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Chemistry > Acids & Bases > Flashcards

Acids & Bases Flashcards

1
Q

Reacting acids and bases

A

1) Acid + Metal → Salt + Hydrogen
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

2) Acid + Metal Oxide → Salt + Water
Li2O(s) + 2HCl(aq) → 2LiCl(aq) + H2O

3) Acid + Metal Hydroxide → Salt + Water
Ca(OH)2(aq) + 2HCl(aq) → CaCl2(aq) +2H2O

4) Acid + Metal Carbonate → Salt + CO2 + Water
CaCO3(g) + 2CH3COOH(aq) → (CH3COO)2Ca(aq) + CO2 + H2O

5) Acid + Ammonia → Ammonium salt
NH3(g) + HCl (aq) → NH4Cl

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2
Q

Strong/ weak acid examples

A

Strong:
HCl = Cl-
H2SO4= SO4 2-
HNO3= NO3-
H3PO4 = PO4 3-

Weak:
CH3COOH= COO-
Any other acid

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3
Q

Strong/ weak base examples

A

Strong:
Metal oxides (MgO, Na2O)
Metal hydroxide ( NaOH, Ba(OH)2 )

Weak:
NH3
Carbonate (Na2CO3) = CO3 2-

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4
Q

What is an acid?

A

An ACID is a proton donor

HCl(aq) → H+(aq) + Cl-(aq)

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5
Q

What is a base?

A

A BASE is a proton acceptor

-OH(aq) + H+(aq) → H2O

*H2O can act as an acid and a base

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6
Q

Dilutions

A
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7
Q

pH expressions

A

pH = -Log10[H+]
[H+] = 10^-pH

  • The smaller the pH, the greater the concentration of H+ ions.
  • A difference of 1 on the pH scale means a 10x difference in [H+]
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8
Q

Strong acids and calculations

A

A STRONG acid FULLY DISSOCIATES
HCl(aq) → H+(aq) + Cl-(aq)

Calculations:
-Log[HA] monoprotic
-Log2[HA] diprotic (H2SO4)
-Log3[HA] triprotic (H3PO4)

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9
Q

Explain why chloroethanoic acid is a weaker acid than ethanoic acid?

A

Chloroethanoic acid is weaker as it is more polar.
Ethanoic acid is stronger, less polar due to the positive inductive effect

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10
Q

Weak acids and calculations

A

A WEAK acid only PARTIALLY DISSOCIATES
CH3COOH(aq) ⇌ CH3COO-(aq) + H+(aq)

Ka = [H+][A-]
[HA]

[H+] = √Ka[HA]
-Log[H+]

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11
Q

How to differentiate if strong or weak acid using a single pH mesurement?

A

strong acid pH=1
weak acid pH= greater than 1

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12
Q

pKa values

A

pKa = -Log10Ka
Ka = 10^-pKa

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13
Q

Pure water and calculations

A

H2O(l) ⇌ H+(aq) + -OH(aq) ∆H = +57.3kJmol-1
very slightly dissociates
always neutral because [H+] = [OH-] at all times

Kw = [H+][-OH]
[H+] = √Kw
-log[H+]

-water conc is constant as it very slightly dissociates
-as temp ↑, equilibrium shifts to the right in endo direction to oppose ↑ in temp
-[H+] increases

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14
Q

Strong alkali and calculations

A

Strong bases fully dissociate
MOH → M+ + OH-

Kw = [H+][-OH]
[H+] = Kw / [nMOH]
-Log[H+]

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15
Q

Buffer Solutions

A

A BUFFER is a solution which can resist changes in pH when a small amount of acid or base is added

An ACIDIC BUFFER is made of a weak acid and a soluble salt of that acid. It maintains a pH below 7.

A BASIC BUFFER is made of a weak BASE and a soluble salt of that BASE. It maintains a pH above 7.

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16
Q

Calculating the pH of buffer

A

Ka = [H+][A-]
[HA]

[H+] = Ka[HA] / [A-] (A- = conc of salt)
-log[H+]

17
Q

Half neutralisation

A

Equal number of moles of acid and salt

Ka= [H+]
pKa = pH

18
Q

Calculating the pH change of a buffer when a small amount of acid is added

A

When acid is added, the equilibrium shifts to the left to oppose increase in [H+]
H+ + CH3COO- → CH3COOH

19
Q

Calculating the pH change of a buffer when a small amount of alkali is added

A
20
Q

How does a buffer resist pH changes when a small amount of alkali is added?

A

-OH + H+ → H2O
CH3COOH → H+ + CH3COO-

equilibrium shifts right

21
Q

2 ways to conduct pH titrations

A

1) normal titration

2) Using a pH probe:
- Add 25cm^3acid into 250ml conical flask
- Measure the pH of the acid solution and record.
- Add 1cm3 of the base solution
- Stir the mixture
- Measure the pH and record.
- Repeat the process until the base is in excess.
- Add base in smaller increments near the end point

It is important to calibrate the pH meter as after storage it may not give an accurate reading. To calibrate it you place the pH meter in a solution of known pH and then adjust the meter accordingly.

22
Q

pH titration curves

A
23
Q

Acid base titration calculations

A

at the equivalence point:
nNaOH= CxV = 25/1000 x 0.1
= 0.0025

nNaOH= nAcid
nAcid= 0.0025
[Acid] = 0.0025/ vol

24
Q

Equivalence point definition

A

when exactly enough acid has been added to neutralise the base

25
Q

End Point definition

A

the exact volume of acid or base which needs to be added to cause an indicator to change colour

26
Q

Indicators

A

A SUITABLE INDICATOR CHANGES COLOUR SOMEWHERE ON THE VERTICAL SECTION OF A pH TITRATION CURVE

27
Q

The Half-neutralisation point

A

The point at which enough base has been added to neutralise exactly half of the acid

Half neutralisation= Ka = [H+]

working out pH when no numbers = -log(Ka)

Half neutralisation = half equivalence

Chemistry (30 decks)

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