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There are many factors that have an affect on the rate of a chemical reaction. The speed of reaction means how fast the reactants change into the desired product. The consequence of this is that more of the products are made in a certain period of time if it has a high rate of reaction. Factors can only have two affects on a reaction making the reaction happen faster or slower depending on how it interferes with the reaction, the factors never change the outcome of the reaction, the final product.
This ability to change the rate of the reaction enables us to control reactions and predict how changing variables affects the experiment. Rate of reaction = 1 _ Time Taken Reacting chemicals must either: Collide with each other. Collide with enough energy to break the existing bonds. The energy required to break these bonds is called the Activation Energy (EA) There are four main factors that affect reaction these are: Temperature Use of a catalyst Concentration Surface area Temperature.
When the temperature is increased the reactant molecules move around faster with a greater amount of energy. This means there will be more successful collisions because it is the amount of energy in the molecules that determines whether a reaction is more successful. More successful collisions resulting in reactants get used up quicker resulting in the reaction time decreasing. Catalyst A catalyst affects the reaction rate by lowering the activation energy for the reaction. This means that more collisions are successful because it is easier to reach activation energy.
A catalyst does not get used up during a reaction. It can then be used again, and again. Surface area Grinding up a solid into smaller chunks creates a larger surface area by making the surface area in contact with a liquid or a solid bigger – Therefore more collisions can occur which increases the number of successful reactions thus speeding up the reaction. Purpose The purpose of this investigation is to determine how one of the above factors affects a reaction. So we choose only one variable. We were given 3 reactions to choose from, each allowing us to vary a certain factor.
After careful consideration we came to the conclusion that due to lack of equipment we could not test catalyst, measuring temperature would be too difficult in the classroom because we could not control the classroom temperature very well it could change over the course of the experiment, and surface area would be difficult to calculate. So we opted for the concentration variable as it was easy to measure accurately the concentration of the reactants. There were three possibilities to choose from and these were: Choice 1.
Hydrochloric Acid + Sodium Thiousulphate –> Sodium Chloride + Water + Sulphur Dioxide. HCl (aq) + Na2 S2 O3 (aq) –> NACl + H2O + S + SO3 Choice 2 Hydrochloric Acid + Magnesium –> Hydrogen + Magnesium Chloride HCl (aq) + Mg(s) –> H(g) + MgCl (aq) Choice 3 Hydrochloric Acid + Calcium Carbonate –> Calcium Chloride + Carbon Dioxide + Water HCl (aq) + CaCO3 (s) –> CaCl2 (aq) + CO2 (g) + H2O (l) Preliminary tests To help me decide which one to choose I decided to do some preliminary tests the First I did the reaction between Sodium Thiousulphate and Hydrochloric Acid.
This test involved putting a considerable amount of Hydrochloric Acid into a clear beaker on top of a piece of plain paper with a (X) clearly marked in the middle of the glass beaker. Sodium Thiousulphate is then dropped into the solution and the reaction forms a cloudy solution and soon the liquid becomes opaque. The timer starts from when the Sodium Thiousulphate is dropped into the solution and is stopped when the (X) under the beaker is no longer visible. How fast the reaction is depends on how quickly the mark under the beaker is no longer visible.
I decided this was not a viable or easily measure test. The measuring comes down to human judgement, which varies from person to person. It would have been possible to use light sensing equipment to accurately measure the time that it takes for the cross to become no longer visible. But as this equipment was not at my disposal I decided that this test would not be accurate enough for me to get a good set of precise results. The second preliminary was the reaction between Calcium Carbonate and Hydrochloric Acid.
It involved putting Hydrochloric acid into a beaker and then adding the Calcium Carbonate in the form of larger ‘chunks’, the beaker would immediately be attached to a ‘gas syringe’, this piece of equipment that measures how much gas the reaction gives off in cm3. This meant we could accurately measure how much gas is given off from the reaction, giving us a precise value rather than estimation like in the first experiment. This experiment was a better possibility because a distinct value could be recorded; human judgement was not an issue which was one of the problems with the first experiment.
Unfortunately surface area is the problem with this experiment, the marble chips were difficult to measure to ensure the size of the surface area accurately. This meant that this was constantly a variable but not a variable that we could measure accurately meaning that the experiment would either have unreliable results through estimating surface area or have unreliable results through having two variable factors. The final set of preliminary tests was the reaction between Magnesium and Hydrochloric Acid. This test was set up in a very similar manner as the experiment above, as the aim in both was to measure the amount of gas given off.
In this case in was Hydrogen was the gas being given off. The reaction took place in a beaker which was in turn connected to a gas syringe, measuring the Hydrogen produced. The experiment allowed us to control each of the variables. Temperature could be controlled using a water bath as with preliminary test number two. The surface area of the magnesium was much easier to control than the Calcium Carbonate of the previous experiment, this was because the magnesium was in ‘strip form’ instead of unevenly sized chips.
The concentration of the Hydrochloric Acid was also easy to vary, using a burette enabled us to measure the amount of water and acid being put into the beaker. Because of this ability to control all the affecting factors I felt that the Magnesium + Hydrochloric Acid was the experiment that would provide the most reliable set of results. The use of a gas syringe meant that the results collected would be both precise and accurate, with minimum human judgement involved. After choosing to do this experiment I then conducted a further set of preliminary tests.
I decided to keep the experiment roughly the same although I improved the method of measuring the magnesium strips because in my original preliminary I just cut them roughly equal sizes but in this preliminary I decided that I would use a ruler to ensure equal sizes this therefore would try and eliminate the factor of surface area so that I am only testing how the concentration affects the rate of reaction by having a similar surface area. Experiment Key 1. 0 Moles 12. 50 cm3 Hydrochloric Acid 12. 50 cm3 Water 1. 1 Moles
13. 75 cm3 Hydrochloric Acid 11. 25 cm3 Water 1. 2 Moles 15. 00 cm3 Hydrochloric Acid 10. 00 cm3 Water 1. 3 Moles 16. 25 cm3 Hydrochloric Acid 8. 75 cm3 Water 1. 4 Moles 17. 50 cm3 Hydrochloric Acid 7. 50 cm3 Water Test number one Moles 10 Seconds 20 Seconds 30 Seconds 40 Seconds 50 Seconds 60 Seconds 1Test Number two.
Moles 10 Seconds 20 Seconds 30 Seconds 40 Seconds 50 Seconds 60 Seconds 2 Show preview only The above preview is unformatted text This student written piece of work is one of many that can be found in our GCSE Patterns of Behaviour section.