Strong Acids & Weak Acids

• Acids produce protons in water
• Acids ionise in aqueous solutions - they produce hydrogen ions, H+:

  HCl -> H+ + Cl-

  HNO3 -> H+ + NO3-

  These acids don't produce hydrogen until they meet water. So, e.g. hydrogen chloride gas isn't acidic
• An H+ ion is just a proton
• Acids can be strong or weak
• Strong acids (e.g. sulfuric, hydrochloric and nitric acids) ionise completely in water. All acid particles dissociate to release H+ ions
• Weak acids (e.g. ethanoic, citric and carbonic acids) do not fully ionise in solution. Only a small portion of acid particles dissociate to release H+ ions
• The ionisation of a weak acid is a reversible reaction, which sets up an equilibrium between the undissociated and dissociated acid. Since only a few of the acid particles release H+ ions, the position of equilibrium lies well to the left

  Strong acid: HCl -> H+ + Cl-

  Weak acid: CH3COOH ⇌ H+ + CH3COO-
• Reactions of acids involve the H+ ions reacting with other substances. If the concentration of H+ ions is higher, the rate of reaction will be faster, so strong acids will be more reactive than weak acids of the same concentration
• pH is a measure of the concentration of H+ ions
• The pH of an acid or alkali is a measure of the H+ ions in the solution
• For every decrease of 1 on the pH scale, the concentration of H+ ions increases by a factor of 10
• An acid that has a pH of 4 has 10 times the concentration of H+ ions of an acid that has a pH of 5
• For a decrease of 2 on the pH scale, the concentration of H+ ions increases by a factor of 100. The general rule is:

  Factor H+ ion concentration changes by = 10^-x
• In the general rule: Factor H+ ion concentration changes by = 10^-x

  • X is the difference in pH. So if pH falls from 7 to 4 the difference is -3, and the factor the H+ ion concentration has increased by is 10^-(-3) = 10^3
• Acid strengh tells you what proportion of the acid molecules ionise in water
• The concentration of an acid is different to the acid strength. Concentration measures how much acid there is in a certain volume of water. Concentration is basically how watered down the acid is
• The larger the amount of acid there is in a certain volume of liquid, the more concentrated the acid is
• You can have a dilute (not very concentrated) but strong acid, or a concentrated weak acid
• pH will decrease with increasing acid concentration regardless of whether it's a strong or weak acid