Physical 2 Section 1 - Unit 21: Acid Base Equilibria
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Equation for pH (1 mark)
pH = -log [H+]
Explain what is meant by a buffer solution (2 marks)
• Solution which resists change in pH /maintains pH - Despite the addition of (small amounts of) acid/base
If an acid is added to a buffer solution, what happens to the moles of reactants in buffer solution?
• Moles of acid increases - Moles of salt decreases
Explain how a mixture of solution can act as a buffer when small amounts of acid or alkali are added (3 marks)
• When acid added, there is an increase in concentration of H+ so equilibrium moves to left • When alkali is added, OH- ions react with H+ ions so equilibrium moves to right to replace H+ ions • Concentration of H+ remains constant
State why calibrating a pH meter just before it is used improves the accuracy of the pH measurement (1 mark)
• Over time / after storage, meter does not give accurate readings
Describe how you would obtain the pH curve for a titration (5 marks
• Measure pH (of the acid) • Add alkali in known small portions • Stir mixture • Measure pH (after each addition) • Repeat until alkali in excess • Add in smaller increments near endpoint
Using the concept of electronegativity, justify why the acid strengths of ethanedioic acid and ethanoic acid are different (6 marks)
Difference in structure of two acids • Acids are in form of RCOOH • But in ethanoic acid, there is a R=CH₃ • Whilst in ethanedioic acid, there’s a R=COOH Inductive effect • The unionised COOH group contains 2 very electronegative oxygen atoms • So there is a negative inductive (electron withdrawing) effect • The CH₃ group has a positive inductive (electron pushing) effect How the polarity of OH affects acid strength • The O-H bond in the ethanedioic acid becomes more polarised • And so there’s more dissociation into H+ ions • Ethanedioic acid is therefore a stronger acid than ethanoic acid
Explain what happens to the pH of water, according to Le Chatelier’s principle, when temperature is increased (3 marks)
• Dissociation of water is endothermic • So equilibrium shifts to the right to decrease temperature again • So higher [H+] produced and lower pH
What is pKa used to measure (1 mark)
• The strength of an acid
State the equation for pKa (1 mark)
pKa = -log [Ka]