This will help ensure that the results of the experiment are reliable and ensure that the acids react with the magnesium and not the formulated oxide layer. The Acids: Choice of acids and their varying strengths was based upon the Bronston-Lowry theory. According to the Nuffield Advanced Chemistry Students’ Book, acids that ionise completely at moderate dilutions are called strong acids. Consequently they tend to have a large Ka (dissociation constant) value. Weak acids interact very little with water therefore do not easily dissociate.
Ethanoic acid is an example of a weak acid as it dissociates very few hydrogen protons to bases.(Nuffield Advanced Chemistry Students Book, Longman, p338).
Hydrochloric acid is an example of a strong acid. This is because it completely donates its protons (H+). Dissolving hydrochloric acid in water will in turn formulate an aqueous equilibrium as indicated in the equation below:
HCl (aq) → H+ (aq) + Cl -(aq),
almost completely dissociated In the reaction above the H+ in the hydrochloric acid acts as the acid and the Cl- acts as the conjugate base.
The HCl loses its protons readily; therefore a high concentration of H+ ions is produced when the reaction reaches its homogenous equilibrium.
The dissociation constants in the reaction is equivalent to the equilibrium constant Kc. However, the difference between Kc and Ka is that Ka is not influenced by changes in the concentration, but it is influenced by changes in temperature. This enables us to accurately measure the extent to which the acid is dissociated (strength of the acid).
Hydrochloric acid is a strong acid and therefore it will have a greater extent of dissociation. Hence, there will be a greater amount of [H+ ] protons in the acid and [Cl -] ions in the conjugate base. This provides a larger Ka value.
The Ka value is the dissociation constant for the acid.
[H+ ] [Cl – ] [HCl (aq)] = 1.7 x 10 – 5mol dm-3.
The Ka value for hydrochloric acid is not available in the book of data. This is primarily due to the fact that that the acid is very strong, which indicates that it will dissociate into hydrogen and chlorine ions.
CH3COOH (aq) → H+ (aq) + CH3COO-(aq),
only partially dissociated When using both acids we will be able to analyse the comparisons in the different rates of the reaction. This will in turn help determine whether the amount of protons donated in the reaction will influence the amount of collisions made and therefore whether it will increase the rate of the reaction.
Reactions between ionic compounds only work in the solution because the ions are mobile and able to interact. The balanced equation for hydrochloric acid is as follows:
2HCl (aq) + Mg (s) → MgCl2 (aq) + H2 (g)
Ignoring the spectator ions, which are the (Cl-) ions.
2H+ (g)+ Mg(s) → Mg2+ (aq)+ H2 (g)
The mechanism of the reaction is a description of the sequence of steps, a form of breaking the reaction down into a number of steps. The rate of the reaction depends on the rate of the slow step, which is the rate-determining step. Therefore, in order to increase the rate of the reaction, the speed of the rate-determining step must be increased.
This can be achieved if reactants in the rate-determining step are first or second ordered. This will change the amount of the concentration of reactants, which will increase the rate in which products are produced for them to be used in the fast step, increasing the rate of the whole mechanism. A proposed mechanism for the reaction is shown in the sequence below:
(Rate determining step) (Slow step) Mg (s) + H+(aq) → Mg+(s) + H
(Rate determining step) (Slow step) Mg+(s) + H+ → Mg2+ (aq) + H
(Fast step) H (g) + H (g) → H2 (g)
The mechanism suggests that the reaction consists of two rate-determining steps.
Each rate-determining step has high activation energy in the reaction. Therefore it determines the overall rate of the reaction. This suggests that the order is second, as indicated in the indices below:
So Rate = k [H+ (aq)] 2
The magnesium ribbon doesn’t feature in the rate equation because it is a solid. This may occur because the hydrogen ion appears in both rate-determining steps. This is because only liquids, gases and ions have a concentrate. The chlorine doesn’t appear in the rate equation. Any step involving chlorine must come after the rate-determining step.
It is reasonable therefore to postulate that the first step involves a proton with magnesium. This is because both the species appear in first order terms in the rate equation. The second step controls the rate of the reaction. It therefore contains the most activation energy. The last step in the mechanism reacts rapidly forming hydrogen gas. The hydrogen molecule appearing twice in the rate determining step indicates that the reaction is second order in respect to the acid, suggesting the result of the graph will be of second order. The mechanism above shows that the hydrochloric acid is an aqueous solution.
Therefore the concentrated solution contains water as does the equation, which means that the chloride ions and the hydrogen ions in the acid have already dissociated. This may imply that the hydrogen ion in the acid reacts with the solid magnesium ribbon. From the magnesium ion an electron is transferred to produce a hydrogen free radical. “A free radical is an uncharged attacking group with an odd number of electrons, so they possesses only one of the electron pairs needed for the formation of a new covalent bond (Nuffield advanced chemistry students book, (Longman) p185).
This reaction is fairly slow in comparison to the rest; this is due to the fact that the hydrogen ion has to react with the larger magnesium ion. The hydrogen ion then gains another electron from a magnesium ion to form a hydrogen free radical. The magnesium ion has a 2+ charge. This stage of the reaction is called propagation. The rate-determining step has two positively charged reactants, which repel one another. After this stage of the reaction the hydrogen free radicals end up colliding against each other. This terminates the reaction to produce hydrogen gas, a process known as homolytic fission.
This reaction occurs at high speeds due to the fact that the free radical molecules contain a set of unpaired electrons forming an attacking group. These electrons readily react so that the molecule can produce a full shell that will then become inert and stable. As both molecules contain an odd number of electrons in their outer shell they readily form covalent bonds in which the electrons are shared between the nuclei. Ethanoic acid is also readily available and is a weak acid; these were strong factors for its choice.
The balanced equation for the weak acid (ethanoic acid) and the magnesium ribbon is shown below:
2CH3COOH (aq) + Mg (s) → (CH3COO-)2 Mg2+ (aq) +H2 (g)
Ignoring the spectator ions
2H+(aq) + Mg → Mg 2+ (aq) + H2 (g)
The mechanism in this reaction will be the same as the one for hydrochloric acid, as indicated above. This is because the spectator ions (Cl-) and CH3COO- are omitted in the ionic equation, therefore they can be ignored. Preliminary Experiments: The aim of the preliminary experiment is to formulate an accurate method in conducting the final experiment. Three experiments will be conducted which will help ensure the validity and reliability of the results.
In this experiment the variables, which may contribute to inaccuracies in the final results, will be explored. The aim of the preliminary experiments is to determine the following: Whether the oxide layer on the magnesium makes a difference in terms of the rate of the reaction. Suitable range of concentrations to use. Suitable temperature range to use. The type of metal which is going to be used Apparatus. If the experiment is exothermic. The Volume of Acid: In the preliminary experiment we are going to use a 10.0cm3 of acid. This is because it is a standard measurement, which can be accurately and easily measured using a pipette.
The volume of acid is in excess in comparison to the length of magnesium ribbon. Therefore, it will ensure that the reaction will be completed methodically and successfully. As the reaction is exothermic the excess volume of acid will act as a temperature regulator absorbing any heat produced.
The apparatus chosen for the preliminary experiment was an important consideration because it would ultimately determine the selection procedure for the apparatus, which would be used in the final experiment.
When conducting the first preliminary experiment it was concluded that using a beaker gave far too much surface area making the reaction quicker. In the second experiment it was then decided to place the acid in a boiling tube that has a diameter of 2.0 cm. The main advantage in using the boiling tube is that it provides an increase in depth ensuring that the reaction takes place. When the physical apparatus had been selected, a suitable acid and concentration of that acid was needed, both strong and weak. The three contenders for the choice of strong acid were Sulphuric, Hydrochloric and Nitric acid.
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