Acid-Base

Question 1

What is a Bronsted-Lowry acid?

Answer 1

Substance that can donate a proton

Question 2

What is Bronsted-Lowry Base?

Answer 2

Substance that accepts a proton

Question 3

What is a conjugate acid-base pair?

Answer 3

Pair of reactants/products that are linked to each other by the transfer of a proton
HA + B ⇌ A- + BH+
E.g
CH3COOH (aq) + H2O (l) ⇌CH3COO- (aq) + H3O+ (aq)
ACID. BASE. Con. BASE Con. ACID

HA donates proton to form its conjugate base, A- (CH3COO-)
B accepts PROTON from HA to form its conjugate acid, BH+

(Substance with bigger Ka will act as acid)

Question 4

Calculate pH and [H+]?

Answer 4

PH = -log[H+]
[H+] = 10^-pH

Question 5

What is meant by ‘the pH scale is logarithmic scale with base 10’?

Answer 5

Each value is 10x the value below it
E.g pH 5 is 10x more acidic than pH 6

Question 6

What is a strong acid and examples?

Answer 6

Strong acid is an acid that dissociates completely in aqueous solutions
E.g HCl - hydrochloric acid
HNO3 - nitric acid
H2SO4 - sulfuric acid

HA —> H+ + A-
HA= Strong acid
H+ ions = formed from dissociation
IRREVERSIBLE REACTION (equilibrium shifted right)

Question 7

In a monoprotic strong acid ————————-
Also what assumption is made when strong acids ionise?

Answer 7

The conc of H+ ions = conc of strong acid

No. Hydrogen ions formed from ionisation of water is very small relative to [H+] due to ionisation of strong acid so NEGLECTED

Question 8

What are weak acids? E.g?

Answer 8

Partially dissociate when dissolved in water, giving an equilibrium mixture
E.g most organic acids (ethanoic acid)
HCN - hydrocyanic acid
H2S - hydrogen sulfide
H2CO3 - carbonic acid

HA —> H+ + A-
Equilibrium is LEFT / equilibrium is ESTABLISHED
Due to PARTIAL DISSOCIATION , more molecules of HA (weak acid) than H+ and A- ions

Question 9

Why is the enthalpy of neurtralsation of strong acids and strong bases very similar?

Answer 9

Acid/bases are fully ionised and the same ionic equation is occurring in all cases:
H+ + OH- —> H2O
- in each strong acid-strong bases reaction (no matter which strong base or strong acid used)

Question 10

Why is the enthalpy of neutralisation less exothermic for weak acids and weak bases?

Answer 10

Only partially ionise so energy needed to fully ionise them —> LESS EXOTHERMIC
- in this example:
energy needed to break O-H bond to release a H+
CH3COOH —> CH3COO- + H+ - shows partial ionisation

Question 11

What is Ka and example of an expression?

Answer 11

Ka= acid dissociation constant (mol dm-3)
- indicate extent of dissociation
HIGHER Ka = more dissociated = stronger acid

Ka = [H+] [A-] / [HA]
For weak acids as an equilibrium is established

Question 12

When writing Ka expression for weak acids, what assumption is made?

Answer 12

Conc of H+ ions due to ionisation of water is NEGLIGIBLE
[H+]eqm = [A-]eqm as they have dissociated in a 1:1 ratio
Amount of dissociation of acid is small so we assume initial conc of undissociated acid has remained constant ( [HA]initial = [HA] equm)
So can simplify to Ka = [H+]^2/[HA]initial

Question 13

What is ionic product of water , Kw and how is it derived?

Answer 13

Equibrium constant for the self ionisation of water
Kc = [H+][OH-] / [H2O]
Rearranged to:
Kc x [H2O] = [H+][OH-]
Bc [H2O] is music bigger than conc of ions, we asssume value is constant so we incorporate it into the constant and make it Kw

Kw = [H+(aq)][OH-(aq)]
Kw =1x10^-14 mol2dm-6

Question 14

Relationship between Kw and pKw?

Answer 14

pKw = -logKw

Question 15

Relationship between pKa and Ka?

Answer 15

pKa = -logKa
- used bc for weak acids, Ka are very small numbers so pKa easier to work with
(pKa values lie with 3 and 7, for weak acids)

Question 16

For strong bases , the [OH-] is = to_________________

Answer 16

Conc of the base

Question 17

Why is pure water/neutral solutions neutral?

Answer 17

Neutral solutions are defined as having equal concs of H+ and OH- : [H+] = [OH-]
Therefore Using Kw = [H+][OH-] when neutral :

Kw=[H+]^2 and [H+] = √Kw
At 25C , [H+] = √1×10^-14 = 1x10^-7
= pH 7

Question 18

What is an acid base indicator?

Answer 18

Weak acid which dissociates to give an anion of a different colour
Weak acid HIn:
HIn ⇌ In- + H+
Colour1. Colour 2

Equilibrium will shift left/right due to Chatelier’s principle if acidity changes
- colour changes over a pH range

Question 19

When is endpoint of a reaction reached?

Answer 19

When:
[HIn] = [In-]

And at this point if Ka = [H+][In-]/[HIn] , then:
Ka = [H+] OR pKa of indicator = pH

Question 20

Best indicators for strong acid-strong base reactions?

Answer 20

PH changes from 4-10 ,so indicator must change colour in this range :
METHYL RED/PHENOLPHTHALEIN

Question 21

Best indicator for weak acid-strong base reaction?

Answer 21

PH changes from 7 to 10 so:
PHENOLPHTHALEIN

Question 22

Best indicator for STRONG ACID-WEAK BASE reactions?

Answer 22

Ph changes from 4 to 7 so:
METHYL RED
Can use METHYL ORANGE TOO

Question 23

Best indicator for WEAK ACID -WEAK BASE reactions?

Answer 23

No sudden pH change so no suitable indicators for these titrations
- end point not easily determined

Question 24

What is a buffer solution?

Answer 24

Solution where pH doesn’t change significantly if small amount of acid or alkali added to it

Buffer can consist of:
Weak acid - conjugate base
Weak base - conjugate acid

Question 25

What happens when H+ ions added to ethanol acid buffer?

Answer 25

CH3COOH ⇌ H+ + CH3COO- Ethanoic acid. Ethanoate ion - reserve supplies of HA (CH3COOH) and A- ions (CH3COO-) **Equilibrium shifts left** - H+ ions react with CH3COO- ions to form more CH3COOH until equilibrium re-established (therefore the added H+ ions are removed from mixture) due to reserve supply of CH3COO-/ CH3COOH, conc of these doesn’t change much **PH remains reasonably constant**

Question 26

What happens if OH- ions are added to ethanoaic acid buffer?

Answer 26

The OH- react with H+ to form water so H+ conc **DECREASES** (therefore OH- ions removed from mixture) **Equilibrium shifts right** - **more CH3COOH molecules ionise to form more H+and CH3COO-** until equilibrium is re-established **large reserve supply of CH3COOH**, conc of CH3COOH doesn’t change much when CH3COOH dissociates to form more H+ ions **Reserves of CH3COO-** the conc of CH3COO- doesn’t change much **PH remains reasonably constant**

Question 27

Ka expression for buffers and how to find [H+]?

Answer 27

Ka = [salt][H+]/[acid] Rearranged: [H+] = Ka x [acid]/[salt]

Question 28

How blood pH controlled?

Answer 28

CO2 from aerobic respiration combines with water in blood to form carbonic acid (H2CO3) H2CO3 ⇌ H+ (aq) + HCO3- (aq) If conc of H+ not controlled —> ACIDOSIS (too much acid in body fluids) —> body malfucntion —> coma Equilibrium shifts left/right to keep pH constant If CO2 present: carbonic acid CONC increases Equilibrium shifts right + produces more H+ and HCO3 - ions Excess HCO3- combine with H+ to control pH

Question 29

What is a half equivalence point?

Answer 29

Stage of titration at which exactly half the amount of acid/base has been neutralised E.g REACTION BETWEEN NaOH and CH3COOH NaOH (aq) + CH3COOH (aq) → CH3COONa (aq) + H2O (l) - can assume **[HA]=[A-]** so [CH3COOH (aq)] = [CH3COO- (aq)] at half equivalence point so means… **PKa = pH at HALF equivalence point**

Question 30

As concentration increases by a factor of 10 _________

Answer 30

PH decreases by 1 unit

Question 31

Compare the pH of a strong acid and weak acid after dilution 10,100 and 1000 times?

Answer 31

STRONG ACID: diluting strong acid by 10x , increase pH by 1 unit , diluting by 100x, increase pH by 2 units WEAK ACID: diluting weak acid by factor of 10, increase pH by 0.5 , diluting by 100x increases pH by 1unit

Question 32

What is the enthalpy change of neutralisation for string acids /bases?

Answer 32

-57/-58 Kj/mol Bc acid/alkali are fully ionised and undergo neutralisation by this reaction: OH- (aq) + H+ (aq) —> H2O (l)

Question 33

What are diprotic acids?

Answer 33

2 mol H+ per mole (Multiply conc of acid by 2 to get [H+] or multiply moles by 2 in calcs)

Question 34

Ways to make acidic buffers?

Answer 34

***Weak acid + its salt of its conjugate base*** (ethanoic acid + sodium ethanoate (CH3COONa)) - weak acid partially dissociates/conj base salt FULLY DISSOCIATES CH3COONa(aq) ➔ CH3COO−(aq) + Na+(aq) CH3COOH(aq) ⇌ H+(aq) + CH3COO−(aq) ***EXCESS weak acid + STRONG BASE*** (excess ethanoic acid + sodium hydroxide) Excess weak acid remains in solution after base fully consumed - remaining weak acid PARTIALLY DISSOCIATES CH3COOH(aq) ⇌ H+(aq) + CH3COO−(aq) In both cases: equm solution contains large amounts of CH3COOH (HA) and CH3COO- (A-)

Question 35

Ways to make basic buffers?

Answer 35

Weak base + salt of its conj acid (ammonia + ammonium chloride) SALT fully dissociates , WEAK BASE partially dissociates NH4Cl(aq) ➔ NH4+(aq) + Cl-(aq) NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq) **solution contains large amount of NH4 + and NH3** EXCESS weak base + STRONG ACID (excess ammonia+ HCl)

Question 36

Example of BASIC BUFFER and how it works?

Answer 36

NH3 + H20 ⇌ NH4 + + OH- Add H+ : H+ conc increases. Some H+ reacts with OH- ions forming H2O Equilibrium shifts right to replace consumed OH- ions Some H+ ions react with NH3 to form NH4 + ***These reactions removes most of added H+ ions - minimise change in pH*** Add OH- : OH- conc increases - react with NH4+ to form NH3 and H2O —> equm shifted left, removes OH- ions from solution - ***minimise change in pH*** In both cases: [B] (NH3) and [BH+] (NH4 +)in reserve so stay constant

Question 37

Devise an experiment to determine acid dissociation constant , Ka, for a solution of ethanoic acid, CH3COOH of unknown conc?

Answer 37

**Titrate** ethanoic acid with strong base Measure **pH at reg intervals** Plot **pH against vol ** of strong base (titration curve) Use graph to find **pH at half equivalence point** At half neutralisation: **PH = pKa so Ka = 10^-pH**

Question 38

CH3COOH + H2O —> CH3COO- + H3O + Give a reason why only the proton from COOH group, and not from the methyl group, is donated to a water molecule?

Answer 38

Loss of hydrogen from methyl group would produce carboanion with no stabilisation

Question 39

Predict with reason, the sign of enthalpy change for the ionisation of water?

Answer 39

Kw increases with temperature So this means more ionisation occurs at higher temperatures So ionisation for water is ENDOTHERMIC (+) (H20 —> H+ + OH- )

Question 40

Why is CH2ClCOOH stronger acid than CH3COOH?

Answer 40

When CH2ClCOOH loses a proton —> CH2ClCOO- The negative charge on carboxylate ion is more stable in CH2ClCOO- than in CH3COO- ions because: Cl atom pulls electron density away (as Cl is electronegative) so helps to spread out the negative charge This stabilises the anion so losing a proton is easier -> stronger acid

Question 41

How to measure pH of solution using a pH meter?

Answer 41

Before using need to calibrate it : Place the bulb of the pH meter in deionised water + allow reading to settle Then adjust reading so it reads 7.0 Do same for a standard solution of pH 4 and pH 10 (make sure to rinse probe with deionised water between each reading) Can now take measurements - pace probe into solution and let reading settle before you record results and make sure to rinse probe in between measurements (Can also attach pH probe to data logger which records data at set intervals for a specified amount of time)