IB Chem topic 8: Acids and Bases

Brønsted Lowry theory of acids and bases

Brønsted Lowry theory of acids and bases

• an acid is a proton donor or any substance that donates a hydrogen ion —must contain hydrogen

• a base is a proton acceptor or any substance that accepts hydrogen ions — therefore must have a lone pair of electrons

• acids only exhibit acid behaviour (donate protons) if a proton acceptor is present

Amphoteric

can act as both an acid and a base (a general term)

Amphiprotic

• a proton-bearing molecule that is capable of donating its protons as well as accepting additional hydrogen ions

• must have both a hydrogen and a lone pair

Amphoteric vs Amphiprotic

• Amphoteric is a more general term

• amphiprotic is specific to Brønsted Lowry definition

• aluminum oxide is amphoteric (can act as acid or base) but is not amphiprotic (its acid/base reactions don’t involve transfer of H⁺ ions)

Examples of amphiprotic species

HSO₄^- , H₂PO₄^- , HPO₄^2- , H₂O

Alkalis

• soluble bases

• produce OH^- ions when dissolved in water

• eg. K₂O_(s)+H₂O_(l) → 2K⁺_(aq)+2OH^-_(aq)

Salt

• Ionic compounds formed when the hydrogen of an acid is replaced by a cation

• Have a parent acid and parent base

Acid/Base Reactions to know (hint: there are 3)

Acid + Metal → Salt + Hydrogen

Acid + Base → Salt + Water

Acid + Carbonate → Salt + Water + Carbon Dioxide

Conjugate pairs

• an acid’s conjugate base forms when the acid loses an H⁺ ion

• a base’s conjugate acid forms when it gains an H⁺ ion

• occurs in equilibrium, conjugate pairs differ by a single proton

Formula for pH

pH = -log₁₀[H⁺]

Formula for hydrogen ion concentration from pH

[H⁺] = 10^-pH

pH probe

reads [H⁺] through a specialized electrode, must be calibrated with a buffer solution and be standardised for a specific temp

K_w

• ionic product constant of water

• =1.00×10¹⁴ at 298K

• =[H⁺][OH^-]

Formula for hydrogen ion concentration from K_w

[H⁺]=√(K_w)

Strong acids and bases

• ionize completely into their ions in aq solutions

• Strong acids are good proton donors and their conjugate bases are bad proton acceptors

• Strong bases are good proton acceptors and their conjugate acids are bad proton donors

Weak acids and bases

• partly ionizes in aqueous solutions, leading to an equilibrium mixture in which the undissociated (reactant side) form dominates the equilibrium

• (ie. eqm will lie to the left)

Strong acids examples

HCl, HNO₃, H₂SO₄

Weak acids examples

CH₃COOH, H₂CO₃, H₃PO₄

Strong bases examples

LiOH, NaOH, KOH, Ba(OH)₂ (ie. group 1 hydroxides)

Weak bases examples

NH₃,C₂H₅NH₂ (ethylamine)

Methods of determining the strength of acids and bases

• Conductivity: electrical conductivity depends on [mobile ions] so strong acids have a higher conductivity

• Rate of reaction: depends on [H⁺], so strong acids have a faster RoR

• pH: higher the [H⁺], lower the pH

  • Can be used to compare the strenth of acids if they are of the same molar concentration (A dilute strong acid and concentrated weak acid may appear to have the same pH

Acid Deposition

All processes by which acidic components, as precipitates or gases, leave the atmosphere

Rain water pH

5.6, slightly acidic due to the presence of dissolved carbon (carbonic acid)

Contribution of sulfur to acid rain

• SO₂ produced is produced from burning fossil fuels and smelting

• SO₂ can be further oxidized to SO₃

• Dissolves in H₂O to from acid

  • H₂O_(l) + SO_2(g)→ H₂SO₃ (sulfurous acid)

  • H₂O_(l) + SO_3(g)→ H₂SO₄ (sulfuric acid)

Contribution of nitrogen to acid rain

• NO internal combustion engines, then can be oxidized to NO₂

• Nitrogen dioxide dissolves in water to form a mixture of nitrous (HNO₂) and nitric (HNO₃) acid

  • nitrous oxide often oxidizes to nitric acid

Wet deposition vs Dry deposition

• Wet deposition involves acid rain (liquids)

• Dry deposition can occur due to gasses falling to the ground as dust and smoke

Impact of acid deposition on materials

• Marble and limestone buildings (CaCO₃) eroded

  • CaCO₃ + H₂SO₄ → CaSO₄ +CO₂ + H₂O

• Metals can be corroded

  • Fe + H₂SO₄ → FeSO₄ + H₂

• Can remove protective coating on metals

Impact of acid deposition on plants

• slower growth, injury, death

• causes some minerals (Mg²⁺, Ca²⁺, K⁺) to become soluble in soil and wash away by leeching

Impact of acid deposition on water

• has caused lakes to die - unable to support life

• fish like trout/perch cannot survive in pH <5

• can cause eutrophication

Impact of acid deposition on human health

• indirect, can react to form fine sulphate/nitrate particles which can cause respiratory issues

Methods of reducing SO₂ emissions

• Hydrodesulfurization - catalytic process that removes sulphur from refined petroleum by reacting it with H₂ (PRE-COMBUSTION METHOD)

• Flue-gas desulphurization - removes 90% SO₂ from flue gas in smoke stacks (POST-COMBUSTION METHOD)

Reduction of NO_X emissions

• catalytic converter in vehicles (coverts CO and NO to CO₂ and N₂)

• Lower temp of combustion — formation of nitrogen monoxide is reduced at lower temps