Acids and Bases EQUATIONS

Question 1

How do we make a buffer from OH- practically

Answer 1

Add a controlled amount of OH⁻ (e.g. NaOH) to excess acid (e.g. ethanoic acid)

CH₃COOH+OH −→ CH₃COO − + H₂O

Now you have
Some CH₃COOH (weak acid) left over

Some CH₃COO⁻ (conjugate base) formed

✅ This mixture = a buffer solution

Question 1

Give 3 examples of strong acids

Answer 1

HCl
H2SO4
HNO3

Question 2

Give an example of a weak base

Answer 2

NH3

Question 3

What is the equation for pH

Answer 3

pH = -log[H+]

Question 4

What is the equation for [H+]

Answer 4

[H+] = 10⁻ᵖᴴ

Question 5

What is the equation for Kᵥᵥ

Answer 5

Kᵥᵥ = [H⁺]²
This is due to
Kᵥᵥ = [H⁺][OH⁻] and [H⁺] = [OH⁻] in the equation:
H2O ⇌ H+ + OH-
So we can write Kᵥᵥ = [H⁺]²

Question 6

Which equations do we use to calculate the pH of strong bases

Answer 6

Kw equation to get H+ conc
pH equation to get pH

Question 7

What equation do we use to calculate the pH of strong acids

Answer 7

Kw equation to get H+ conc
pH equation to get pH

Question 8

What do we use Kₐ for

Answer 8

Calculating H+ conc for weak acids

Question 9

What is the equation for Kₐ

Answer 9

Question 10

What is the units for Kₐ

Answer 10

moldm-3

Question 11

What is the weak acid approximation equation for Kₐ

Answer 11

Question 12

What is the equation for pKₐ

Answer 12

-logKₐ

Question 13

What is the equation for Ka if we know pKa

Answer 13

10⁻ᵖᴷᵃ

Question 14

If the value of Ka is smaller, what does this mean about the strength of a weak acid

Answer 14

It is a weaker, weak acid

Question 15

If the value of pKa is smaller, what does this mean about the strength of a weak acid

Answer 15

It is a stronger, weak acid

Question 16

If the value of Ka is larger, what does this mean about the strength of a weak acid

Answer 16

It is a stronger, weak acid

Question 17

If the value of pKa is larger, what does this mean about the strength of a weak acid

Answer 17

It is a weaker, weak acid

Question 18

Where is the equivalence point on a pH curve

Answer 18

At the start of when the curve becomes completely vertical

Question 19

What is the half neutralisation point on a pH curve and what do we use it for

Answer 19

It is the half way point between the equivalence point and 0
We can find the pH at this point to calculate pKa of a WEAK ACID

Question 20

Explain the relationship between pKa and pH for weak acids at the half neutralisation point on a pH curve

Answer 20

Question 21

Why do diprotic acids have two equivalence points in their pH curves

Answer 21

The 2 protons are released from the molecule separately (they don’t leave at the same time)

All the molecules release the same proton on the same part of the molecule first, once all are done, then second proton comes off

Question 22

What is a buffer solution

Answer 22

A solution that resists changes in pH on addition of small amount of acid, base or on dilution with water

Question 23

What is the dissociation equation for method 1 (acidic buffer) of making a buffer

Answer 23

Question 24

What is the equation for calculating the H+ concentration of a buffer

Answer 24

Then -log(H+)

Question 25

What is the dissociation equation for a buffer made by method 2 (basic buffer)

Answer 25

Question 26

What are the 5 steps in calculating the pH of a buffer made from method 2 (basic buffer) (assuming we know the concentration and moles of HA & A-)

Answer 26

1 - Calculate the moles of HA(start) (C =n/V) 2 - Calculate the moles of OH- (C=n/V of base, but check ratio of base to OH) 3 - Calculate moles of HA(end) (nHA(start) - nOH-) 4 - Write down nA- (nA- = nOH-) 5- Use buffer equation to find concentration of H+, then put into log equation

Question 27

What are the steps in calculating the pH of a buffer made by method 1 (acidic buffer)

Answer 27

Then -log(H+)

Question 28

What is the equation to calculate H+ concentration of a buffer solution after adding an acid

Answer 28

Question 29

What is the equation to calculate H+ concentration of a buffer solution after adding a base

Answer 29

Question 30

Which way does equilibrium shift when we add an acid to a buffer solution, what effect does this have on the concentration of HA

Answer 30

Upon addition of an acid, equilibrium shifts left, and HA concentration increases

Question 31

Which way does equilibrium shift when we add a base to a buffer solution, what effect does this have on the concentration of HA

Answer 31

Upon addition of a base, equilibrium shifts to the right and HA concentration decreases

Question 32

How do we make a buffer from a salt practically

Answer 32

Mix a salt of a weak acid or base with the corresponding weak acid or base e.g. Mix CH₃COONa and CH₃COOH CH₃COONa (aq)→CH₃COO− (aq) + Na+(aq) Lies to the right as dissociates fully in H2O CH₃COOH (aq)⇌CH₃COO−(aq) + H+(aq) Lies to the left as is a weak acid ✅ Final mixture contains: CH₃COOH (weak acid) CH₃COO⁻ (from salt) → This is a buffer solution

Question 33

What's a conjugate acid and conjugate base

Answer 33

A conjugate acid is the species formed when a base gains a proton (H⁺). A conjugate base is the species left behind when an acid loses a proton (H⁺).

Question 34

Why do we aim for higher percentage yield, atom economy and lower uncertainty

Answer 34

Percentage yield - Higher yield Atom economy - Less waste of reactants/raw materials Uncertainty - Ensures data is more reliable

Question 35

Why can we not perform a titration using an indicator with a weak acid and weak base

Answer 35

pH change is too gradual at the equivalence point so colour change of indicator is difficult to judge