Acid and magnesium Essay Example for Free

Acid and magnesium Essay B unit for rate constant is not definite, because I am not certain what order the reaction is So Rate = 0. 0490 [HCl] 1. 90 This shows the rate is very dependent on the [HCl], the concentration against time is illustrated in graph 8. The reaction for H2SO4 is: H2SO4(aq) + Mg(s) H2(g) + Mg SO4 (aq) Graph 4 is Rate against [H2SO4]. I found the line of best fit is roughly a straight line. Graph 5, I plot Rage against [H2SO4] 2 and found the line of best fit is a curve. N. B the error bars shows the uncertainty is higher than I purposed Using the log method again, I found the line is y = 1. 33x 0. 889 So overall order is 1. 33 and k is 0. 129 So Rate = 0. 129 [H2SO4] 1. 33 However, the order of a reaction must be a integer, because number of particles are discrete. So the order I obtain must be an average order of the reaction. From graph 3 and 6, the points are not fitted with the line of best fit that means the gradient is not constant, hence the order is not constant throughout the reaction. The gradient of the first few points is big and decrease hence the order is high at the beginning, and decrease as the reaction proceeds. Integrated Rate Laws. Using the integrated Rate Laws, I can see how [acid] varies with time. I assume [HCl] is second order and [H2SO4] is first order. F According to the graph, the reactions seems stop after 50seconds, which is the same results in table 4. However, these two graphs are only estimations, because I have to assume the order of the reaction in the first place. For Experiment 2: HCL and Mg Using table 6, I plotted graph   and Mg Using table 7, I plotted graph 8, do some calculation as above, line of best fit is y = -1800x + 1. 71 EA= 18008. 31=15000j=+15kJ According to the rate equation, Rate = k[Acid] a, a would affect the rate, and I used Ln K for the calculation above, where I assume a is a constant. My results from experiment 1 shows that the order may have changed as concentration changes , according to graph 1 and 2, because the rate slowing down. Just look at the last two results on graph 1 and 2, it seems the order has decrease to zero, because the line of best fit is a horizontal line. This can due to the experiment error I described in page 12. Same case applied sulphuric acid, the order may has slow down as concentration increases. For experiment 3 Enthalpy HCl and Mg Q=cmT = specific heat capacity of solution x mass of the solution x temperature change. Having work out the enthalpy change for the whole reaction, the energy liberated during 20% of the reaction can be calculated. Table 11 Change of temperature during the 20% of the reaction for HCl and Mg Concentration/ moldm-3 Volume/ cm3 Mole Energy liberated during the 20% reaction/j Change in Temp/ Table 12 Change of temperature during the 20% of the reaction for H2SO4 and Mg Concentration/ moldm-3 Volume/ cm3 Mole Energy liberated during the 20% reaction/j Change in Temp/ oThe material below is reference to Chemistry in Context, it talks about the effect of temperature rise(below). From the kinetic theory, we can predict the relative increase in number of collisions when the temperature rises by 10K. The kinetic energy of a particle is proportional to its absolute temperature: 1/2mvi ? T But the mass of a given particle remains constant Vi ? T Therefore Vi 1 = T1 Vi 2 T2 Where V1 is the velocity at temperature T1, and V2 is the velocity at temperature T2. Now, suppose that the average speed of a particle is V at 300K. We can work the average speed at 310K by using the equation. This therefore shows that the average speed at 310K is only 1. 016 times greater than that at 300K i. e. it has only increased by 1. 6%. So as the calculation above, I can work out the increase in speed of particles, using the temperature change in table 12 and 13. Temperature in the solution is 24 oc, as I measured. Table 14 % increase in velocity of the particles during the reaction for HCl and Mg Concentration/ moldm-3 Temp before(T1)/ oc Temp after(T2)/ oc T2/T1 % increase in V. Table 15 % increase in velocity of the particles during the reaction for H2SO4 and Mg, Concentration/ moldm-3 Temp before(T1)/ oc Temp after(T2)/ oc T2/T1 % increase in V Entropy. The entropy of this reaction also tell us how likely the reaction would occur. HCl and Mg 2HCl(aq) + Mg(s) H2(g) + MgCl2 (aq) ?Stotal=? Ssurrouding+ ? Ssystem ?Ssurrouding= -? H/T, (T=298K, room temp) and we worked out ? H before, -1. 17kJ, Ssurr = +3. 93 Jmol-1K-1 ?Ssystem=Product Reactant , values are obtained from the databook, H2 (65. 32)+MgCl2 (89. 6)- 2(H+(0)+Cl-(56. 5))-Mg(32. 7)=+74. 5 Jmol-1K-1 ?Stotal= +78. 43 Jmol-1K-1 We can deduced that the reaction would happen spontaneously in 298K H2SO4 and Mg, H2SO4 (aq) + Mg(s) H2(g) + Mg SO4(aq) Same calculation as above, ? Stotal=? Ssurrouding+ ? Ssystem ?Ssurr=+3. 79 Jmol-1K-1 ?Ssys= Mg SO4 (91. 6) + H2 (65. 32) Mg(32. 7) H2SO4 (0+ 20. 1)=+169. 2 Jmol-1K-1 ?Stotal= +173 Jmol-1K-1 Confidently, we can deduced that the two reaction would happen spontaneously in 298K So far, for HCl and Mg, I have found Rate = 0. 0490 [HCl] 1. 90 EA =+16. 6kJ ?H= 1. 17kJ per mole ?Stotal= +78. 43 Jmol-1K-1 For H2SO4 and Mg, Rate = 0. 129 [H2SO4] 1. 33 EA= +15kJ ?H= 1. 30kJ per mole ?Stotal= +173 Jmol-1K-1 The likelihood of the reaction The reaction is very likely to happen. This is because ? Stotal is positive, suggest spontaneous reaction and its =+16. 6kJ is not very high. When the concentration increases, the temperature increases as in graph 10, hence a increase in speed of particles (graph 12), and 0. 5xMxv2 is KE, so KE increase by the square of v, which 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 + 164 = 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 Show preview only The above preview is unformatted text This student written piece of work is one of many that can be found in our GCSE Patterns of Behaviour section.