Acids and Bases

Last updated Jun 23, 2023 Edit Source

# ← Chemistry Home

• Check out Pearson Chapter 17
• Acids and bases are important substances used in agriculture and industry with many diverse roles
  • Acids and bases interact in many environmental and biological systems undergoing many different chemical reactions

Soluble bases are known as Alkali

• Acids and bases have the ability to change the colour of certain plant extracts which are then used as indicators to reveal the acidity of a substance
  • Litmus is a purple dye obtained from lichen used as an indicator for acids and bases

• The acidity of an aqueous substance is measured using a pH value
  • The pH value is dependent on the number of hydrogen ions in the solution
  • Acids have a low pH value, bases have a high pH value
• Universal indicator is widely used to estimate pH of a solution
  • It is an estimate → not super accurate
  • Used in conjunction with a pH colour scale
  • Colour goes from red to violet (acid to base)
• pH is calculated as: $pH$ $= -log_{10}[H^+]$
  • $H^+$ is the concentration of $H^+$ in mol L$^{-1}$
  • pH is measured on a log scale → a pH of 5 is 10 times more acidic than a pH of 6
  • pH values are usually given to two decimal places
• The pH of an acid depends on both its strength and concentration

• $pH$ is an indicator
• $pH$ $= -log_{10}[H^+]$
  • $pH$ does not have units
• To find the concentration of $H^+$ using $pH$ → $10^{-pH} = [H^+]$
  • Measured in mol L$^{-1}$
• Using the above calculations we can find information about $pH$ when dealing with acids, however when dealing with bases (Bronsted-Lowry bases) we are dealing with the concentration of $OH^-$, not $H^+$
  • However we can use the concentration of $OH^-$ to calculate the concentration of $H^+$
    • This is because water can act as a weak Bronsted-Lowry acid or base
      • $H_2O + H_2O \rightleftharpoons H_3O^+{(aq)}\\ +\\ OH^-{(aq)} \rightleftharpoons H^+{(aq)} + OH^-{(aq)}$
      • Water is a proton donor and acceptor

$K_w$ → Ionisation Constant of Water The ionisation constant at 25$\degree$C → $K_w = 1\\ ×\\ 10^{-14}$

$K_w = [H^+][OH^-] = 1\\ ×\\ 10^{-14}$

• Concentration in mol $L^{-1}$
• $K_w$ is written without units

Using the concentration of $H^+$ or $OH^-$ we can find the concentration of the other ion and thus convert between $pH$, moles and concentration!

• When $pH$ is neutral, the concentration of $H^+$ and $OH^-$ are equal

• # Properties of Acids

  • Turns litmus paper red
  • Usually corrosive
  • Taste sour
  • React with bases
  • Solutions have a pH less than 7
  • Solutions conduct an electric current

• # Properties of Bases

  • Turn litmus paper blue
  • Caustic and slippery
  • Taste bitter
  • React with acids
  • Solutions have a pH greater than 7
  • Solutions conduct an electric current

• Acids and bases are difficult to define and have several different definitions
  • The definitions for these substances have changed overtime
  • Arrhenius, Bronsted-Lowry, Davy and Lewis are all different definitions

• An Arrhenius acid is any compound that increases the concentration of H$^+$ in aqueous solution
• Classifying Arrhenius Acids
  • Polyprotic acids
    • Some acids can form multiple hydrogen ions per molecule in water, these acids are known as polyprotic acids
    • The number of hydrogen ions an acid can donate depends on the structure of the molecule
      • This can be difficult to tell as ethanoic acid ($CH_3COOH$) is monoprotic and only forms one hydrogen ion in water
    • Diprotic and triprotic acids are also polyprotic acids
  • Diprotic Acids
    • Diprotic acids can donate two protons
    • They can form up to two hydrogen ions per molecule in water
    • Sulfuric acid ($H_2SO_4$) and carbonic acid ($H_2CO_3$) are diprotic acids
    • Diprotic acids usually ionise in two stages, however the Arrhenius model represents them as ionising at once
      • e.g. $H_2SO_4 (l) → SO_4^{2-} (aq) + 2H^+ (aq)$
  • Triprotic Acids
    • Triprotic acids can donate three protons
    • They can form up to three hydrogen ions per molecule in water
    • Phosphoric acid ($H_3PO_4$) and boric acid ($H_3BO_3$) are triprotic acids
    • Triprotic acids usually ionise in multiple stages, however the Arrhenius model represents them as ionising at once
      • e.g. $H_3PO_4 (l) \rightleftharpoons PO_4^{3-} (aq) + 3H^+ (aq)$
        • Note the double arrow because phosphoric acid is a weak acid
• An Arrhenius base is any compound that disassociated in water forming hydroxide ions increasing the concentration of OH$^-$ in aqueous solution
• Limitations:
  • There are many acids and bases that are not soluble

• Acids are defined as substances that contain replaceable hydrogen
  • Hydrogen could be replaced partially or completely with metals
  • When acids reacted with metals or bases, they produced salts
• This is flawed as many compounds contain hydrogen and are not acids

• An acid is a proton donor
  • HCl + H$_2$O → H$_3$O$^-$ + Cl$^+$
    • H$_3$O$^-$ - hydronium ion
    • Water can accept or donate protons; it can act as an acid or a base
• A base is a proton acceptor

• Acids and bases of the same concentration in controlled conditions do not ionise at the same rate
  • We describe the rate/amount an acid or base ionises as its ‘strength’

• The strength of an acid refers to the amount of acid that ionises in solution
• Strong acids dissociate fully
  • e.g. $HCl,\\ H_2SO_4,\\ HNO_3,$ etc.
  • $HCl → Cl^-{(aq)} +\\ H_3O^+{(aq)}$
• Weak acids dissociate partially
  • e.g. $CH_3COOH$
  • $CH_3COOH \rightleftharpoons CH_3COO_{(aq)}^- + H_3O_{(aq)}^+$

• The strength of a base refers to the amount of base that ionises in solution
• Strong bases dissociate fully
  • e.g. $NaOH,\\ KOH,\\ Ca(OH)_2$, etc.
  • $NaOH → Na^+{(aq)} + OH^-{(aq)}$
• Weak bases dissociate partially
  • e.g. $NH_3$
  • $NH_3 \rightleftharpoons NH_{4\\ (aq)}^+ + OH_{(aq)}^-$

• Salt is a substance that forms when hydrogen in an acid is substituted by metal ion
  • e.g. $HCl$ (acid) → $NaCl$ (salt)
    • Hydrogen is substituted by sodium
  • General equation: $H$A + M$OH$ → MA + $H_2O$
    • A → Anion, M → Metal Ion
• When Arrhenius acids and bases interact, they undergo a neutralisation reaction
  • This produces water and a salt
  • Acid + Base → Salt + $H_2O$