OCR A Level Chemistry

Revision Notes

5.3.1 Acids & Bases

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Brønsted-Lowry Acids & Bases Theory

  • A Brønsted acid is a species that can donate a proton
    • For example, hydrogen chloride (HCl) is a Brønsted acid as it can lose a proton to form a hydrogen (H+) and chloride (Cl-) ion

            HCl (aq) → H+ (aq) + Cl- (aq)

  • A Brønsted base is a species that can accept a proton
    • For example, a hydroxide (OH-) ion is a Brønsted base as it can accept a proton to form water

   OH- (aq) + H+ (aq) → H2O (l)

Weak acids dissociating

  • In an equilibrium reaction, the products are formed at the same rate as the reactants are used
  • This means that at equilibrium, both reactants and products are present in the solution
  • For example, ethanoic acid (CH3COOH) is a weak acid that partially dissociates in solution
  • When equilibrium is established there are CH3COOH, H2O, CH3COO- and H3O+ ions present in the solution
  • The species that can donate a proton are acids and the species that can accept a proton are bases

         CH3COOH (aq)   +  H2O (l)    ⇌  CH3COO- (aq)     +   H3O+ (aq)

                                    acid                   base            conjugate base      conjugate acid 

  • The reactant CH3COOH is linked to the product CH3COO- by the transfer of a proton from the acid (CH3COOH) to the base (CH3COO-)
  • Similarly, the H2O molecule is linked to H3O+ ion by the transfer of a proton
  • These pairs are therefore called conjugate acid-base pairs
  • A conjugate acid-base pair is two species that are different from each other by an H+ ion
    • Conjugate here means related
    • In other words, the acid and base are related to each other by one proton difference

Monobasic, dibasic & tribasic acids

  • Acids can be classified by the number of bases that they can donate protons to in a reaction, which depends on how many H+ per molecule that they can give up in a reaction
  • Acids such as HCl, HNO3, and HCN that contain one ionisable hydrogen atom in each molecule are called monobasic (or monoprotic) acids
    • When HCl reacts with NaOH we can see that one hydrogen is replaced by a sodium atom

HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (aq) 

  • Dibasic (or diprotic) acids contain two ionisable hydrogen atoms per molecule, for example H2SO4
    • Ionisation of such acids occurs in two steps
    • When H2SOreacts with NaOH we can see that two hydrogens are replaced by two sodium atoms

H2SO4 (aq) + 2NaOH (aq) → Na2SO4 (aq) + 2H2O (aq) 

  • Tribasic (or triprotic) acids contain three ionisable hydrogen atoms per molecule, for example H3PO4
    • Ionisation of such acids occurs in three steps
    • When H3POreacts with NaOH we can see that three hydrogens are replaced by three sodium atoms

H3PO(aq) + 3NaOH (aq) → Na3PO4 (aq) + 3H2O (aq) 

The Role of Hydrogen Ions in Equations

  • As we have seen previously, metals, alkalis, metal oxides and metal carbonates react with acids to form salts
  • We can represent the active species, H(aq), by writing ionic equations
  • Once we have written all the ions in the reaction, we can then cancel them out

Acids and metals

  • The typical reaction of a metal and an acid can be summarised as

acid + metal    salt + hydrogen

  • For example:

2HCl (aq) + Zn (s)   ZnCl2 (aq) +   H2 (g)

Becomes 

2H+ (aq) + 2Cl- (aq) + Zn (s)  Zn2+ (aq) + 2Cl- (aq) + H2 (g)

2H+ (aq) + Zn (s)  Zn2+ (aq) + H2 (g)

Acids and metal oxides 

  • The reaction of an acid with a metal oxide forms two products:

acid + metal oxide   salt + water

  • For example:

2HCl (aq) + CaO (s)  CaCl2 (aq) + H2O (l)

Becomes 

2H+ (aq) + 2Cl- (aq) + CaO (s)  Ca2+ (aq) + 2Cl- (aq) + H2O (l)

2H+ (aq) +  CaO(s)  Ca2+ (aq) + H2O (l)

Metals and carbonates

  • The reaction between a metal carbonate and an acid produces three products:

acid + metal carbonate salt + water + carbon dioxide

  • For example:

    2HNO3 (aq) + CuCO3 (s) → Cu(NO3)2 (aq) + H2O (l) + CO2 (g)

Becomes

2H+ (aq) + 2NO3- (aq) + CuCO3 (s)  Cu2+ (aq) + 2NO3- (aq) + H2O (l) + CO2 (g)

2H+ (aq) + CuCO3 (s) → Cu2+ (aq) + H2O (l) + CO2 (g)

  • If the carbonate is soluble, e.g. Na2CO3 

2H+ (aq) + 2NO3- (aq) + 2Na+ (aq) + CO32– (aq) → 2Na+ (aq) + 2NO3- (aq) + H2O (l) + CO2 (g)

2H+ (aq) + CO32– (aq)  H2O (l) + CO2 (g)

Acids and alkalis

acid + alkali   salt + water

  • For example:

HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l)

Becomes 

H+ (aq) + Cl- (aq) + Na+ (aq) + OH- (aq) Na+ (aq) + Cl- (aq) + H2O (l)

H+ (aq) + OH- (aq)  H2O (l)

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Philippa

Author: Philippa

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.