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To investigate the differences in order of reaction and activation energy of the reactions between magnesium and weak and strong acids.

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Introduction

Plan Aim - To investigate the differences in order of reaction and activation energy of the reactions between magnesium and weak and strong acids. Background Information Activation Energy and collision theory Collision theory is a model used to explain the dependence of rate of reaction on temperature. The theory is that reactant particles are moving and colliding with one another. Collisions may be successful i.e. resulting in a reaction depending on whether their combined kinetic energy is equal to or greater than the activation energy for that specific reaction. The activation energy, or EA is the amount of energy required to break the bonds in reactant chemicals before a reaction can take place between them. It provides the reactants with enough energy to reach the transition state. It is expected then that, at higher temperatures, reactant particles will have more kinetic energy resulting in more successful collisions. This being true, it can further be expected that, as the temperature is increased, there will be a proportionate increase in the number of successful collisions and thus an increase rate. The Maxwell Bolzman distribution curve below shows how at higher temperatures more particles have a kinetic energy greater than the activation energy. Therefore at higher temperatures more collisions will be between particles with energy greater than or equal to the the activation energy, therefore more collisions will be successful therefore the rate of reaction will be higher. The Arrhenius equation gives a link between the factors involved in the kinetics of a reaction between one mole of colliding particles: Where: const. = constant, k = rate constant of the reaction, R = the gas constant, 8.31 J K-1mol-1, EA = the activation energy of the reaction, J mol-1 , and T = temperature in kelvins. A graph which uses this information is: The gradient of which is: -EA / R Rate and order of a Reaction For a reaction between two substances A and B the rate of the reaction can be taken as the rate at which A is used up. ...read more.

Middle

Repeat three times for each concentration of both acids and record data in a table like those on the following page marked 'order of reaction'. Activation energy Remove any magnesium oxide from the surface of the magnesium ribbon using wire wool and prepare 2cm pieces using clean scissors. Set up equipment as show in diagram 2 above except with the bunson off and without the test tube. Make sure the gas syringe is set to 10cm3. Measure 20cm3 of 2M Ethanoic acid in a measuring cylinder and transfer to the test tube. Clamp the conical flask in place and place the thermometer in the solution and record the temperature in kelvins after a 1 minute wait. Remove the thermometer, add 2cm of magnesium ribbon, seal the test tube with the bung securely and start the stop clock. Again, stop the clock as soon as the gas syringe is at 30cm3 and 20cm3 of hydrogen had been collected. Wash the test tube with tap water and dry. Make sure the gas syringe is set to 10cm3. Repeat the experiment, this time heating the water bath so that the temperature of the soution is approximately 70, 60, 50 and 40�C (343, 333, 323 and 313 kelvins). Recording the temperature and time taken for 20cm3 of gas to be collected. Repeat the whole experiment above using 2M hydrochloric acid. Only one value is taken at each temperature because it is highly unlikely that the solution will be at that exact temperature in a second attempt. Justification of Quantities and Concentrations It was decided that 2cm of magnesium ribbon would provide a substantial mass and surface area for reaction. It was also decided that concentrations of acid less than 0.5M would have reaction time which would be too slow to be practical in the given time and concentrations of more than 3M would react to quickly - inviting more chance of error in the recording of reaction time. It was also required that about five concentrations were investigated. ...read more.

Conclusion

(see anomoly assessment graphs) These graphs suggested that the reaction was likely to be second order - but none are really conclusive. ACTIVATION ENERGY Main calculation: gradient = -EA / R = -EA / 8.314 therefore: EA = -(gradient x 8.314) Ethanoic Acid gradient = -2403.8 therefore EA = -(-2403.8 x 8.314) = 19'985.19 J mol-1 = 20.0kJmol-1 Hydrochloric Acid gradient = -2021.8 therefore EA = -(-2021.8 x 8.314) = 16'809.25 J mol-1 = 16.8kJmol-1 The results for the acitvation energy have been much more successful. There were no anomolies - all points fell quite close to the lines. Concluding Summary Ethanoic acid is a weak monobasic acid which only partially ionises in aqeuous solution. Hydrochloric acid is a strong monobasic acid which fully ionises in aqueous solution. The exothermic reaction between ethanoic acid and magnesium metal is 2nd order with respect to the ethanoic acid. The investigation provided no conclusive evidence of a possible difference between the mechanisms of the reactions of ethanoic acid and hydrochloric acid with magnesium. If the reaction of hydrochloric acid is taken to be 2nd order (although results inconclusive) then it is probable that the mechanisms of the two reactions are similar or the same - with the same rate determining step. The investigation has demonstrated that the activation energy for a reaction between a weak acid and a metal is likely to be larger than that of a strong acid and a metal because a weak acid is only partially ionised in solution and needs energy to be put in to break bonds and form ions. This can be seen from the results of this investigation - ethanoic acid is a weak acid and the activation energy of the reaction between ethanoic acid and magnesium metal is 20.0kJmol-1 whereas the activation energy of the reaction between hydrochloric acid (a strong acid) and magnesium metal is much lower at 16.8kJmol-1. this big difference is due to the fact that ethanoic acid exists mainly as CH3COOH molecules in solution which need energy to break into H+ and CH3COO- but hydrochloric acid exists entirely as ions in solution. ...read more.

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