Acids and Bases

Question 1

What is a Brønsted-Lowry acid?

Answer 1

a species that can donate a proton

Question 2

What is a Brønsted-Lowry base?

Answer 2

a species that can accept a proton

Question 3

What is the formula for pH?

Answer 3

pH = - log10[H+]

Question 4

What is the equation for concentration of H+ ions if the pH is known?

Answer 4

[H+] = 10^(-pH)

Question 5

Is the forward reaction in the equilibrium of water endothermic or exothermic?

Answer 5

endothermic

Question 6

Does water become more or less acidic as temperature increases?

Answer 6

more acidic as more H+ ions are produced

Question 7

What is the equilibrium dissociation constant for?

Answer 7

weak acids and bases

Question 8

What is the equilibrium dissociation constant (Ka)?

Answer 8

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

Question 9

If HA is in excess, how do you find pH?

Answer 9

Use [HA] and [A-] along with Ka to find [H+], then pH

Question 10

If [A-] is in excess, how do you find pH?

Answer 10

Use Kw to find [H+] then pH

Question 11

If HA = A-, how do you find pH?

Answer 11

pKa is equal to pH, therefore find pKa

Question 12

What is the definition of a strong acid?

Answer 12

An acid that completely dissociates to its ions when in solution with pH 0-1.

Question 13

What is the definition of a weak acid?

Answer 13

An acid that only partially dissociates when in solution with pH 3-5.

Question 14

What is the definition of a strong base?

Answer 14

A base that completely dissociates to ions when in solution with pH 12-14.

Question 15

What is the definition of a weak base?

Answer 15

A base that only partially dissociates when in solution with pH 9-11.

Question 16

Give some examples of weak acids

Answer 16

Most organic acids (e.g. ethanoic acid), HCN (hydrocyanic acid), H2S (hydrogen sulfide) and H2CO3 (carbonic acid)

Question 17

Give some examples of strong acids

Answer 17

Hydrochloric, nitric and sulfuric acid

Question 18

On a pH titration curve, how do we identify the neutralisation or equivalence point?

Answer 18

the large vertical section and the equivalence point is approximately in the middle of the large vertical section

Question 19

How do we investigate the neutralisation point?

Answer 19

alkali is slowly added to an acid and the pH measured with a pH probe or vice versa

Question 20

How do we make a titration curve more accurate?

Answer 20

add smaller volumes of either acid or alkali

Question 21

What is the neutralisation point around for a strong acid and strong base reaction?

Answer 21

pH 7

Question 22

What is the neutralisation point around for a strong acid and a weak base reaction?

Answer 22

less than pH 7 (more acidic)

Question 23

What is the neutralisation point around for a weak acid and a strong base reaction?

Answer 23

more than pH 7 (more basic)

Question 24

What is the neutralisation point around for a weak acid and a weak base reaction?

Answer 24

normally pH 7 but hard to determine

Question 25

Why do specific indicators have to be used for specific reactions?

Answer 25

they can only indicate a pH change within a certain range

Question 26

When is methyl orange used?

Answer 26

for reactions with a more acidic neutralisation point

Question 27

What is the colour change for methyl orange?

Answer 27

orange in acids and turns yellow at the neutralisation point

Question 28

When is phenolphthalein used?

Answer 28

for reactions with a more basic neutralisation point

Question 29

What is the colour change for phenolphthalein?

Answer 29

pink in alkalis and turns colourless at the neutralisation point

Question 30

What do acidic buffer solutions contain?

Answer 30

a weak acid and the salt of that weak acid

Question 31

What do basic buffer solutions contain?

Answer 31

a weak base and the salt of that weak base

Question 32

What is the definition of a buffer solution?

Answer 32

a solution which is able to resist changes in pH when small volumes of acid or base are added

Question 33

What is the equation that links the concentration of hydrogen ions and pH?

Answer 33

pH = -log10[H+] (aq)

Question 34

The higher the concentration of hydrogen ions, the ____ the pH

Answer 34

lower

Question 35

The pH of strong acids can be determined with what and why?

Answer 35

pH = -log10[H+] (aq) because we can assume that the acid is fully dissociated; that is, all its hydrogen ions have been released into solution. Therefore, the concentration of the hydrogen ions is equal to the concentration of the acid if there is one H atom in one molecule of the acid, or to twice the concentration of the acid if there are two H atoms in one molecule of the acid.

Question 36

How can the pH of weak acids be calculated and why?

Answer 36

using the acid dissociation constant Ka (in moldm-3). We assume that for any weak acid HA, the hydrogen ions only partially dissociate from the acid anion, A- : HA (aq) <-> H+ (aq) + A- (aq). Since this is in equilibrium, we can write an expression for it: Ka = ([H+(aq)][A-(aq)])/[HA(aq)]

Question 37

What does Ka vary with?

Answer 37

temperature