Topic 12 Acids Flashcards
Define pH
-log [H+]
How to find [H+] ?
10 ^-pH
What is a strong acid?
Dissociates completely
What is a weak acid?
Hardly dissociates in water
What is the Bronsted-Lowry definition of acid and alkali?
Acid = proton donors
Alkali = proton acceptors
How to find Kw?
[H+][OH-]
What is the Kw of water at room temperature? Always the same.
1 x 10^-14 mol2 dm-6
What is the pH of 0.1, 0.01 and 0.001 moldm-3 of an HCl solution?
pH 1, 2, 3 respectively
Calculate the pH of a 0.05moldm-3 H2SO4 solution
H2SO4 -> 2H+ + SO4 2-
1:2 ratio
0.05x2 = 0.1
= pH 1
Why is H (a bare proton) more unlikely to exist in an aqueous solution but instead react w water to form H3O+?
They are highly reactive, very unstable on their own, so will react with water
Why is the Ka calculation an approximation?
- ignores H+ made by water
- ignores dissociation in acids
What’s a conjugate pair?
A species before and after losing/gaining an H+ ion
What does p mean?
-log
Why is pure water not alkaline in certain conditions?
[H+] =[OH-] , not exceeded [H+]
What is the pH of magnesium hydroxide compared to calcium hydroxide?
Calcium hydroxide is more alkali
Because G2 hydroxide solubility increases down
More solubility = more OH- dissolved and released
Hence detected by pH probe
So more alkali
How to calculate Ka of weak acid?
[H+]^2 / [acid]
How to compare Ka values and determine which ones more acidic?
The greater the Ka value for an acid is relative to the strength of the acid
Due to Bigger numerator in equation
How to find pH of buffer?
[H+] = Ka x ([acid]/[salt])
Then
-log[H+]
What are 3 things that affect pH? (In buffers)
- ratio of [acid]:[salt]
- type of acid (diff Ka)
- temp (changes Ka)
How to find pH of weak acid?
Ka = [H+]^2 / [acid]
rearrange and -log(ans)
What is the role of buffers in blood?
- to keep pH of blood plasma constant at 7.35-7.45
- used in respiration and metabolism
What is the equilibrium equation of buffers in blood?
H2CO3 ⇌ HCO3- + H+
all (aq)
What is the buffer named in blood?
carbonic acid-hydrogencarbonate buffer mixture
Why is chloroethanoic acid a stronger acid than ethanoic acid?
- more stable anions
- Cl- draws e-s to itself (more chloros=more stable)
- reduces negative charge on O
- more chloros = more stable (less attractive) = stronger acid (as happier to dissociate H+)
Write the reaction of ammonia and water.
NH3 + H2O ⇌ NH4+ OH-
Explain why the pH of solution 1.0 moldm-3 ammonia is less than pH 14
It is a weak base
So equilibrium lies to the left (NH3 + H2O ⇌NH4+ + OH-)
OH- produced is nowhere close to 1moldm-3 (as it hardly dissociates already)
What is a buffer solution meant?
Solution that keeps pH of mixture relatively constant when small volumes of acid/alkali are added
Suggest a buffer solution for ammonia
Ammonium chloride
- salt of base
Hydrogen carbonate (HCO3-) act as a weak acid in aqueous solution. Write an equation for this equilibrium
HCO3- ⇌ H+ + CO3-
How does buffer work when small amounts of acids and alkali are added?
the equilibrium of the buffer solution will shift to oppose the change
When to use buffer/Kw/weak acid equations to find pH in titration questions?
When stated buffer, use buffer
When acid in excess (alkali reacts to make salt) use buffer eq
When alkali in excess, subtract excess moles and use Kw
Know what acids are strong and weak. If obscure, assume weak
Why is the salt of weak acid alkaline?
Because it accepts protons from water and forms OH- and acid
What must be assumed when calculating Ka acid (2 marks)
- that [H+] and [HCOO-] are equal, no further dissociation took place
- [HCOOH] at equilibrium is equal to original [HCOOH]
How to find pH of unknown conc weak acid + strong base? (5 marks)
Titration
Use pH meter to record regularly
+ colour indicator
Plot results on graph
Find pH of half equivalence point
At that point pH = pKa (magic moment)
So Ka =10^-pH
Give a reason why the proton donated from the CH3COOH is from carboxylic group not methyl group? (1 mark)
C-H is non polar
O-H bond is polar so
Enthalpy of hydration outweighs energy needed to break O-H bond
When methanoic acid is added to propanoic acid, an eqilibrium is set up containing 2 acid-base pairs.
CH3CH2COOH + HCOOH ⇌
HCOO- + CH3CH2COOH2+
HCOOH is the stronger acid so it loses H+
Show that when the solution has been half-neutralised, the acid dissociation constant is given by the expression Ka=10^-pH
(3 marks)
At half equivalence point
[A-] = [HA]
where in Ka = [H+][A-]/[HA]
[A-] and [HA] cancel out
so Ka = [H+]
so pKa = pH
Ka = 10^-pKa = 10^-pH
Predict, with a reason, whether water is acidic, alkaline or neutral at 310K. (2 marks)
Neutral
Because [H+] and [OH-] conc increase proportionally
Predict, with a reason, the sign of the enthalpy change for the ionisation of water. (1 mark)
Endothermic bcs bond breaking
Explain how a buffer system
H2CO3 <—> H+ + HCO3-
Helps control the pH when extra carbon dioxide is present due to strenuous exercise. (3 marks)
- CO2 dissolved in blood forms carbonic acid.
- So shifts to RHS to produce more H+
- Excess hydrogen carbonate joins combine with H+
Describe without calculation, how you use your graph to determine Ka for propanoic acid. (2 marks)
At half equivalence point
Ka = 10^-pH
Deduce, by referring to Kp, how number of SO2 molecules will change if more oxygen is added to the equilibrium mixture. (2 marks)
2SO2 + O2 <—> 2SO3
Equilibrium shift to RHS
But only temp changes Kp value so remains unchanged
What is Kw? (1 mark) (not its expression)
Ionic product of water
Glycolic acid has an acid dissociation constant of 1.5 × 10–4 mol dm–3 compared
with a value of 1.7 × 10–5 mol dm–3 for ethanoic acid.
(i) Give a possible explanation as to why the value of Ka for glycolic acid is
approximately ten times larger than that of ethanoic acid. (2)
- the hydroxy group (O-H) attracts electrons
- so H+ more easily dissociates
What does Ka indicate about the strength of an acid?
the higher Ka, the stronger acid
What is the dilution factor when pH increases by 1 in strong and weak acids?
About
10fold increase in strong acid
100fold increase in weak acid
