5.1.3 Acids, Bases and Buffers: Module 5: Physical Chemistry & Transition Elements: Chemistry OCR A A Level

Define a Bronsted-Lowry acid.

Proton donor.

Define a Bronsted-Lowry base.

Proton acceptor.

Define Lewis base.

Electron pair donor.

Define Lewis acid.

Electron pair acceptor.

What ion causes a solution to be alkaline?

-OH (hydroxide ion)

Write an equation for the ionisation of water.

2H2O (l) ⇌ H3O + (aq) + -OH (aq)

OR H2O (l) ⇌ H + (aq) + -OH (aq)

Give example of a monobasic acid.

HCl

Give example of a dibasic acid.

H2SO4

Give example of a tribasic acid.

H3PO4

Define strong acid.

Acids dissociate completely.

Give some examples of strong acids.

● Hydrochloric acid

● Sulfuric acid

● Nitric acid

What is the difference between concentrated and strong?

Concentrated means many mol per dm3, strong refers to amount of dissociation.

Define weak acids.

Acids that only partially dissociate.

Give some examples of weak acid.

Methanoic acid, any organic acid.

What is constant that is used to measure the extent of acid dissociation called?

Acid dissociation constant.

What is the symbol of acid dissociation constant?

Ka

What does a larger Ka value mean?

Larger the Ka - greater the extent of dissociation.

Write the equation used to convert Ka into pKa

p•Ka = -log10•Ka

What is the relationship between pKa and strength of the acid?

Smaller the pKa stronger the acid.

Write the equation used to convert concentration of H + into pH.

pH = -log[H+]

Write the equation used to convert pH into concentration of H+.

[H+] = 10^-pH

Why is a pH scale usefulcompared to concentration of H+?

pH scale allows a wide range of H + concentration to be expressed as simple positive values.

What is the relationship between pH and [H+]?

High pH value means a small [H+].

If two solutions have a pH difference of 1, what is the difference in [H+]?

A factor of 10.

[H+] of a strong acid is equal to what?

[H+] = [HA]

What is the assumption made when calculating pH of weak acids?

It is assumed that the concentration of acid at equilibrium is equal to the concentration of acid after dissociation. This is because only very little of the acid dissociates

Write the expression for ionic product of water, Kw.

Kw = [H+][OH-]

What is the units for Kw?

mol^2•dm^-6

What is the value of Kw at 298 K?

1.0 x 10^-14

What physical factors affect the value of Kw? How do they affect it?

Temperature only - if temperature is increased, the equilibrium moves to the right so Kw increases and the pH of pure water decreases.

Indices of of [H + ] and [OH-] always adds up to what value?

-14

Define the term strong base.

Base that dissociates 100% in water.

Give examples of some strong bases.

NaOH

KOH

Ca(OH) 2

Give example of a weak base.

Ammonia

Write the equation used to calculate [H+] of strong bases.

[H+] = Kw / [OH- ]

Define a buffer solution.

A mixture that minimises pH change on addition of small amounts of an acid or a base.

What are the 2 ways in which buffers can be made?

● Weak acid and its conjugate base.

● Weak acid and a strong alkali.

In which direction does the equilibrium shift when an acid is added to a buffer solution? Why?

Equilibrium shifts to the left because [H+] increases and the conjugate base reacts with the H+ to remove most of the H+.

In which direction does the equilibrium shift when an alkali is added to a buffer solution? Why?

Equilibrium shifts to the right, because [OH-] increases and the small concentration of H+ reacts with OH-. To restore the H+ ions HA dissociates shifting the equilibrium.

What is a titration?

The addition of an acid/base of known concentration to a base/acid to determine the concentration. An indicator is used to show that neutralisation has occurred, as is a pH meter.

Define the term equivalence point.

The point at which the exact volume of base has been added to just neutralise the acid, or vice-versa.

What is the end point?

The point at which pH changes rapidly.

What are the properties of a good indicator for a reaction?

Sharp colour change (not gradual) - no more than one drop of acid/alkali needed for colour change End point must be the same as the equivalence point otherwise titration gives wrong answer. Distinct colour change so it is obvious when the end point has been reached.

What indicator would you use for a strong acid-strong base titration?

Phenolphthalein or methyl orange, but phenolphthalein is usually used as clearer colour change.

What indicator would you use for a strong acid-weak base titration?

Methyl Orange

What indicator would you use for a strong base-weak acid titration?

Phenolphthalein

What indicator would you use from a weak acid-weak base titration?

Neither methyl orange or phenolphthalein is suitable, as neither give a sharp change at the end point.

What colour is methyl orange in acid?In alkali?

Red in acid; yellow in alkali.

What colour is phenolphthalein in acid? In alkali?

Colourless in acid; red in alkali.

What colour is bromothymol blue in acid? In alkali?

Yellow in acid and blue in alkali.

Describe how to use a pH metre.

● Remove the pH probe from storage solution and rinse with distilled water.

● Dry the probe and place it into the solution with unknown pH.

● Let the probe stay in the solution until it gives a settled reading.