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Determine the effect temperature has on the rate of reaction.

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Introduction

Aim * This experiment aims to determine the effect temperature has on the rate of reaction. It will be a comparative study between a weak acid, ethanoic and a strong acid, hydrochloric, when added to magnesium ribbon. * This experiment aims to determine if the concentration has an effect on the rate of reaction. It will be a comparative study between a weak acid, ethanoic and a strong acid, hydrochloric, when added to magnesium ribbon. Introduction Kinetics refers to the study of rates of chemical reaction. This is very important as in organic chemistry you can often get several different products from the same mixture of reagents, but by controlling the reaction conditions the rate of formation of each product may be changed. Knowledge of the kinetics of a reaction helps to produce larger quantities of desired products while reducing the presence of unwanted by products. The rate of reaction is the change in concentration of reactant or product in a given time. For the general reaction; A + B C The rate of a chemical reaction, R, quoted in units of mol dm�� s�� may be given by the expression; R = k [A]x [B]y [C]z Where [A], [B] and [C] are the concentrations of the reactants in mol dm-� and k is the rate constant. The order of reaction is the number of concentration factors in the rate equation. In the example above the order with respect to A is 1 and with respect to B is 2; the overall order is therefore 3. However, it is important to remember that the order of a reaction can only be found by experiment and cannot be worked out from the equation of the reaction - reactant orders can't be predicted; we must determine them by experiment. Eg. 2NO2 (g) + 2H2 (g) � 2H2O (l) + N2 (g) Where rate = k [NO]� [H2]� Common orders Zero order: rate is unchanged with time: rate ? ...read more.

Middle

100cm� gas syringe - should be appropriately accurate for measuring the gas produced since it is accurate to 1cm3 of gas. 10cm�-measuring cylinder - I have chosen to use 10cm�-measuring cylinder to measure the volumes of substances used since it is more accurate than a pipette. Weighing balance - I will use a three-figure balance to measure the mass of magnesium. Side-arm test-tube Bung Thermometer Stop-watch Diagram Table of Results Table for reaction with HCl and magnesium to produce 20 and 40cc - Temp �C Molar Volume of HCl Time/sec to collect 20cc of gas Average time/sec to collect 20cc of gas Time/sec to collect 40cc of gas Average time/sec to collect 40cc of gas Table for varying concentrations of HCl and ethanoic acid - Temperature/ �C Concentration of HCl Time/sec to collect 40cc of gas Average time/sec to collect 40cc of gas Table for gas produced for ten second intervals for HCl and ethanoic acid - Volume of Hydrogen gas produced at Time/sec 0.5 Molar 1.0 Molar 1.5 Molar 2.0 Molar Table for Vf - Vt for HCl and ethanoic acid - Time/sec Volume of Hydrogen gas given off cm� Average Volume cm� Vt Vf - Vt Preliminary data The preliminary work that I will be conducting is to find out the correct amount of hydrochloric acid to use. To do this, I will measure out a volume of hydrochloric acid and react with in excess of 0.1g of magnesium ribbon and if there is still some magnesium left when the reaction has stopped effervescing then the volume of hydrochloric acid will have to be increased. Apart from trying to find the optimum rate of reaction I also have to find out how to keep the temperature change down. As the reaction is taking place the temperature will rise because the reaction is exothermic, and this could cause my results to be inaccurate as the temperature change will heat up the acid and give the acid particles more energy so they will move faster and collide with the magnesium with greater force causing more successful collisions per second. ...read more.

Conclusion

Let's assume activation energy of 50 Kj mol��, 50,000 J mol��. The value of the gas constant, for example, is 8 JK�� mol��. At 20�C (293K) the value of the fraction is: e-EA/RT = e (-50000/(8 x 293)) = 5.45 x 10^�8 By raising the temperature by 10�C (303K), the effect on the rate of reaction is: e-EA/RT = e (-50000/(8 x 303)) = 1.1 x 10^�9 You can see that the fraction of molecules able to react has almost doubled by increasing the temperature by 10�C. that causes the rate of reaction to almost double. This is the value used in rate of reaction work. Determining reaction Mechanism In organic chemistry, an organic reaction can happen in a number of successive steps. These steps are known as the mechanism of the reaction and there is no reason that all the steps take place at the same rate. However, it is clear that the whole reaction goes at the rate of the slowest of the steps in the mechanism. This slowest step is called the rate-determining step. A major factor in interpreting results is the absence of an accepted mechanism for the reaction. Our effort at mechanistic theory argues by analogy with heterogeneous catalysis, and goes as follows; * Hydrogen ions migrate to the metal surface * Hydrogen ions accept electrons from the metal surface * Hydrogen atoms combine to give hydrogen molecules * Metal ions hydrate and diffuse away from the metal surface We propose that steps 2 and 3 are surely fast steps and that, given efficient agitation step 4 should also be fast. This leaves step 1 as the most likely rate-determining step. If the mechanism is correct it is important to remember that the reaction is between H+ (aq) and Mg(s). A strong acid is completely ionised but a weak acid is not. Energy will be needed to break bonds and produce the H+ ions in a weak acid, so I would expect the activation energy to be higher. This necessitates a change to step 1 of the mechanism. ...read more.

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