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Planning This investigation looks at the rate of reaction between a metal and three different acids of different protonisties. There are two different ways that this can be investigated, the first way is the ‘Half Life’ or ‘Vf-Vt’ method, and this looks at the decreasing half life in the reaction. The other is the ‘Order’ reaction, this looks at the order of the acids in relationship to the time it takes to react a certain volume to concentration. The acids being used are hydrochloric acid, sulphuric acid and phosphoric acid. The metal being used is magnesium. Both methods are going to be trialled to see which would be best to carry out the actual investigation. To do this the experiment was set up as shown on the next page. [pic 1][pic 2][pic 3][pic 4][pic 5][pic 6]Diagram:Half Life Method –The greatest and smallest concentrations of acid were used, 2.0m and 0.5m. A piece of magnesium was cut off the magnesium reel and measured to 0.5g before being rolled into a small ball so it could be totally immersed in the acid and fit into the side arm test tube. This was placed to the side as the acid was measured out (2.1cm ³ for 2.0m and 8.4cm ³ for 0.5m). The acid was then placed in the test tube along with the magnesium, the bung in place on top and the timer started. Once the gas volume had reached 80cm³ the time taken to reach that volume was noted in my results. The apparatus was then washed out and the equipment set back up to do the other experiments. This method was repeated for the other concentration and with the other acids. The results are on graphs TG1, TG2 and TG3.Order Method – The quantities of acid used in this reaction were not specified so the use of 20cm³ seemed appropriate enough for the experiment. Again the use of 2.0m and 0.5m concentrations were used. The acid was placed in the side arm test tube and another 0.5g ball of magnesium dropped into the acid. This time, though, the reaction was timed for 20 seconds then the amount of gas collected was measured. After the first concentration the equipment was washed and the experiment repeated for the other acids and the concentrations used for them. These results are on graphs TG4, TG5 and TG6. After completing both of these methods the decision to use the ‘Order’ method had been made because of the better results it showed, but also because flaws were noticed while the ‘Half Life’ method was being carried out, the main imperfection being that, as the reaction was taking place, the side arm test tube was heating up, this was because the reaction taking place was exothermic. This was a problem because if this method was to be used for the actual investigation a way of keeping the temperature constant (to keep the reaction fair and the results as accurate as possible) would need to be found. There are a few ways that the temperature could be kept constant, for example the peak temperature of the reaction could be measured and the acid heated up to this temperature before being reacted to with the magnesium; however this is dangerous because the use of hot, concentrated acid would now be used. Another way would be to pack ice around the experiment to keep the temperature down. However this would require watching the temperature closely and would mean repeating several experiments if the temperature fluxuated. Here are the calculations used to work out the exact amount of acid that would be needed for the trial method of ‘Half Life’.Mg + H₂SO₄ → H₂ + Mg + SO₄m = c x v H₂ → 100cm → 24000 ATPm = 1 x 4.2 x 10⁻³ → 100max moles = 4.2 x 10⁻³ 24000 = 4.2 x 10⁻³v = m 4.2 x 10⁻³ = 2.1cm³ ← this is for 2moles concentration c 2v = m 4.2 x 10⁻³ = 8.4cm³ ← this is for 0.5 moles concentration c 0.5__________________________________________________________________________________3Mg + 2H₃PO₄ → 3H₂ + 3Mg²⁺ + 2PO₄³⁻m = c x vm = 2 x 4.2 x 10⁻³ 3max moles = 2.8 x 10⁻³v = m 2.8 x 10⁻³ = 1.4cm³ ← This is for 2moles concentration c 2 v = m 2.8 x 10⁻³ = 5.6cm³ ← This is for 0.5moles concentration c 0.5__________________________________________________________________________________Mg + 2HCl → H₂ + Mg²⁺ + 2Clm = c x vm = 2 x 4.2 x 10⁻³max moles = 8.4 x 10⁻³v = m 8.4 x 10⁻³ = 4.2cm³ ← This is for 2moles concentration c 2v = m 8.4 x 10⁻³ = 16.8cm³ ← This is for 0.5moles concentration c 0.5__________________________________________________________________________________Trial Results:Half LifeMg + H₂SO₄ → H₂ + Mg + SO₄ 2 moles 2.1cm³ acid
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