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].
We find the rate equation through experiment, then find out the order.. Together we can deduce its mechanism. Activation energy, EA Activation energy is the energy needed for the reactants to reach some inter-mediate state. This tells us how likely the reaction would happen. Particles have to have sufficient energy(greater than EA). In the case, the energy is mostly kinetic energy, since ke=1/2 mv2, and the distribution of speeds can be determined by Zartmann, so the energy distribution is determined as the graph below.
Enthalpy change, ? H Definition: standard enthalpy change of a reaction as the amount of heat absorbed or evolved when the molar quantities of reactants as stated in the equation react together under standard conditions, i. e. at pressure of 1 atm, temperature of 298K, with substances in their normal physical states under these condition and solution having unit activity. Enthalpy is the changes in the heat content. These changes can be shown through the change in temperature. ?H= Enthalpy after – enthalpy after Enthalpy change of a reaction sometimes can be a rough guide to the likelihood that the reaction will occur.
We can see if ? H is negative, hence energy lost, so the product is more stable, means the reaction would happen. Ref, ‘Chemistry in context’ Integrated Rate Laws The rate of reaction is proportional to the rates of change in concentrations of the reactants and products; that is, the rate is proportional to a derivative of a concentration. Rate equation is a differential equation that relates the rate of change in a concentration to the concentration itself. Integration of this equation produces the corresponding integrated rate law, which relates the concentration to time.
Reaction Order Differential Rate Law Integrated Rate Law Characteristic Kinetic Plot Slope of Kinetic Plot Units of Rate Constant Zero vs t k Dm mole-1 sec-1 Ref:’http://www. chm. davidson. edu/ChemistryApplets/kinetics/IntegratedRateLaws. html’ The second order can be also rearranged to 1/[A] – 1/[A]0 = kt Ref: http://www. cartage. org. lb/en/themes/Sciences/Chemistry/Miscellenous/Helpfile/Kinetics/secondorder. htm Method.
Two acids I have chosen are Hydrochloric acid HCl and Sulphuric acid H2SO4, one monobasic acid and one dibasic acid. Planning Order and Rate(experiment 1) The product are hydrogen gas and a salt, and gas can be easily measured and collected by using a syringe, and the rate of the reaction is proportional to the gas produced, so by know how much gas is produced per unit time, rate can be found. It can be done by two methods: Continuous Rate Method, or Initial Rate method. Continuous Rate Method involves tracking how much gas is produced at a certain time interval until the reaction has stopped.
Initial Rate Method is only to measure the 10% to 20% of the reaction where we can assume that the concentration of the reactant has not changed. I was provided with acids with 2M concentration, so I had to dilute myself. Mg comes with ribbon or powder. Preliminary work I tried with Continuous Rate Method, because it seems would shows me how the rate very with time.
Mg can either be powder or ribbon, I choose ribbon because I can see the reaction more clearly and the rate would not be too fast. The reaction for HCl is: 2HCl(aq) + Mg(s) H2(g) + MgCl2 (aq) I used 20cm3 of acid solution. It was my initial guess of the acid quantities needed.
I used 100% excess of Mg, 0. 96g. It produced 100ml of gas in 3 seconds, (in theory it would produced, 480ml of gas), so 21% of the reaction. That is too fast. I did another experiment with 1M acid solution with 100% excess Mg,0. 48g, 100ml of gas only takes 5 seconds. That is too fast too. I adjusted the solution to 10 cm3, it is the minimum value, because less than 10 cm3 the Mg can not be excess.