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Hydrogen bromide forms a strong acid when dissolved in water whereas hydrogen fluoride forms a weak acid. Distinguish between the terms strong acid and weak acid. State equations to describe the dissociation of each acid in aqueous solution. [3]

Here, we have to focus on the term strong acid. It is defined as an acid that is dissolved completely in an aqueous solution. We say that the reaction of dissociation of the acid goes to completion, and we write it as HA(aq) -> H+(aq) + A-(aq). Notice that the arrow points only in one direcition, and this is because for strong acids, the dissociated form H+ + A- is preferred, therefore the reaction doesn't go the other way.

Based on this knowledge, we can define the the term weak acid, which can be seen as any acid, which is not strong acid. This means that the acid is not completely dissolved in the aqueous solution, but only partially. The reason why I derived the weak acid from the strong is because strong acid has to dissolved completely, but any acid, which dissolves only a tiny bit, or quite a lot but not completely, is considered a weak acid, so the range of the weak acid is much bigger. The equation of dissociation is HA(aq) <–> H+(aq) + A-(aq), because as it is not completely dissolved, the two forms of the acid create an equilibrium, hence the two way arrow.

With this knowledge, we can move on to the question. First notice, that this question is for 3 points, and asks us for one definition and 2 equations, therefore it is sensible to expect that the definition is for one point and so is each equation.

The distinction between strong acid and weak acid is that strong acid is dissolved completely in an aqueous solution, while weak acid is dissolved only partially. (it is safer to involve both terms strong acid and weak acid in the answer to receive the point for it.)

Hydrogen bromide, HBr, is a strong acid, hence it's equation of dissociation is HBr(aq) -> H+(aq) + Br-(aq). Hydrogen flouride, HF, forms a weak acid, therefore the equation is HF(aq) <–> H+(aq) + F-(aq).

Always write also the state of the molecules! Keep in mind that weak acids form equilibri, hence the two way arrow!

Juraj O. IB Biology tutor, IB Chemistry tutor, IB Maths tutor, IB Fur...

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