Acids Taste sour Can be weak (ethanoic acid, citric acid) Can be - - PowerPoint PPT Presentation

Acids

• Taste sour
• Can be weak (ethanoic acid, citric

acid)

• Can be strong (Hydrochloric, nitric,

sulphuric)

• Release H+ ions into solution

Hydrochloric acid is hydrogen chloride gas dissolved in water: HCl(g) H+(aq) + Cl-(aq)

Basically, its an alkali

• Bases (including alkalis) neutralise acids, e.g. metal
• xides, metal hydroxides
• Insoluble in water
• Soluble hydroxides are called alkalis
• Form OH- ions when dissolved in water

Sodium hydroxide is an alkali formed by dissolving solid sodium hydroxide in water NaOH(s) Na+(aq) + OH-(aq)

Ionic Equation for Neutralisation

H+(aq) + OH-(aq) H2O(l)

The pH scale

• Indicators are chemicals that change colour when added to acids or alkalis
• E.g. litmus – red in acids, blue in alkalis
• Universal indicator has a range of colours to show how acidic or alkaline a solution is.

Measuring pH

• pH meter – digital display – automatic readings – more accurate –

monitor over time (attached to data logger/computer)

Examples of lab acids and alkalis

• pH meters must be calibrated before use, and rinsed in de-ionised

water in between readings.

Solution pH Colour with universal Indicator Acidic, alkaline or neutral?

0.1 mol/dm3 hydrochloric acid 1 red strongly acidic 0.01 mol/dm3 hydrochloric acid 2 red strongly acidic 0.001 mol/dm3 hydrochloric acid 3

• range

weakly acidic 0.1 mol/dm3 nitric acid 1 red strongly acidic 0.1 mol/dm3 sulfuric acid 0.7 red strongly acidic 0.1 mol/dm3 ethanoic acid 3

• range

weakly acidic 0.1 mol/dm3 citric acid 4 yellow weakly acidic distilled water 7 green neutral 0.1 mol/dm3 sodium chloride solution 7 green neutral 0.1 mol/dm3 sodium hydroxide 13 purple strongly alkaline

Neutralisation curve

Strong vs Weak

Strong acids

• Hydrochloric, nitric, sulphuric
• React quickly
• Can be concentrated or dilute
• Low pH (0,1)
• Ionise completely - all H+ ions go

into solution HCl(aq) H+(aq) + Cl-(aq)

• Higher concentration of H+ ions

Weak acids

• Ethanoic, citric, carbonic
• React slower
• Can be concentrated or dilute
• Higher pH (2-6)
• Only some molecules ionise

(reversible reaction) – reaches equilibrium CH3COOH ⇌ CH3COO– + H+

• Lower concentration of H+ ions

pH Values

• Reducing the concentration of H+ ions by a factor of 10 increases pH by 1

Concentration of H+ ions (mol/dm3) Concentration of H+ ions (mol/dm3) pH Value 0.10 1 x 10-1 1.0 0.010 1 x 10-2 2.0 0.0010 1 x 10-3 3.0 0.00010 1 x 10-4 4.0

Salts

• When acids are used in

reactions, we replace the hydrogen with a metal particle.

• The new compound is called a

salt.

• Salts form crystals of different

colours and shapes

Naming Salts

• Name comes from the

metal and the acid

• The metal replaces the

hydrogen ions in the acid

• Hydrochloric acid = chloride
• Sulphuric acid = sulphate
• Nitric acid = nitrate

Sodium hydroxide + hydrochloric acid _________ + _______

Challenge – symbol equations

• Hydrogen = H2
• Hydrochloric acid = HCl
• Sulphuric acid = H2SO4
• Nitric acid = HNO3