pH and Indicators

What is Kw?

Kw = [H⁺][OH^-]

Why is Kw temperature dependent?

Its value changes when temperature changes

What ions are produced when water ionises?

H+ and OH-

In pure water what is true about [H⁺] and [OH^-]?

They are equal

What happens to [H⁺] when acid is added to water?

It increases

What happens to [OH^-] when acid is added to water?

It decreases

What happens to [OH^-] when base is added to water?

It increases

What happens to [H⁺] when base is added to water?

It decreases

Is pure water neutral at all temperatures?

Yes because [H⁺] = [OH^-]

Is pH 7 neutral at all temperatures?

No only at 25 C

What is the pH?

pH = -log₁₀[H⁺]

What is the pOH formula?

pOH = -log₁₀[OH^-]

What is pH plus pOH at 25 C?

14

What does the pH scale measure?

Acidity or alkalinity

What pH is acidic at 25 C?

Less than 7

What pH is neutral at 25 C?

7

What pH is alkaline at 25 C?

Greater than 7

Limitations of the pH scale

Does not work at extremely low concentrations, It is based on 25 C

What is a strong acid?

An acid that fully dissociates in solution

What is a strong base?

A base that fully dissociates in solution

How do you find pH of a strong acid?

Find [H⁺] from the mole ratio then use pH = -log10[H⁺]

How do you find pH of a strong base?

Find [OH^-] then pOH then pH = 14 - pOH

What does weak mean?

Does not fully dissociate in solution

What must be given for weak acid or weak base pH questions?

Ka or Kb

What does Ka show?

Extent to which a weak acid dissociates into ions

What does Kb show?

Extent to which a weak base dissociates into ions

What is the weak acid formula in the notes?

[H⁺] = sqrt(K_a[HA])

What is the weak base formula in the notes?

[OH^-] = sqrt(K_b[BOH])

What do you use when pH is given?

10^-pH = [H⁺]

What is an indicator?

A substance that changes colour depending on pH

Most indicators are what type of substance?

Weak acids

Why do indicators change colour?

Undissociated and dissociated forms have different colours

What is the indicator equilibrium?

HIn ⇌ H⁺ + In^-

In the notes what colour is HIn?

Red

In the notes what colour is In^-?

Blue

What colour appears when acid is added to HIn equilibrium?

Red

What colour appears when base is added to HIn equilibrium?

Blue

What is the range of an indicator?

The pH range over which a noticeable colour change occurs

How big a pH change is needed for visible indicator change?

At least two pH units

Which indicator can be used for strong acid strong base?

Phenolphthalein or methyl orange

Which indicator suits strong acid weak base?

Methyl orange

Which indicator suits weak acid strong base?

Phenolphthalein

Why is weak acid weak base not normally titrated?

No indicator gives a visible colour change

What colour change is methyl orange in the notes?

Red to yellow (acid to base)

What colour change is phenolphthalein in the notes?

Colourless to pink (acid to base)