Write the net acid-base reaction that occurs when HBr is added to water. (Use the lowest possible coefficients. Omit states of matter in your answer.) (b) What is the relative Ka value for this reaction? -Ka ≈ 1-Ka >> 1-Ka << 1 (c) Which are the only species present? – Hydronium ions – conjugate base ions – strong acid molecules – water molecules
The Correct Answer and Explanation is:
(a) Net acid-base reaction:
HBr + H2O → H3O+ + Br−
(b) Relative Ka value:
Ka >> 1
(c) Only species present:
Hydronium ions, conjugate base ions, water molecules
Explanation:
When hydrobromic acid (HBr) is added to water, a proton transfer reaction occurs. This is known as an acid-base reaction based on the Brønsted-Lowry theory, where an acid donates a proton (H+) and a base accepts it. HBr is a strong acid, meaning it completely dissociates in water. So, one hydrogen ion (H+) from HBr is transferred to a water molecule (H2O), producing hydronium ion (H3O+) and bromide ion (Br−).
This leads us to the net equation:
HBr + H2O → H3O+ + Br−
Because HBr fully ionizes in water, its acid dissociation constant (Ka) is extremely high. The Ka >> 1, indicating that nearly all of the acid molecules donate their protons to water. A high Ka means the reaction lies far to the right, favoring product formation (H3O+ and Br−). Strong acids like HBr do not exist in undissociated form once mixed with water.
In the resulting solution, the only species that remain are:
- Hydronium ions (H3O+) from proton transfer,
- Conjugate base ions (Br−) from HBr losing a proton, and
- Water molecules (H2O), which are always present in aqueous solution.
No undissociated HBr molecules are left, because strong acids completely dissociate. Therefore, the strong acid molecules are not among the present species after reaction.
This complete dissociation and the presence of only ions (and water) is what distinguishes strong acids like HBr from weak acids, which only partially dissociate and would have a Ka value much less than 1 (Ka << 1).
