Acids, bases and pH A2

Question 1

What are the Bronstead-Lowry model definitions for acids and bases?

Answer 1

Acid = proton (H+) donor
Base = proton (H+) acceptor

Question 2

Where can protons dissociate? And what ions do they form?

Answer 2

In solution.

They form H3O+ (hydroxonium) ions with water.

Question 3

What is a conjugate acid-base pair?

Answer 3

A pair of species that can be interconverted by proton transfer.

Question 4

HCl + H2O <–> H3O+ + Cl-

Label the conjugate acid-base pairs

Answer 4

HCl + H2O <–> H3O+ + Cl-
A1 B2 A2 B1

Question 5

What are the names that describe how many H+ ions are replaced in an acid?

Answer 5

Monobasic = one proton (e.g. HCl)
Dibasic = 2 protons (e.g. H2SO4)
Tribasic = 3 protons (e.g. H3PO4)

Question 6

A1 recap:

What do you get when you react metals, carbonates and alkalis with acids? What are these reactions called?

Answer 6

Metal + acid = salt + water
REDOX REACTION

Metal Carbonate + acid = salt + water + CO2
NEUTRALISATION

Alkali + Acid = Salt + water
NEUTRALISATION

Question 7

What kind of scale is pH? What does it measure?

Answer 7

A logarithmic scale for measuring [H+] in solution.

(each increasing pH value is a magnitude of 10 smaller)

Question 8

What is the equation to work out pH from [H+]?

Answer 8

pH = - log [H+]

Question 9

How can [H+] be determined from pH?

Answer 9

[H+] = 10^ - pH

Question 10

What is a strong acid? What does this mean about the [HA] and [H+]?

Answer 10

Strong acids completely dissociates in water.

So the concentration of HA is equal to the concentration of H+.

Question 11

What is the equation for dilution factor?

Answer 11

Original volume / total volume

Question 12

Weak acids form an equilibrium so their dissociation can be represented by the acid dissociation constant ___.

Answer 12

ka

HA <–> H+ + A-

Question 13

What does a higher/lower value of Ka suggest about equilibrium position?

Answer 13

High ka = shift to the right

Low ka = shift to the left

(ka<1, ka>1)

Question 14

As the values of Ka are very _____, its is easier to give their negative logarithm (__) instead. What is the equation? What is the inverse?

Answer 14

Small
pKa

pKa = -logKa

Ka = 10^ - pKA

Question 15

The weaker an acid the _____ the Ka and the _____ the pKa.

Answer 15

Smaller the Ka
(becouse less H+ dissociated so shift to left)

Larger the pKa
(greater power of -10)

Question 16

The [H+] of a weak acid depends on the value of ___ and ___.

Answer 16

Ka and [HA]

Question 17

What are the 2 approximations that need to be made about weak acid dissociation?

Answer 17

1) HA dissociation forms equal [H+] and [A-] (so becomes [H+]^2)

2) The change in [HA] is extremely small / negligible so [HA] at eqm = [HA] at start

Question 18

What equation can you use to find Ka for weak acids?

Answer 18

Ka = [H+]eqm^2 / [HA]

Question 19

How do you calculate pH for weak acids?

Answer 19

[H+] = square root(Ka x [HA])

pH = -log[H+]

Question 20

How can the value for Ka be determined experimentally?

Answer 20

By using a pH meter. Then calculate [H+] and then Ka.

Question 21

When calculating values for Ka, what are the issues with the approximations made?

Answer 21

1) At pH values >6, water dissociation is significant (releasing some H+ into solution so [H+] does not equal [A-]). So it doesn’t work for very weak acids / dilute solutions.

2) If [H+] concentration is significant there will be a difference between [HA] at eqm and [HA] at start. So doesn’t work for stronger weak acids with Ka > 10^-2 mol dm^-3 or very dilute solutions.

Question 22

Water is amphoteric. What does this mean?

Answer 22

It acts as an acid and a base.

because it is able to be ionised slightly:
H2O <–> H+ + OH-

Question 23

Where does Kw come from?

Answer 23

As the dissociation of water is very small, the [H2O] remains constant, so bringing both Ka and [H2O] to the same side of the equation gives:
Ka x [H2O] = Kw = [H+] [OH-]

Question 24

What alters the Kw value?

Answer 24

Temperature

Question 25

What is the difference between a strong and weak base?

Answer 25

Strong bases completely dissociate in solution whereas weak bases only partially dissociate.

e.g. NaOH -> Na+ + OH-