Question

1) Why can we generally assume that the equilibrium concentration of a weak acid equals its initial concentration?

2) Why is pure water a poor conductor of electricity?

3) Describe the differences and similarities between weak and strong acids.

Answer 1

1) Because a weak acid will not ionize very much.

Consider the weak acid HCN (Ka = 6.17x10^-10). When HCN dissolves in water, the following equilibrium is established:

HCN(aq) + H2O(l) <=> CN-(aq) + H3O+(aq)

But the equilibrium so strongly favors the left side of the reaction that the equilibrium concentration of HCN is essentially the same as the starting concentration of HCN. Let's throw in some math so we can see what we're talking about. Assume that you prepare a solution that is initially 0.1 M HCN.

Reaction: HCN(aq) + H2O(l) <=> CN-(aq) + H3O+(aq)
Initial:.......0.1 M ... ... ... ....... ... 0 M... ... ..0M
Change:.... - x ... ... ... ... ... ... ...+ x ... ... .+ x
Equil: ... .. 0.1 - x ... ... ... ... ... ... x ... ... ... x

So now let's write the equilibrium constant expression:

Ka = [CN-][H3O+] / [HCN]

6.17x10^-10 = (x)(x) / (0.1 - x)

6.17x10^-10 = x² / (0.1 - x)

6.17x10^-11 - (6.17x10^-10)x = x²

0 = x² + (6.17x10^-10)x - 6.17x10^-11

Using the quadratic formula gets you two roots:

x = 7.85x10^-6 M and x = -7.86x10^-6 M

We can discard the negative root as extraneous, which means that the value of x at equlibrium is 7.85x10^-6 M. But 7.85x10^-6 M is *tiny* with respect to the initial [HCN] (which was 0.1 M), and 0.1 M - x is essentially equal to 0.1 M. When we're calculating the equilibrium concentrations, we can make a very good simplifying assumption by replacing 0.1 M - x with 0.1 M

6.17x10^-10 = (x)(x) / (0.1 M)

6.17x10^-11 = x²

x = 7.85x10^-6 M

Q.E.D. I hope that helps. Good luck!

2)

Water is a poor conductor of electricity because it is only very slightly ionized in which hydrogen and hydrogen ion have the equal concentrations, and it is therefore neutral.

HOH->H+ + OH-

3)  Differences

Firstly, the definitions:

Acids dissolve in water to form H+ ions.

Bases dissolve in water to form OH- ions.

Lets take a look at acids - A strong acid (HCl) vs a weak acid (CH3COOH).

A strong acid will fully dissociate in water to form H+ ions.

HCl + H2O---> H3O+ + Cl-

This reaction is non-reversible. After dissolution, only a very very minute concentration of HCl itself remains in the solution, as most of the diluted HCl has dissolved into ions.

Ka = [H+] [Cl-] / [HCl]

A weak acid, however undergoes a reversible reaction as it only partially dissociates.

CH3COOH + H2O <---> CH3COO- + H3O+

Ka = [CH3COO-] [H3O+] / [CH3COOH]

As Ka is a constant, as more CH3COOH is added, by Le Chatelier's Principle, in this reversible reaction, the system will aim to shift the equilibrium as such to reverse the change. Therefore, less CH3COOH than added will dissociate into its constituent ions.

Similarities

They both dissociate to give H⁺ ions in aqueous solution.