, which could have produced major in accuracies. This is particularly evident in Graph 1. The amounts of gas produced after 5 minutes should be evenly spaced between concentrations. There is a big gap between 1.25 molar and 1.5 molar acid, and a very small gap between 1.5 and 1.75 molar acid, suggesting that the 1.5 molar acid was actually at a higher concentrate than we first thought. One possible reason that several of the points for the 15-second mark are below the lines drawn for them is that we lost some of the gas as we put the bungs in. This resulted in the difference between the x-axis and the first point being smaller than it should have been. The differences between the first and second points are correct because the bung then stayed in for the rest of the experiment. Cambridge Co-ordinated Chemistry by Jones, Jones and Acaster agrees that increasing the concentration of an acid will indeed speed up a reaction. It does not give a formula or equation for this, so I cannot check my graph 3 against anything else to see if it is correct. On the whole I think that this has been a successful experiment. I have collected the results I predicted, and proved that increasing the concentration of a reactant will increase the rate of the reaction. If I had more time and resources, I would take more measurements of this reaction from different concentrations, both above and below 2 molar acid. This would allow me to see if my prediction for the line on Graph 3 is correct or not. -Mike Stead ...read more.

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