Chemistry IB SL Acids and Bases

Theory of acids and bases

The role of acids and bases

• Acids and bases are opposites

• Many theories describe how they work

1. Arrhenius Theory of acids and bases

   • Acids produce H⁺ ions, bases produce OH^– ions

   • Neutralization is the process of combining an acid and a base

   • Limitation: weak base ammonia and hydrogen chloride gas could not be explained, as ammonia does not contain OH^–

2. Bronsted-Lowry acids and bases

   • Refer to hydrogen ion as a proton;

   • Acid is proton donor, base is proton acceptor

Conjugate acids and bases

• In a reversible reaction between acid and bases, there are conjugates as well

Amphiprotic species

• Amphiprotic species: Substances which can be both Bronsted-Lowry acids or bases depending on the reaction

  • Examples:

Properties of acids and bases

Properties of acids and bases

Reactions of acids with metals, bases, and carbonates

• Most acids react with metals, metal oxides, hydroxides, hydrogencarbonates and carbonates

• All these reactions produce a salt which is a compound of anion and cation

• Standard enthalpy change of neutralization: the energy change associate with the formation of 1 mol of water from the reaction between a strong acid and a strong base under standard conditions

  • Negative value because neutralization is exothermic

• For acids and bases:

  

  For metals higher than hydrogen on activities series:

  

  For metal carbonates and hydrogencarbonates:

  

Acid-Base titrations

• Refer to section 22 of data booklet to see colour changes associate with indicators

The pH scale

The pH scale

• pH scale is effective way of representing concentration of hydrogen ions [H⁺] in a solution

• It is a logarithmic scale with base 10, easy way for non-scientist to understand safety of materials

• pH distinguishes between acidic, neutral and alkaline

Calculating pH

• The concentration of an acid with one proton is the same as the concentration of hydrogen ions [H⁺]

  • [HCl] = [H⁺]

• For more protons:

  • [H₂SO₄] = 2[H⁺]

Ionization of water

Strong and weak acids and bases

Strengths of acids and bases

• Strength of an acid or base depends on the degree to which it ionizes (dissociates) in water

• In the following reactions, an acid or a base reacts with water to produce conjugates

  • Strong acid is an effective proton donor that is assumed to completely dissociate in water.

    • Examples: HCl, H₂SO₄ and HNO₃

    • Conjugate base of strong acid is weak base

  • Weak acid only partially dissociates in water; it is a poor proton donor.

    • Examples: CH₃COOH and H₂CO₃

    • The dissociation of a weak acid is a reversible reaction that reaches equilibrium

    • Only small portion of acid molecules have dissociated at equilibrium

    • Conjugate base of weak acid is strong base

  • Strong base completely dissociates in water

    • Examples: KOH, NaOH (Any group 1 metal hydroxides)

    • Note: Metal hydroxides don’t act as Bronsted-Lowry bases, however in solution the hydroxide ion OH^– acts as a base

  • Weak base partially dissociates in water

    • Example: NH₃

• Words strong and concentrated, and weak and diluted have very different meanings.

  • Strong: completely dissociated into ions

  • Concentrated: a high number of moles of solute per litre of solution

  • Weak: slightly dissociated

  • Diluted: a low number of moles of solute per litre of solution

Experimental determination of the strength of acids and bases

• Techniques to compare strength of acids and bases of equal concentration

• Conductivity

  • All acids and bases dissociate in water to form ions. The conductivity of an aqueous solution depends on the concentration of ions present. Voltage is applied to different solutions (of equal concentration), and the voltage reflects on concentration of ions, and thus strength as well.

  • Strong acids and bases have higher conductivities

  • Weak acids and bases have lower conductivities

Energy changes on neutralization

• Neutralization: occurs when an acid and base react together

• This reaction is exothermic, ΔH < 0

• The enthalpy change of neutralization for a strong acid is almost identical to that of weak acid

  • For strong acid or base:

    • They are completely dissociated in water, thus only enthalpy change to consider is exothermic formation of water from H⁺ and OH^– ions

  • For weak acid or base:

    • Dissociation of weak acid or base is mildly endothermic

    • Enthalpy change for strong base-weak acid is less than strong base-strong acid

    • The weaker the acid, the more endothermic, and thus lower enthalpy change of neutralization

Acid deposition

• Acid deposition: the process by which acid-forming pollutants are deposited on the earth’s surface.

• Increased industrialization have led to rapidly increasing emission of nitrogen and sulfur dioxides which cause acid rain, most prevalent form of acid deposition

Acid rain

• Pure water has pH of 7.0. Rainwater is acidic due to presence of dissolved carbon dioxide which forms weak carbonic acid. Typical pH is 5.6

• Acid deposition is formed when nitrogen or sulfur oxides dissolve in water to form HNO₃, HNO₂, H₂SO₄ and H₂SO₃

• The sources of nitrogen and sulfur are:

  • Volcanic eruptions

  • Decomposition of vegetation

  • Combustion of fossil fuels

• Go over nitrogen and sulfur cycles which form the acids

Pre- and post-combustion technologies

Pre-combustion methods: refer to techniques used on fuels before their combustion

Post-combustion methods: focus on several complementary technologies to remove sulfur dioxide, nitrogen oxides and other gases from combustion gases before they are released into the atmosphere.

How to find parent acid

Alkalis 

Soluble bases that release an OH^- ion when they react with water: K₂O(s) + H₂O(l) → 2K⁺(aq) + 2OH^–(aq)  

• NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH^–(aq)  

• CO₃^2–(aq) + H₂O(l) ⇌ HCO₃^–(aq) + OH^–(aq)  

• HCO₃^–(aq) ⇌ CO₂(g) + OH^–(aq) 

Note: All alkalis are bases, but not all bases are alkalis 

Acid Reaction Types (p 351-353) 

Simplest: Acids react with bases to form water + a salt (an ionic compound) 

1) Acid + Metal → Salt + Hydrogen gas 

Samples:  

2) Acid + Base → Salt + Water 

Samples:  

3) Acid + Carbonate → Salt + Water + Carbon dioxide Samples: