Monobasic/Dibasic?

Letter 15572

Monobasic/Dibasic?

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Hi there,

I'm working on my A-level investigation which is looking at magnesium and acids but this time with activation energy and arrhenius equation. I'm investigating the activation energy of Magnesium reacting with a monobasic and a dibasic acid at various temperatures, so that I have to variable, temperature and type of acid. I've chose HCL for my mono and H2SO4 for my dibasic, I wanted to know how a dibasic reacts more rapidly than monobasic what actually happens to the magnesium and the H+ ions from the acids at a molecular level.

Callum Parr
- Crawley, Sussex, England

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The Arrhenius Equation is K = A.e(-Ea/RT), where K is the reaction rate constant, Ea is the Activation Energy, R is the Gas Constant and T is the temperature in degrees Kelvin. The Activation Energy is the minimum amount of energy needed for the reaction to occur and is affected by the stoichiometry of the reaction (is it 1st, 2nd or 3rd order?) and hence is dependent on collision theory; it is also related to the rate laws, so the concentration of reactants will be important.

If you take natural logs of both sides you get ln(K) = ln(A) – Ea/RT. You can hence calculate the Activation Energy for any reaction if you know the rate of the reaction at different temperatures from ln(K2) - ln(K1) = -Ea/R(1/T2 -1/T1). In this case the K1 and K2 could be the amount of hydrogen produced in a given time. Plot a graph of ln(K) vs 1/T and the slope will be -Ea/RT. However, for the test to work properly, you need to have equal molarities of the acids and the same amount of magnesium surface area (Now tell me why!). It is also important that the reaction does not passivate by a reaction product becoming insoluble and slowing down the acid/metal reaction. As far as molecular chemistry is concerned, the overall reaction is ionic and one of electron transfer, with magnesium rendering up two electrons to become Mg2+ and the hydrogen ions from the dissociated acid accepting one electron to become nascent hydrogen. As you should be aware, the hydrogen ions (or protons) are solvated and exist as H3O+, so they must lose their solvation sheath before they can react on the magnesium surface; this is another factor that will affect the Activation Energy. Two nascent hydrogen atoms combine to form a hydrogen molecule that will be released as a gas bubble. The rate at which this occurs is dependent on the rate and ease of adsorbtion of the hydrogen ion onto the magnesium and the rate of electron transfer. This is also linked to the hydrogen overpotential of magnesium. Offhand I do not recall whether the reaction is SN1 or SN2, but perhaps you can work that one out by applying thermodynamics to the problem! Arrhenius was a clever guy and also came up with his theory of electrolytic dissociation. This states that any molecule of an electrolyte can give rise to 2 or more electrically charged ions or atoms. The degree of dissociation is the level to which this occurs at any given concentration, but at infinite dilution, it is considered to approach unity (ie all the molecules have dissociated). He also tried to calculate the degree of dissociation from the conductivity of electrolytes at different concentrations and found the numbers to come out very close to each other. However, it is now thought that this was more by luck than judgement!

Trevor Crichton
R&D practical scientist - UK

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Hi, I'm currently in the midst of writing up my A2 Chemistry practical and I'm investigating the activation energy of the reaction. I've been doing some research but I can't seem to find anything on how the number of H+ ions in an acid affects the activation energy. Could you give me some help on how the H+ ion attacks the magnesium? Also, would I be correct in saying that in dibasic acids, there are more H+ ions so the reaction should be faster, but may not necessarily have a different activation energy? Thank you very much.

Jasveen Kaur
- Hong Kong

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Dibasic acids have double the amount of H+ ions in the solution and these are what attack the magnesium during a reaction. They speed up the rate at which the gas is given off but not the volume because there is still a limited amoutn of magnesium to react with it. They should double the rate of the reaction in comparison to monobasic because you still have to take into account the surface area of the magnesium which it is attacking, depending on if you are using magnesium powder or ribbon (powder means it will have less of an effect than the ribbon cos it has more surface area). Good luck with the write up!!!!!

Katie S
- UK

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