P- Acid-Base Equlibria Flashcards by Katherine Choi

State Brønsted–Lowry theory

acid = proton donor
base = proton acceptor

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How does an acid produce H+ ions in solution?

H+ reacts w/ H2O molecules by giving a proton

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Equation for reaction between a strong acid and water.

HA (g) + H2O (l) –> H3O+ (aq) + A- (aq)
- H3O = hydroxonium ion
- simpler ver: HA (aq) –> H+ (aq) + A- (aq)

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def + examples of strong acids

completely ionise
eg. HCl, H2SO4, HNO3, H3PO4

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def + examples of weak acids

partially ionise
eg. all organic acids (carboxylic) ethanoic CH3COOH, only inorganic weak acid- HF

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In a reaction, how much of a strong acid is turned into products?

100%

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Give an example of a monoprotic acid

HCl –aq–> H+ (aq) + Cl- (aq)
1 mole of HCl gives 1 mole of H+ –> monoprotic

HNO3- strong monoprotic

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Give an example of a diprotic acid

H2SO4 (l) –aq–> 2H+ (aq) + SO4 2- (aq)
1 mole of H2SO4 gives 2 moles of H+

has a ↓ pH for the same conc as monoprotic ∵ gives ↑ moles of H+

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Give an example of a triprotic acid

H3PO4 –> 3H+ (aq) + PO4 3- (aq)
1 mole of H3PO4 gives 3 moles of H+

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Define pH.

pH = -log10 [H+]
- [ ]- conc in moldm^-3

p: -log10

ALWAYS GIVE TO 2dp

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Equation for reaction between a weak acid and water

HA (aq) ⇌ H+ (aq) + A- (aq)

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[H+] = ?

rearrange pH formula

[H+] = 10 ^-pH

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Give an equation for the dissociation of HF.

HF (g) ⇌ H+ (aq) + F- (aq)

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Calculating pH from [H+]:
What is the pH if [H+] = 1 x 10^ 4

4
1 x 10^4 –> 4

check both values are roughly the same

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Formula for [H+] of diluted acid.

[H+] = original [H+] x org. vol/ diluted vol (cm3)

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HA (l) ⇌ H+ (aq) + A- (aq)
Ka = ?

[H+] x [A-]
Ka = ——————–
……………..[HA]
- only for weak acids!!
- [H+] = [A-]
- ↓ Ka - weaker acid (↓pKa)

pKa = ?

pKa = -log10 Ka
- Ka = 10^ -pKa

What limits the range of pH scale?

solubility - saturated solution- x dissolve any more

Is water an acid or a base?

Amphoteric- both

What is the equation for water at equilibrium?

H2O (l) ⇌ H+ (aq) + OH- (aq)

What is the conc of H+ at room temp?

1 x 10^ -7 moldm-3
pH 7.00

Kw = ?

Kw = [H+][OH-]
1 x 10^ -14 mol2dm-6 at stp

stp = 298K, 100kPa

pH value for neutral

varies w/ temp

def for neutral

[H+] = [OH-]

Is the dissociation endothermic or exo + why?

endo - ↑ temp ↑ Kw -- eq. shifted to RHS

What is a strong base + give examples?

fully ionises in water - eg. NaOH, KOH, Ca(OH)2, Ba(OH)2

What is a weak base + give an example.

partially ionises in water - eg. NH3 {NH3 (g) + H2O (l) ⇌ NH4 + (aq) + OH- (aq)}

What are [H+] and [OH-] in strong bases?

they are different! Kw still 1 x 10^-14

How do you compare strengths of bases?

see how **readily** they **produce OH- in solution**- 1. **contain OH-** (eg. NaOH (s) --> Na+ (aq) + OH- (aq) ) 2. **take H+ from H2O** (eg. CaO (s) + H2O (l) --> Ca2+ (aq) + 2OH- (aq) )

Formulas for the half neutralisation of a weak acid

when half of HA molecules reacted w/ OH-: - [HA] = [A-] - Ka = [H+] - pKa = pH

What are acid buffers made of?

**weak acid + salt of weak acid** eg. CH3COOH + CH3COO- (both at high conc. >> [H+]

How does an acidic buffer work?

**when H+ (acid) added:** - CH3COO- + H+ --> CH3COOH - **H+ removed by CH3COO-** --> x much change in [H+] ---> pH stable **when OH- added:** - CH3COOH + OH- --> CH3COO- + H2O - **OH- removed by CH3COOH** --> pH stable

What is a basic buffer made of?

weak base + salt of weak base

How does a basic buffer work? eg. NH3 + NH4+

**H+ added:** - NH3 + H+ --> NH4+ - **H+ removed by NH3** --> [H+] barely changes --> pH stable **OH- added:** - OH- + NH4+ --> NH3 + H2O - **OH- removed by NH4+** --> pH stable

What is the equivalence point?

when solutions are mixed in **exactly the right proportions as in the equation**

What is the end point?

when **indicator changes colour**