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# Study the reaction kinetics and find out evidence about the mechanism between the reaction of acid and magnesium.

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Introduction

Investigation of acid and magnesium This investigation is to study the reaction kinetics and find out evidence about the mechanism between the reaction of acid and magnesium. It can be done by finding out the rate, order, activation energy and enthalpy change of the reaction. Acid What is an acid? Definition: an Arrhenius acid is a compound containing hydrogen which will form hydrogen ions in water. What is a strong acid? Acids that donate their acidic protons almost completely to water are classified as strong acids; acids that interact very little with water are classified weak acids. Ref, 'Nuffield Chemistry Students' Book' Ionic equation for acid and Mg: 2H+(aq) + Mg(s) --> H2(g) + Mg2+(aq) Rate equation and order of reaction Reactants are converted to products during a reaction and the rate of reaction is a measure of how fast the reaction is. It is an indication of how much reactants are converted into product in a given time. It can be affected by the concentration. The rate equation can be expressed as: Rate ? [A] a [B] b Rate = k [A] a [B] b Rate = k[Acid] a N.B.[Mg] is not needed in the rate equation: because Mg is a solid, so its concentration does not vary. k is the rate constant a is the order of reaction Order with respect to any one reactant is the index. The sum of the order of reaction is calculated by add the index of the reactants, in this case, is only the concentration of acid [acid]. Order means how much dependence of the rate is on the concentration of the reactant. i.e. n is usually 0,1,2 when n=0, so [a]0=1, so Rate = k ; when n=1,Rate = k[a] ; when n =2, Rate = k [a]2 From the rate equation, we can see that [acid] involve in the Rate Determining Step (that means it is the slowest step within the reaction), because the rate depends on [acid]. ...read more.

Middle

Enthalpy (experiment 3) Diagram: I used 0.06g of Mg react with 10cm3 of acid in a test tube. I measured the temperature before and the maximum temperatures, which is when the reaction finished. Results: Table 7 HCl and Mg Concentration/moldm-3 Temp Before/oc Temp after/oc Change in temp/oc 1.00 23 50 27 1.50 24 51 27 2.00 22 50 28 Table 8 H2SO4 and Mg Concentration/moldm-3 Temp Before/oc Temp after/oc Change in temp/oc 1.00 23 50 27 1.50 22 54 30 2.00 24 55 31 Then I realized that the enthalpy change should be the same for each concentration, because the same amount of Mg has reacted. But the low concentration has a lower rate, so it would lose more heat to the surrounding during the reaction that may explain why the change in temperature is slightly lower. So I suggest the change in temp for the HCl is 28 oc and for H2SO4 is 31 oc, simply just take the highest value. Analysis: Uncertainty Experiment 1 has measuring error, i.e. reading on the syringe, they are +or-1ml out of 100ml, that is 2%, the Mg is 400% excess, is about 5% error??(roughly)explain, the reading on the thermometer is +or-1,that is 2%. So for experiment 1, error is roughly 2%(syringe) +5%(Mg excess) is 7% For experiment 2, error is roughly 2%(syringe)+2%(temp before)+2%(temp after)=6% For experiment 1: The reaction for HCl is: 2HCl(aq) + Mg(s) H2(g) + MgCl2 (aq) Table9 Effect of [HCl] on rate (Mg in excess) Concentration/moldm-3 Time/s Rate(1/s) Log(HCL) Log rate Con2 (mol2dm-6) 0.25 292.00 0.00 -0.60 -2.47 0.06 0.50 87.66 0.01 -0.30 -1.94 0.25 0.75 38.58 0.03 -0.12 -1.59 0.56 1.00 15.37 0.07 0.00 -1.19 1.00 1.25 11.85 0.08 0.10 -1.07 1.56 1.50 9.47 0.11 0.18 -0.98 2.25 1.75 7.34 0.14 0.24 -0.87 3.06 2.00 6.78 0.15 0.30 -0.83 4.00 Table 10 Effect of [H2SO4] on rate (Mg in excess) Concentration/moldm-3 Time/s Rate(1/s) ...read more.

Conclusion

is a big gain in energy, so the activation energy curve is shifted to the right hand said and more particles has the KE to go over the energy barrier. This means the higher the concentration the higher increase in temperature, so higher velocity of particles, higher KE, hence more particles have enough energy to complete the reaction, so more likely the reaction would happen. Difference in rate Two acids are ionic Ionic equation for acid and Mg: 2H+(aq) + Mg(s) --> H2(g) + Mg2+(aq) H2SO4 is dibasic and HCl is monobasic. They are all strong acid, means the H+ would dissociated fully. H2SO4 has 2 acid protons and HCl only has 1 acid proton. So 0.1mol wrt [HCl], is 0.1mol wrt [H+], and 0.1mol wrt [H2SO4] and 0.2mol wrt [H+]. The rate for sulphuric acid ought to be higher than hydrochloric acid, because the more the acid proton, the quicker the acid proton dissociate. However, when we consider about the spectator ions, SO42+ and Cl-, there is a significant difference in size, Cl- has ionic radius of 0.180nm, S8+ has ionic radius of 0.102nm, covalent radius of 0.102nm, and O2+ has ionic radius of 0.140nm and covalent radius 0.073nm. For SO42+ overall is roughly about 0.140 0.103 0.101 0.140 the length of SO42+ is about 0.140 + 0.102 + 0.102 = 0.344 is about 2 times bigger the Cl ions. The atomic weight of SO4 is 32 + 16x4 = 96, and Cl is 35. SO4 is 2.7 times heavier. From these evidence, I can suggest that during the collision, the spectator ions SO4 and Cl affect the rate of reaction as well. The SO4 ion is bigger than the Cl, so the sulphuric acid reaction, the SO4 slows down the rate of collision because of its size. But for hydrochloric aicd reaction, the Cl is relatively small and lighter, so the collision is easier to happen, hence a quicker rate of reaction. ?? ?? ?? ?? Keith Li 111 - 1 - ...read more.

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