Acids and Bases

Question 1

What is a Brønsted-Lowry acid?

Answer 1

A substance that can donate a proton (H+ ion).

Question 2

What is a Brønsted-Lowry base?

Answer 2

A substance that can accept a proton.

Question 3

What is an oxonium ion?

Answer 3

H3O+. It exists because a hydrogen ion (proton) is never found isolated so in aqueous solutions it is always bonded to at least one water molecule. For simplicity, protons in an aqueous solution are represented by H+.

Question 4

What is the expression for Kw?

Answer 4

[H+(aq)]x[OH-(aq)]

Question 5

What is Kw at room temperature?

Answer 5

1.0x10^-14 mol^2dm-6

Question 6

How to calculate pH

Answer 6

-log10[H+]

Question 7

How to find pH of strong bases

Answer 7

Find [OH-]. Divide Kw by this to get [H+] and use to find pH.

Question 8

What are strong and weak acids?

Answer 8

Strong acid is fully dissociated into ions in solution.

Weak acid is only slightly dissociated into ions in solution.

Question 9

What is the Ka (acid dissociation constant) of weak acids?

Answer 9

[H+][OH-]/[HA]. Units are moldm^-3

Question 10

How to find pH of weak acids

Answer 10

Ka = [H+]^2 / [HA]. Ka is given. Use this to find [H+] and therefore pH.

Question 11

What is pKa?

Answer 11

-log10Ka

Question 12

Equivalence point

Answer 12

The point in a titration at which the reaction is just complete (acid or base has just been neutralised). The ph here is not always exactly 7.

Question 13

Titration curve for strong acid and strong base added

Answer 13

pH starts low and curves up gradually until just enough volume of base is added and very steep line up. Then levels off.

Question 14

Titration curve for strong acid and weak base added

Answer 14

Starts at low pH and curves up gradually until just enough volume of base is added and very steep line. Then levels off very shortly after equivalence point pH.

Question 15

Titration curve for weak acid and weak base added

Answer 15

pH starts fairly low. Curve starts steep and gets shallower until just enough volume of base is added and quite steep line near equivalence point pH but only for short period. Then pH levels off very shortly after equivalence point pH

Question 16

Titration curve for weak acid and strong base added

Answer 16

pH starts fairly low and curve is steep at start and gets shallower until near equivalence point pH and there is a very steep line. Then levels off quite far beyond equivalence point pH.

Question 17

What is the end point of an acid base titration?

Answer 17

The point in a titration when the volume of reactant added just causes the colour of the indicator to change.

Question 18

What makes a good indicator?

Answer 18

Colour change sharp not gradual. End point given by indicator must be same as equivalence point. Distinct colour change.

Question 19

What is the half-neutralisation point?

Answer 19

Half way between the zero and the equivalence point.

Question 20

What is useful about the half-neutralisation point?

Answer 20

The pH at this point is the pKa of the weak acid.

Question 21

What are buffers?

Answer 21

A solution that resists change of pH when small amounts of acid or base are added or on dilution.

Question 22

How do acidic buffers work?

Answer 22

They are weak acids and a soluble salt of the acid. If a little alkali is added, the OH- ions react with the HA to produce water and A-. This removes the added OH- ions. Or could react with H+ so more HA ionises. If a little acid is added, the equilibrium of dissociation shifts to the left as the H+ ions combine with the A- ions to form HA which removes the added H+ ions.

Question 23

How do basic buffers work?

Answer 23

They are a weak base and a salt of that base. The aqueous base removes added H+ ions. The salt removes added OH- ions.

Question 24

How to calculate the pH of a buffer solution when nothing is added.

Answer 24

Use the Ka expression for the dissociation of the weak acid (the base will be A-). Find concentrations. Rearrange to find [H+] and calculate pH.

Question 25

How to calculate pH of buffer solution formed when things are reacted together.

Answer 25

Write out equation of reaction. Do ICE with moles. Find concentrations. Do normal thing for finding pH of buffer solution.

Question 26

How to find pH of buffer solution when acid is added

Answer 26

All of the added acid reacts with the salt to form the weak acid so moles of salt decreases and moles of HA increases. Use ICE to find final moles then final concentrations. Do same as normal to find pH of buffer solution.

Question 27

How to find pH of buffer solution when base is added

Answer 27

All of the base with react with the H+ ions and more HA will ionise so fewer moles of HA and more moles of salt. Use ICE to find these values then concentrations and do same as normal to find pH of buffer solution.

Question 28

How to obtain a pH curve for a titration

Answer 28

Measure the pH of the acid. Add small know quantities of alkali stirring the mixture throughout. Between each addition, measure the pH of the contents of the conical flask. Add alkali in smaller increments near the end point. Repeat until the alkali is in excess.

Question 29

Why does calibrating a pH probe just before it is used improve the accuracy of the pH measurement?

Answer 29

Over time, a pH probe doesn’t give accurate readings. Also, temperature variations affect readings.

Question 30

Why doesn’t the Kw (ionic product of water) expression include [H2O]?

Answer 30

[H2O] is very high compared to the concentration of H+ and OH- ions as it only very slightly dissociates. Therefore [H2O] is effectively constant and is incorporated into a modified equilibrium constant Kw rather than Kc

Question 31

What is a strong or weak acid?

Answer 31

Strong acids are fully dissociated into ions in solution. Weak acids are only slightly dissociated into ions in solution.

Question 32

Why does a buffer solution not change in pH when small amounts of water are added?

Answer 32

The ratio of [A-] to [HA] remains the same. [H+] is proportional to [HA] over [A-] (by Ka) so [H+] remains constant.