AQA A Level Chemistry 3.1.12 - Acids and Bases

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Calculating [H⁺]

-pH [H⁺] = 10

Calculating Ka from pKa

-pKa Ka = 10

How is a pH titration carried out?

1. Calibration curve of pH meter and electrode using buffer solutions. 2. Prepare graph of pH (y-axis) against volume of alkali added (x-axis). 3. Pipette 25cm³ 0.1M acid into 250cm³ beaker. Measure pH and record as initial pH. 4. Add 0.1M alkali from burette in appropriate portions, measuring and plotting pH after each addition. 5. Repeat up to 50cm³. 6. Plot curve of best fit.

Low pH means

Acidity

High pH means

Alkalinity

Buffer region

Around pa where pH changes very little, despite acid/base being constantly added.

An indicator colour change must...

Be rapid Occur at exactly the same time as the equivalence point.

Strong acid

Fully dissociates in water.

Dissociation of water

H₂O ⇌ H⁺/H₃O⁺ + OH⁻

Oxonium ion

H₃O⁺

Ka weak acid dissociation constant

Ka = [H⁺] [A⁻] [HA]

The ionic product of water (at 298K)

Kw = [H⁺] [OH⁻] = 1.0 x 10⁻¹⁴

pH

Measure of acidity by measuring concentration of H⁺ ions.

Equivalence volume/endpoint

Mi-point of "vertical section".

How can an acidic buffer be made?

Mix HA with Na⁺A⁻. Part-neutralise HA with NaOH.

How can a basic buffer be made?

Mix weak base with its salt (e.g. NH₃ with NH₄Cl). Part-neutralise weak base with HCl.

When H⁺ added to basic buffer...

OH⁻ + H⁺ → H₂O Unbalances equilibrium. Higher concentration of H₂O. Le Chatelier's principle. NH₃ + H₂O → NH₄⁺ + OH⁻

When OH⁻ added to acidic buffer...

OH⁻ + H⁺ → H₂O Unbalances equilibrium. Lower concentration of H⁺. Le Chatelier's principle. HA → H⁺ + A⁻

Weak acid

Only very slightly dissociates in water.

What colour changes does phenolphthalein show?

Pink - pH10 Pale pink - pH9 Colourless - pH8

Bronsted-Lowry base

Proton acceptor

Bronsted-Lowry acid

Proton donor

Buffer

Solution that maintains an almost constant pH on addition of small amounts of strong acid or strong base.

The more a weak acid dissociates...

The more H⁺ are formed. The stronger the acid. The larger the Ka value.

What does an acid/base reaction involve?

Transfer of the H⁺ from the acid to the base.

When H⁺ added to acidic buffer...

Unbalances equilibrium. Higher concentration of H⁺. Le Chatelier's principle. H⁺ + A⁻ → HA

When OH⁻ added to basic buffer...

Unbalances equilibrium. Higher concentration of OH⁻. Le Chatelier's principle. NH₄⁺ + OH⁻ → NH₃ + H₂O1.

When water boils what happens to it's pH?

Water dissociating is endothermic, and the reverse reaction is exothermic. Le Chatelier's principle. Equilibrium shifts to right to lower temperature. More H⁺ so pH lower (more acidic).

What does an acidic buffer consist of?

Weak acid (HA) with its salt (Na⁺A⁻)

Acid-base indicators

Weak acids. Conjugate acid and base forms have different colours.

What does a basic buffer consist of?

Weak base with its salt.

When can you NOT use methyl orange?

With a weak acid.

When can you NOT use phenolphthalein?

With a weak base.

What colour changes does methyl orange show?

Yellow - pH5 Orange - pH4 Red - pH3

At half equivalence...

[A⁻] = [HA] Ka = [H⁺] pKa = pH

Calculating pH of a base

[H⁺] = Kw [OH⁻]

Calculating pH

pH = -log [H⁺]

Calculating pKa

pKa = -log (Ka)


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