Acids and bases Module 5

Question 1

What are acids?

Answer 1

Proton donors
Release H+ ions when mixed with water in form of hydroxonium ions
HA + H2O = H3O+ + A-

Question 2

What are bases?

Answer 2

Proton acceptors
When mixed with water taken hydrogen ions from water molecules
B + H2O = BH+ + OH-

Question 3

What do you call acids in terms of how many protons they can release?

Answer 3

Monobasic, HCl
Dibasic H2SO4
Tribasic, H3PO4

Question 4

Describe the equilibrium set up by an acid reacting with a base in terms of conjugate pairs?

Answer 4

HA(acid) + B(base) ⇋ BH+(acid) + A-(base)

In forward reaction HA acts as acid as donates a proton to B
In reverse reaction A- acts as base to accept proton from BH+
HA and A- are a conjugate pairs, HA being conjugate acid and A- being conjugate base
B and BH+ are a conjugate pairs, BH+ being the conjugate acid, and B being conjugate base

Question 5

What are conjugate pairs?

Answer 5

Species linked by the transfer of a proton

Always on opposite sides of the equation

Question 6

Water is a special case, what happens if you react it with an acid?

Answer 6

Forms a conjugate acid (H3O)+

Question 7

Water is a special case, what do you get if you react it with a base?

Answer 7

Forms a conjugate base (OH-)

Question 8

What do you get if react an acid with a reactive metal?

Answer 8

Hydrogen gas

Question 9

What do you get if react an acid with carbonates?

Answer 9

CO2 and water

Question 10

What are Alkalis?

Answer 10

Type of base which releases OH- ions in water

Question 11

What do you get if react an acid with an alkali?

Answer 11

Water

Question 12

What’s a metal oxide?

Answer 12

An insoluble base

Question 13

What do you get if react an acid with a metal oxide?

Answer 13

Water

Question 14

Formula for pH if you know it’s H+ concentration?

Answer 14

pH = -log10 (H+)

Question 15

If you know the pH of solution how do you find it’s H+ concentration?

Answer 15

(H+) = 10^-pH

Question 16

What’s an important rule for monobasic acids or bases?

Answer 16

H+ concentration = Concentration of acid

OH- concentration = Concentration of base

Question 17

Equation for water dissociating

Answer 17

H2O ⇋ H+ + OH-

Question 18

How do you find the ionic product of water Kw?

Answer 18

Kw = (H+)(OH-)

If water is pure then ratio between them is 1:1, so Kw is the same as (H+)^2

Question 19

For a weak aqueous acid what equilibrium do you get?

Answer 19

HA ⇋ H+ + A-

Question 20

What’s important about a weak acid?

Answer 20

Don’t fully ionise in solution so H+ conc isn’t the same as acid concentration

Also it’s concentration is the same at start and at equillibrium

Question 21

Formula for Ka the acid dissociation constant?

Answer 21

Ka = ((H+) x (A-)) / (HA)

Or Ka = (H+)^2 / (HA)

Question 22

Units for Ka?

Answer 22

mol dm^-3

Question 23

Formula to find pKa?

Answer 23

pKa = -log10 Ka

Question 24

Formula to find Ka when you know pKa?

Answer 24

Ka = 10^-pKa

Question 25

What's a buffer?

Answer 25

A solution that minimises changes in pH when small amounts of acid or base are added

Question 26

What do acidic buffers contain?

Answer 26

A weak acid and it's conjugate base

Question 27

What are the 2 ways to form an acidic base?

Answer 27

Mix a weak acid with the salt of it's conjugate base Mix an excess of weak acid with a strong alkali

Question 28

What is the equilibrium for a weak acid and it's conjugate base?

Answer 28

HA ⇋ H+(aq) + A- Eg CH3COOH(aq) ⇋ H+(aq) + CH3COO-(aq)

Question 29

How do acidic buffers resist changes in pH?

Answer 29

If you add a small amount of acid the H+ ion increases,, meaning the position of equilibrium shifts to the left reducing H+ ion concentration. pH doesn't change much If Alkali is added OH- ion concentration increases, they react with H+ to form water, removing H+ ions from solution. Causing equilibrium to shift right so more CH3COOH dissociates, H+ ion concentration increases, pH doesn't change much

Question 30

What are the 2 buffer systems that occur in our blood?

Answer 30

H2CO3 ⇋ H+ + HCO3- | H2CO3 ⇋ H2O + CO2

Question 31

How are levels of H2CO3 controlled in our blood?

Answer 31

Respiration, when we breah out CO2 it's concentration reduces so less H2CO3 formed

Question 32

How are levels of HCO3- controlled?

Answer 32

The kidneys, with the excess being excreted in urine

Question 33

How to calculate the pH of a buffer solution?

Answer 33

Assume that initial concentration of salt is the same the equilibrium value of A- HA initial concentration is the same as it's equilibrium concentration Use and rearrange the Ka formula if necessary

Question 34

What can you use to measure the pH of a solution?

Answer 34

pH meter

Question 35

How do you calibrate a pH probe?

Answer 35

Put in distilled water and adjust reading to 7 | Do the same with pH 4 solution and pH 10 solution rinsing it with distilled water each time

Question 36

What do titrations allow you to do?

Answer 36

Find out exactly how much alkali is needed to neutralise and acid

Question 37

How to draw strong acid / strong base curve?

Answer 37

Starts near 0, ends near 14 | vertical part in centre

Question 38

How to draw strong acid/ weak base

Answer 38

Starts near 0 ends near 10 | Vertical part 3/4 on x axis

Question 39

How do you draw a pH plot?

Answer 39

pH on y axis | volume of alkali added on x axis

Question 40

How to draw weak acid/strong base?

Answer 40

Starts near 5 ends near 14 | Vertical part 1/4 way down x axis

Question 41

How to draw weak acid/ weak base?

Answer 41

Starts near 5 ends round 10 | No vertical part it's S shaped

Question 42

What's the vertical part of a pH plot?

Answer 42

The equivalence point | The point where the acid is neutralised and (OH-)=(H+)

Question 43

How do you chose which indicator to use for a titration?

Answer 43

The pH range of the indicator must lie completely within the vertical part of the pH plot

Question 44

Describe features f Methyl orange indicator?

Answer 44

Colour at low pH is red Colour at high pH is yellow Approx pH of colour change is between 3 and 4.5 So useful for strong acid/ strong base and strong acid/ weak base

Question 45

Describe phenolphthallein?

Answer 45

Colour at low pH is colourless Colour at high pH is pink Approx pH of colour change is 8 to 10 So useful for strong acid/ strong base and weak acid/ strong base

Question 46

Why can indicators be thought as weak acids?

Answer 46

They have differently coloured conjugate pairs, so as H+ or OH- concs change the equilibrium positions also do phenolphthallein-H (colourless) ⇋ phenolphthallein-(pink) +H+ Methyl orange-H (red) ⇋ Methyl orange- (yellow) + H+