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Investigating the kinetics involved in the reaction of metals with acids.

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Introduction

PLAN Background In this coursework, I will be investigating the kinetics involved in the reaction of metals with acids. I will be concentrating on two aspects of the reaction, the order and activation energy of the reaction. I will be studying whether the order of the reaction changes with the type of acid used, and also the effect the type of acid used has on the activation energy. The order of a reaction shows how the reaction rate is affected by the concentrations of the reactants. Considering the equation shown below. A + B + C --> D If the order is zero with respect to reactant A, the rate is unaffected by changes in concentration of A. If the order is one with respect to reactant B, the rate is doubled by doubling the concentration of reactant B. Finally, if order is two with respect to a reactant C, the rate is quadrupled by doubling the concentration of C. Therefore, the overall order of the reaction is given by adding the orders. Rate = k[A]0[B]1[C]2 = 0 + 1 + 2 = 3 The rate constant of the reaction is k. The reaction rate can only be calculated from experimental values. Therefore, in this investigation I will be plotting a concentration/time graph to show the overall order of the acid-metal reaction. The shape of the graph will indicate the order by measuring the half-life of the reactant. The shapes of graphs for zero, one and second order reactions are shown below. Then plotting the rate against the concentration will confirm the order of the reaction. Rate against concentration will be a horizontal line for zero order, directly proportional line for first order and rate increasing at an increasing rate against concentration for second order. (See blue lines) The activation energy of a reaction is the minimum energy required for the reaction to occur. ...read more.

Middle

The maximum volume is more than that required for the experiment. Therefore, there will not be any risk of not being able to collect the required volume. Concial Flask - This will be used to contain the reacting substances and will have a lower chance of the magnesium strip sticking to the side of the container. If I were to use a test tube, the width is equal all the way down the tube. However, a conical flask's width increases, therefore reducing the chance of magnesium sticking to the sides. Stop Clock - This will be used to measure how long it takes for 10cm3 of gas to be collected in the experiments. Test tubes - This will be used in the activation energy experiment as the conical flask will not fit into the glass beaker. Measuring cylinders - These will be used to measure 10cm3 of acids and to create the mixtures of acids for a specific concentration such as 1.5M. Thermometer - This will be used to measure the temperature of the acid in the activation energy experiment. Beaker - This will be used to contain water in the activation energy experiment. Together they will act as a water bath to maintain and lower the temperature of the acids in the activation energy experiment. Clamp, boss, tripod - This will be used to hold the gas syringe steady in both experiments. Justification of Method Dry conical flask - The flask needs to be dry with no water. This is because the water will dilute the acid and therefore, alter the concentration of acid. Also if the conical flask contains other substances, it might result in unwanted effects when the reaction occurs. Reset gas syringe & stop clock - Making sure both of these read zero will reduce the errors that occur when the experiment is carried out. If the gas syringe or stop clock doesn't read zero, the time or volume of gas recorded will be incorrect. ...read more.

Conclusion

The thermometers were only accurate to 0.5oC, meaning this might lead to errors in the experiment. This will affect the results, as the actual precise temperature is not known. In the experiment I am allowing a range between 4-5oC temperature drop. However, if the thermometer is only accurate to 0.5oC, the actual temperature drop might be greater. The only way to improve the accuracy of temperature recordings is to use a digital thermometer. This will provide temperatures readings with greater accuracy and therefore provide trends on the graphs that are more obvious. Another source of error is the fact that the reaction was exothermic, meaning heat was given out during the reaction. Therefore, in the activation energy experiment the acid temperatures could be incorrect due to the fact that the water temperature was being measured. The water temperature was assumed to be the acid temperature, however since the reaction was exothermic, the temperature of the acid could have been greater. Therefore using the actual acid temperature will give points on the graph that are slightly different to the present results. The improvement that could be made is to actually have the theremometer in the test tube containing acid and magnesium. Having a bung that has a hole big enough for a thermometer (ordinary or digital) will be able to measure the temperature of the acid accurately. This will provide temperatures readings with greater accuracy and therefore provide trends on the graphs that are more obvious. The measuring cylinders and thermometer caused the greatest errors that can be be improved upon. When I measured 10cm3 of acid in the measuring cylinder, I can be sure that I had more than 9.5cm3 but less than 10.5cm3. The error would be be: 0.5/10 x 100 which equals 5%. Using the thermometer would provide a percentage error of 2.5% for a reading of 20oC. Therefore, if I am measuring a temperature change, and I measure one temperature too high and the other too low, the total error would be twice the error for each reading. ...read more.

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