Introduction to acids and bases

ie; HA (aq) → H+ (aq) + A-

(aq)

An acid is a proton donor.

Strong acids

X–

H+

X–

X–

X–

X–

X–

H+

H+

H+H+

H+

Eg; HCl (aq) → H+(aq) + Cl-

(aq)

Strong acids are fully dissociated, giving up all their protons.

Other strong acids include sulphuric and nitric, together referred to as mineral acids.

Weak acids

H+

X–

H X

H X

H X

H X

H X

H X

Weak acids are only partially dissociated.

Eg; CH3CO2H ⇌CH3CO2- + H+

The equilibrium position is to the LHS.

So most ethanoic acid is in the undissociated form

Organic acids, based on C/H, are generally all weak acids.

Bases

• A Base is a proton acceptor.

• B(aq) + H+(aq) → BH+

(aq)

• Strong bases completely react

• Eg NaOH(aq) → Na+(aq) + OH-

(aq)

• OH-(aq)

+ H+(aq) → H2O(l)

• Weak bases partially react.

• NH3 (aq) + H+(aq) ⇌ + HNH3 (aq)

pH

Neutral is set at pH 7

Acids have a pH below 7

Alkalis have a pH above 7

Investigating pH

• You are provided with three boiling tubes. A has 10mls of HCl, B 10mls of NaOH and C 10 mls of distilled water.

• Add a few drops of Universal Indicator to each tube. Note the colour and use the charts to determine the pH of each.

Typical reactions of acids1) With metals

Acids react with many metals to give hydrogen and a salt.

The gas is seen as effervesence.

Eg; HCl + Na→ ½H2 + NaCl

Reactions of acids with metals

• Refill your tube of acid.

• Using tweezers carefully add one of the small pieces of calcium provided to the acid.

• Replace the bung.

• When the reaction has finished take a lighted splint and test the gas.

• Note any change.

Salts

• Salts are compounds where the H+ of an acid have been replaced by a metal or an ammonium ion.

• Eg; Sulphates are formed from sulphuric acid.

• Chlorides from hydrochloric acid.

• Nitrates from nitric acid.

• Phosphates from phosphoric acid

2) With bases/alkalis

Acids neutralise alkalis to produce a salt and water

Eg; HCl +NaOH↓

H2O + NaCl

Eg; Sulphuric acid is neutralised by sodium hydroxide.

SO42–

SO42–SO4

2–

SO42–

SO42–

Na+ Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Demonstrating neutralisation

• Use a teat pipette to gradually add the alkali NaOH from tube B into the acid in tube A.

• Note what happens to the pH.

• Continue until there is no further change.

• Refill tube A with acid, add a few drops of indicator then repeat the experiment adding solid magnesium oxide in place of the alkali.

3) Reactions with carbonates

Acids react with carbonates to give a salt, carbon dioxide and water.

Gas is visible as effervesence.

2HCl + CaCO3 → CaCl2 +CO2 + H2O

Eg; Reaction of sulphuric acid and sodium carbonate.

SO42–

SO42–SO4

2–

SO42–

SO42–

Na+ Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Reactions of acids with carbonates

• Refill you tube of acid, then using a teat piptte add it to the solution of sodium carbonate in tube D.

• Make sure you put the bung in.

• When the reaction has stopped take some of the gas using a teat pipette and bubble it through the lime water solution in tube E.

• Repeat, but this time test the gas produced with a lighted splint.