Acids - Summary

Acids

Disciplinary Ideas

• Acids contain H atoms that release H+ ions in water.
• Acid-base reactions involve the transfer of H+ ions from an acid to a base.

Learning Objectives

• Define acids in terms of ions produced in aqueous solutions and their effect on Universal Indicator.
• Describe neutrality, relative acidity, and alkalinity:
  • Relative $H^+$ and $OH^−$ ion concentrations.
  • Color in Universal Indicator.
  • The pH scale.
• Describe characteristic properties of acids in reactions with metals, bases, and carbonates to form salts.
• Describe neutralization reaction: $H^+ + OH^− → H_2O$.
• Describe the importance of controlling pH in soils and treating excess acidity with calcium hydroxide.
• Classify oxides as acidic, basic, amphoteric, or neutral.

Examples of Acids

• Mineral acids:
  • Hydrochloric acid (HCl)
  • Nitric acid ($HNO_3$)
  • Sulfuric acid ($H<em>2SO</em>4$)
• Organic acids:
  • Citric acid ($C<em>6H</em>8O_7$)
  • Ethanoic acid ($CH_3COOH$)

Properties of Acids

• Sour taste.
• Turn blue litmus paper red; Universal Indicator orange or red.
• pH less than 7.
• Conduct electricity in aqueous solutions.
• Acids show their properties in the presence of water
• Acids produce $H^+$ when dissolved in water.

Chemical Properties of Acids

• Acids react with metals to produce salt and hydrogen gas.
• Acids react with metal carbonates to produce salt, water, and carbon dioxide gas.
• Acids react with bases (metal hydroxide / metal oxide) → salt + water

Chemical Reactions of Acids

• Acid + Metal → Salt + Hydrogen gas
  • Example: $2HCl + Mg → MgCl<em>2 + H</em>2$
  • Test for Hydrogen: Use a lighted splint; it extinguishes with a pop sound.
• Acid + Metal Carbonate → Salt + Water + Carbon Dioxide gas
  • Example: $2HNO<em>3 + Na</em>2CO<em>3 → 2NaNO</em>3 + H<em>2O + CO</em>2$
  • Test for Carbon Dioxide: Pass gas into limewater; a white precipitate forms.
• Acid + Base (Metal Hydroxide / Metal Oxide) → Salt + Water
  • Example: $H<em>2SO</em>4 + 2NaOH → Na<em>2SO</em>4 + 2H_2O$
  • Example: $H<em>2SO</em>4 + MgO → MgSO<em>4 + H</em>2O$

Uses of Acids

• Food stuffs: carbonic acid in fizzy drinks, acetic acid in vinegar, citric acid in fruit juices
• Electrolyte in car batteries: sulfuric acid
• Manufacture of soaps and detergents: sulfuric acid

Acids & the Environment

• Factories burning coal liberate sulfur dioxide gas ($SO_2$).
• Burning petrol in car engines produces oxides of nitrogen ($NO_2$).
• $SO<em>2$ and $NO</em>2$ dissolve in rainwater to form acid rain.
• Acid rain lowers the pH of water, corrodes metals and buildings, and removes essential minerals from the soil.