Acids, Bases & Buffers

Define Bronsted Lowry acid

Proton (H+) doner

Define Bronsted Lowry base

Proton (H+) acceptor

What occurs in a Bronsted Lowry acid-base reaction?

Proton is transferred from acid to base

Define strong acid with 3 examples

• acid that completely dissociates

• HCl, H2SO4, HNO3

Write the expression for the ionic product of water (kw) and its units

Kw=[H+][OH]

Mol2dm-6

Why does kw value change with temperature?

Dissociation of water is endothermic, increasing temp shifts equilibrium to the right to produce more H+ and OH-

Why is pure water neutral?

[H+]=[OH-]

Write the ionic product of water for pure water

Kw=[H+]²

What happens to [H+] when [OH-] increases?

[H+] will decrease proportionally

Acidic solution in terms of H and OH?

[H+]>[OH-]

Alkaline solution in terms of H and OH?

[OH]>[H+]

Define pH

pH=-log[H+]

Useful re-arrangement of pH?

[H+]=10^-pH

Define monoprotic acid

One mole of acid dissociates to give one mole of H+

Define diprotic acid

One mole of acid dissociates to give two moles of H+

Define weak acid

Acid that partially ionises/dissociates

Write an expression for the acid dissociation constant for the weak acid HA

Ka=[H+][A]/[HA]

What are the units for Ka?

Moldm-3

Write the simplified expression for Ka for a weak acid with nothing added

Ka=[H+]²/[HA]

State the two assumptions used to simplify the Ka expression for a weak acid with nothing added

• [H+]=[A-]

• [HA] does not alter significantly after dissociation

Define pKa

pKa=-logKa

Useful re-arrangement of pKa?

Ka=10^-pKa

Stronger the weak acid the ___ the value of Ka and the ___ the value of pKa

1. Higher

2. Lower

Describe a method to be used to continuously measure the pH change of a solution during ma titration

1. Use a pH probe and reader and place it in the conical flask

2. Add acid/base from a burette in small intervals and record the pH reading

Strong/weak acid/base titration curves

Define equivalence point in a titration. Where is it found on the titration curve?

When moles of H+ = moles of OH- in the conical flask. Found at central point of the vertical section on the graph

Define end point of titration

Point where indicator changes colour

How can you find a suitable indicator for an acid-base titration using a titration curve?

pH range over which indicator changes colour lies within pH range of vertical section of the titration curve

Where is half neutralisation point on a pH curve and why is it useful?

• Half the volume needed to reach the equivalence point

• pKa of weak acid= pH of solution in conical flask

Define buffer solution

Solution that minimises changes to pH upon the addition of small amounts of acid/alkali

State 2 methods of producing an acidic buffer

1. Mix weak acid with salt of weak acid

2. Mix an excess of weak acid with a strong base

What happens to a buffer solution when a little acid is added? (Terms of HA and A-)

[A-] decreases and [HA] increases

[HA]/[A-]»[H+]

What happens to a buffer solution when a little base is added? (Terms of HA and A-)

[HA] decreases and [A-] increases

[HA]/[A-]»[H+]

General overall equation for reaction of a strong acid with strong base?

H+ + OH- —>H2O

General overall equation for the reaction of a weak acid with a strong base?

HA + OH- —> A- + H2O