Chapter 12 - Acid-Base Equilibria

What is a Bronsted-Lowry acid?

A proton (H⁺) donor

What is a Bronsted-Lowry base?

A proton acceptor

What does an acid base reaction involve?

The transfer of protons

What is a Bronsted-Lowry conjugate base pair?

A conjugate acid–base pair consists of two species that differ by a single proton (H⁺). When an acid donates a proton, the conjugate base is what remains after the acid has lost a proton.

Why can it be useful to identify conjugate acid-base pairs?

Tracking conjugate pairs helps us follow proton transfer in acid–base reactions.

How can you identify conjugate acid-base pairs?

In any acid–base reaction:

• The acid and its conjugate base differ by one H⁺.

• The base and its conjugate acid differ by one H⁺.

When trying to determine conjugate pairs, always look for whether a proton has been lost or gained. Don’t worry about the rest of the formula or how complicated something might look, you are only interested in whether it has gained or lost a H+ ion.

e.g. NH₄⁺ ⇌ NH₃ + H⁺
NH₄⁺ acts as the acid (donates H⁺).
NH₃ is the conjugate base (can accept H⁺).

What is a strong acid/base ?

Strong acids and bases (almost) fully dissociate in water (into H⁺ ions for acids or OH^- ions for bases). Though these are still reversible reactions the position of equilibrium sit very far to the right and so a normal arrow can be used in equations.

What is a weak acid/base?

Weak acids and bases only partially dissociate, forming an equilibrium system. The equilibrium contains the original weak acid, its conjugate base and H⁺ ions (for a weak acid) or the original weak base, its conjugate acid and H⁺ ions.

How do pH of strong and weak acids compare?

Even at the same concentration, strong acids have a lower pH than weak acids because they release more H⁺ ions.

What is a monoprotic acid?

An acid that releases one H+ ion per molecule. E.g. HCl.

What are diprotic and polyprotic acids?

Diprotic acid = acid that releases two H⁺ ions per molecule e.g. H₂SO₄

Polyprotic acid= acid that releases more then one H⁺

What is pH?

The pH scale measures the concentration of hydrogen ions (H⁺) in solution, on a logarithmic scale. A lower pH indicates a higher concentration of hydrogen ions (i.e. a more acidic solution).

Why is it useful that pH is logarithmic?

This allows us to express a wide range of H⁺ concentrations more simply.

The pH scale is a logarithmic scale with base 10 meaning that each value is 10 times the value below it. For example, pH 5 is 10 times more acidic than pH 6.

How can pH be calculated?

pH = -log[H⁺]

(where [H⁺] is the concentration of hydrogen ions in mol dm^–3)

pH should also always be given to two decimal places

How can H⁺ ion concentration be calculated using pH?

[H⁺] = 10^-pH

For strong monoprotic acids this is useful to calculate as the H⁺ concentration is the same as the acid concentration

Calculate the pH of 0.500 mol dm^-3 HNO3

[H⁺] = 0.500

pH = -log 0.500

pH = 0.30

Calculate the concentration of HCl with pH 1.70?

[H+] = 10^-1.70 = 0.0200

[HCl] = 0.0200 mol dm^-3

Calculate the pH of the solution formed when 100 cm³ of water is added to 50 cm³ of 0.100 mol dm^-3 HNO₃?

[H⁺] in original HNO3 solution = 0.100 dm³

[H⁺] in diluted solution = 0.100 x (old volume / new volume) = 0.100 x (50/ 150) = 0.0333

pH = -log 0.0333 = 1.48

Why is it harder to calculate the pH of weak acids?

Because they don’t fully dissociate so the H⁺ concentration isn’t the same as the whole concentration of the acid.

How can we calculate the pH of weak acids?

Using the