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UACE Chemistry > Ionic Equilibria > Flashcards

Ionic Equilibria Flashcards

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1
Q

What is a Solution?

A

This is a homogeneous mixture of two or more substances.

Example sentence: A saltwater solution is a mixture of salt and water.

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2
Q

What is an Unsaturated solution?

A

This is a solution that can still dissolve more solute at a fixed temperature.

Additional information: Unsaturated solutions are commonly used in chemistry experiments.

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3
Q

Define a Saturated solution

A

This is a solution that can dissolve no more solute at a fixed temperature.

Example sentence: When no more sugar can dissolve in a cup of tea, it is a saturated solution.

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4
Q

What is a Super saturated solution?

A

This is a solution that has dissolved more solute than it can hold at a particular temperature.

Additional information: Super saturated solutions are unstable and can crystallize easily.

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5
Q

Define Molarity

A

Molarity of a solution is the number of moles of solute that dissolve in sufficient solvent to make 1000cm3 (1dm3) of solution.

Example sentence: The molarity of a sugar solution is calculated based on the amount of sugar dissolved in water.

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6
Q

What is Solubility?

A

This is the amount of solute in moles required to saturate 1000g (1Kg) of solvent at a fixed temperature.

Additional information: Solubility is often expressed in units of moles per liter.

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7
Q

Describe the variation of solubility of with temperature

A
  • Solubility of ionic compounds generally increases with increase in temperature
  • Solubility of gases decreases with increase in temperature
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8
Q

What factors affect solubility of an ionic compound at a constant temperature

A
  • Lattice energy
  • Hydration energy.
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9
Q

Give factors affecting solubility of an ionic compound

A

• Temperature change
• Presence of a common ion
• Complex ion formation

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10
Q

Define Solubility product

A

This is the product of the molar concentrations of the ions of a saturated solution of a sparingly soluble ionic compound each raised to its appropriate stoichiometric co-efficient.

Example sentence: The solubility product of silver chloride is calculated based on the concentrations of silver and chloride ions in the solution.

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11
Q

State how the value of Ksp is affected by temperature

A

It is constant at a constant temperature and increases with increase in temperature.

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12
Q

What will occur in a solution in which KSP > [Pb2+ ][Cl− ]2 ?

A

Solution is not saturated and more salt can still dissolve in it

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13
Q

What will occur in a solution in which KSP = [Pb2+ ][Cl− ]2?

A

Solution is saturated and addition of a solution with a common ion (either Pb2+ or Cl- ions) will lead to precipitation of the salt (PbCl2) from the solution.

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14
Q

What is a solution in which KSP < [Pb2+ ][Cl- ]2 called?

A

Supersaturated.

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15
Q

What is Common ion effect?

A

This is the reduction in the solubility of an ionic compound in water caused by the addition of a solution of a compound with a common ion.

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16
Q

State the application of solubility product

A

a) Gravimetric analysis e.g. the concentration of sodium sulphate can be determined by precipitating the sulphate ions as barium sulphate. After filtration the barium sulphate is washed with dilute sulphuric acid instead of water to avoid loss of some of the barium sulphate

b) Qualitative analysis
In inorganic qualitative analysis the identification of metal ions present in aqueous solutions involves the application of common ion effect. E.g. certain metal sulphides may not form in acidic conditions

c) Salting out effect
Soap, sodium stearate (sodium octadecanoate), C17H35CO2Na is precipitated out of solution by adding sodium chloride. The increased Na+ (common ion) causes the product [C17H35CO2-][Na+] to exceed the Ksp value of soap and thereby causing its precipitation.
(All the above are also applications of common ion effect)

d) Predicting precipitation
Solubility product enables chemists to predict the maximum concentration of ions of a solution. This way they are able to know whether a precipitate will or will not form

e) Selective precipitation
This involves taking advantage of the different solubilities of different salts as means of separating them e.g. a solution of barium chloride and magnesium chloride can be separated by adding potassium chromate solution forming magnesium chromate and barium chromate. Barium chromate is insoluble in water and can be removed by filtration

f) Complex ion formation
The concept of solubility product can be used to explain complex ion formation by sparingly soluble compounds when some other reagents are added to them.

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17
Q

Describe the use of potassium chromate in silver nitrate solution titration

A
  • This is one of the applications of selective precipitation.
  • In silver nitrate titrations, potassium chromate is used to serve as the indicator.
  • When silver nitrate solution is titrated with an aqueous solution of chloride ions containing a little potassium chromate, only silver chloride is precipitated as a white solid as long as there are still chloride ions in the solution.
  • A red precipitate of silver chromate only forms when all the chloride ions have been precipitated out of the solution.
  • This serves as the endpoint of the titration.
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18
Q

State the factors affecting solubility

A
  1. Temperature change
  2. Addition of a common ion
  3. Complex ion formation
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19
Q

What are the methods of determination of solubility product?

A
  • Conductivity measurements
  • Electrochemical cell e.m.f measurements
  • Titration method
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20
Q

Describe an experiment to determine the solubility product of calcium iodate, Ca(IO3)2

A
  • To a known volume of distilled water of in a glass bottle and its temperature T determined using a thermometer.
  • To the water a large excess of calcium iodate is added, the bottle stoppered and the mixture vigorously shaken until no more salt dissolves.
  • The mixture is left to stand for an hour for the
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21
Q

What are the methods used to determine solubility product?

A

Conductivity measurements
Electrochemical cell e.m.f measurements
Titration method

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22
Q

Describe the experiment to determine the solubility product of calcium iodate, Ca(IO3)2.

A

To a known volume of distilled water in a glass bottle and its temperature T determined using a thermometer.
To the water a large excess of calcium iodate is added, the bottle stoppered and the mixture vigorously shaken until no more salt dissolves.
The mixture is left to stand for an hour for the equilibrium to be established between the calcium iodate and its ions.
The mixture is then carefully filtered to obtain a saturated solution of calcium iodate at temperature T.
The electrical conductivity, K of the saturated solution is measured using a conductometer.
The molar conductivity at infinite dilution of calcium and iodate ions are obtained from tables and they are used to calculate the molar conductivity at infinite dilution of calcium iodate as follows: Λ∞ Ca(IO3)2 = Λ∞ Ca2+ + 2(Λ∞ IO3-)

Example: Λ∞ Ca2+ = 150 S cm2 mol-1, Λ∞ IO3- = 120 S cm2 mol-1

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23
Q

Explain why Lead (II) chloride is only sparingly soluble in cold water, even less soluble in dilute hydrochloric acid but readily dissolves in concentrated hydrochloric acid?

A

Lead(II) chloride is sparingly soluble in water its hydration energy is not high enough to offset its high lattice energy.
In dilute hydrochloric acid the solubility decreases due to the common ion effect of the chloride ions from dilute hydrochloric acid which shifts the equilibrium of lead(II) chloride to the left causing more precipitation of lead(II) Chloride out of solution.
In concentrated hydrochloric acid, the lead(II) chloride reacts with the high concentration of chloride ions from the acid forming a soluble complex of tetrachloroplumbate(II).

Example: PbCl2(s) + 2Cl-(aq) ⇌ PbCl4^2-(aq)

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24
Q

Explain why Calcium phosphate is only slightly soluble in water but readily dissolves in dilute hydrochloric acid?

A

Calcium phosphate is sparingly soluble in water its hydration energy is not high enough to offset its high lattice energy.
In dilute hydrochloric acid, the hydrogen ions from the acid react with phosphate ions from the calcium phosphate forming phosphoric acid, a weak acid which largely exist as molecules in solution.

Example: Ca3(PO4)2(s) + 6HCl(aq) → 6Cl-(aq) + 6H+(aq) + 2H3PO4(aq)

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25
Q

Explain why Silver chloride is only slightly soluble in water, less soluble in dilute hydrochloric acid but readily dissolves in excess ammonia solution?

A

Silver chloride is sparingly soluble in water its hydration energy is not high enough to offset its high lattice energy.
In dilute hydrochloric acid the solubility decreases due to the common ion effect caused by the chloride ions from dilute hydrochloric acid which shifts the equilibrium of silver chloride from right to the left causing more precipitation of silver chloride out of solution.
In ammonia solution, the silver ions from silver nitrate react with ammonia molecules forming a soluble complex of silver(I) diammine.

Example: AgCl(s) + 2NH3(aq) → Ag(NH3)2+(aq)

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26
Q

Why is Copper(II) hydroxide insoluble in water but readily dissolves in excess ammonia?

A

Copper(II) hydroxide is insoluble in water because its hydration energy is not high enough to offset its lattice energy.

In ammonia solution, the copper(II) ions from copper(II) hydroxide react with ammonia molecules forming a soluble complex of tetraamminecopper(II) ions. This reduces the concentration of copper(II) ions and shifts the equilibrium of copper(II) hydroxide from the left to right causing more copper(II) hydroxide to dissolve and ionise. The copper(II) ions are removed by ammonia molecules as soon as they form causing all the copper(II) hydroxide to dissolve if there is enough ammonia solution.

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27
Q

What assumption in made in the ionic product of water?

A

Water is a pure liquid and is in large excess meaning that its concentration remains constant and not included in the Kw expression.

None

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28
Q

Define ionic product of water

A

The ionic product of water, Kw is the product of the molar concentrations of the hydronium and hydroxide ions from self-ionization of water.

None

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29
Q

Define pH

A

The pH of a solution is the negative logarithm to the base ten of the molar hydrogen ion concentration.

None

30
Q

What is the relationship between pKw, pH and pOH?

A

pKw = pH + pOH.

None

31
Q

The pKw for water is 14.3 at 288 K and 14.0 at 298. Explain whether the ionisation of water is exothermic or endothermic.

A

The ionization of water is endothermic since pKw decreases with increase in temperature meaning that more ionizes as temperature increases.

None

32
Q

What is degree of ionization?

A

The fraction of the originally undissociated molecules that will have dissociated into ions.

None

33
Q

What assumption is made in the dissociation of weak monobasic acids?

A

The concentration of water remains constant in dilute solutions and not included in the Ka expression.

None

34
Q

True or false: The larger the Ka value the lower the degree of ionisation and the weaker the acid.

A

False. The larger the Ka value the greater the degree of ionisation and the stronger the acid.

None

35
Q

Why is Chloroethanoic acid a stronger acid than ethanoic acid?

A

In chloroethanoic, the chlorine atom exerts a negative inductive effect (-I). The chlorine atom being highly electronegative it has a strong attraction for electrons in the C-Cl bond. As the chlorine draws the bonding electrons towards itself, the oxygen-hydrogen bond (O-H) is weakened and readily broken to release hydrogen ions. This makes chloroethanoic acid a better proton donor and stronger acid.

In ethanoic acid, CH3COOH, the methyl group exerts a positive inductive by pushing electrons towards the oxygen-hydrogen bond. This increases the electron density and the strength of the oxygen-hydrogen bond. Therefore ethanoic acid is a poor proton donor and a weaker acid.

36
Q

Why is dichloroethanoic acid a stronger acid than chloroethanoic acid?

A

The two chlorine atoms in dichloroethanoic acid exert a stronger negative inductive effect than the one chlorine atom of chloroethanoic acid. Therefore, more electrons are pulled away from the carboxylic group in dichloroethanic acid than in ethanoic acid. This means the oxygen-hydrogen bond is more weakened in dichloroethanoic acid making it a better proton donor and a stronger acid than chloroethanoic acid.

None

37
Q

Why is ethanoic acid is a weaker acid than methanoic acid?

A

In ethanoic acid, the methyl group exerts a positive inductive effect by pushing electrons towards the oxygen-hydrogen bond. This increases the electron density and the strength of the oxygen-hydrogen bond.

None

38
Q

Why is ethanoic acid is a weaker acid than methanoic acid?

A

In ethanoic acid, the methyl group exerts a positive inductive effect by pushing electrons towards the oxygen-hydrogen bond.
This increases the electron density and the strength of the oxygen-hydrogen (O- H) bond making it a poor proton donor and a weaker acid.

Example sentence: Ethanoic acid, also known as acetic acid, is commonly found in vinegar.

39
Q

Why is Propanoic acid is a weaker acid than ethanoic acid?

A

The alkyl group of propanoic acid is longer and exerts a stronger positive inductive effect than the alkyl group of ethanoic acid.
Therefore, more electrons are released by alkyl group towards the carboxyl group in propanoic acid than in ethanoic acid.
This means the oxygen-hydrogen bond is stronger in propanoic acid making it a poor proton donor than ethanoic acid.

Example sentence: Propanoic acid is used in the production of pharmaceuticals and food preservatives.

40
Q

Why is Benzoic acid is a stronger acid than aliphatic acids e.g. ethanoic acid?

A

This is because the negative charge on the carboxylate group (–CO2-) group in benzoic acid is delocalized by interaction with the pi-electron cloud of the benzene ring and is less available for attracting a proton.
This weakens the oxygen – hydrogen (O-H) bond in the carboxyl group of benzoic acid making it a stronger proton donor (stronger acid) than aliphatic acids.

Additional information: Benzoic acid is commonly used as a food preservative.

41
Q

Explain why Methyl amine is a stronger base than ammonia?

A

The methyl group in methylamine exerts a positive inductive effect on the rest of the molecule by pushing electrons towards the nitrogen atom.
This increases the electron density around the nitrogen atom making more nucleophilic and a stronger proton acceptor; hence a stronger base.

Example sentence: Methylamine is used in the production of certain pharmaceuticals.

42
Q

Why is Trimethylamine a weaker base than methylamine?

A

Trimethylamine has three alkyl groups bonded to nitrogen which overcrowd the nitrogen atom causing steric hindrance.
Therefore, despite the higher electron density on the nitrogen atom, the lone pair of electrons on the nitrogen atom is less available for accepting protons making trimethylamine a weaker base.

Additional information: Trimethylamine is responsible for the fishy odor associated with spoiled fish.

43
Q

Why is Phenylamine a weaker base than ammonia?

A

In phenhylamine, the lone pair of electrons on the nitrogen atom interacts with delocalised pi electrons of the benzene ring making it less available for accepting protons.
Hence phenyl amine is a poor proton acceptor and a weaker base.

Example sentence: Phenylamine is used in the production of dyes and pharmaceuticals.

44
Q

Why is Ethylamine is a stronger base than methylamine?

A

The ethyl group in ethylamine is longer and exerts a more positive inductive effect than the methyl group in methylamine.
Therefore, the electron density around the nitrogen atom is higher in ethylamine making it a better proton acceptor and a stronger base than methylamine.

Example sentence: Ethylamine is used in the synthesis of various chemicals and pharmaceuticals.

45
Q

Explain why the pKb values of equimolar solutions of diethylamine and ethylamine are 3.01 and 3.27 respectively.

A

Diethylamine has a smaller pKb value and therefore a stronger base than ethylamine.
This is because dimethylamine has two ethyl groups directly bonded to nitrogen compared to ethylamine which has one.
The two alkyl groups in diethylamide exert more positive inductive effect by pushing electrons towards the nitrogen than the one ethyl group of ethylamine.
Therefore the electron density on the nitrogen atom is higher in dimethylamine making it a better proton acceptor and a stronger base than ethylamine.

Example sentence: Diethylamine is used as a catalyst in various chemical reactions.

46
Q

What is a buffer solution?

A

This is one that can resist change in pH due to addition of small amounts of acid or alkali.

Additional information: Buffer solutions are commonly used in laboratory experiments and in various industrial processes.

47
Q

What is an acidic buffer?

A

Buffer solution consisting of a weak acid and a strong salt of the acid e.g. ethanoic acid and sodium ethanoate

Example sentence: An acidic buffer is used in biochemistry labs to maintain a stable pH for enzymatic reactions.

48
Q

What is a Basic buffer?

A

Buffer solution consisting of a weak base and a strong salt of the base e.g. ammonia and ammonium chloride

Example sentence: A basic buffer is often used in analytical chemistry to control the pH of solutions.

49
Q

How does an ethanoic acid-sodium ethanoate buffer solution work?

A

An ethanoic acid-sodium ethanoate buffer solution works by the equilibrium between the weak acid (ethanoic acid) and its conjugate base (sodium ethanoate) which helps maintain a stable pH when small amounts of acid or base are added.

Example sentence: The ethanoic acid-sodium ethanoate buffer solution is commonly used in biochemical and pharmaceutical research.

50
Q

What is a buffer solution?

A

One that can resist change in pH due to addition of small amounts of acid or alkali.

Example: Buffer solutions are commonly used in laboratory experiments.

51
Q

What is an acidic buffer?

A

Buffer solution consisting of a weak acid and a strong salt of the acid e.g. ethanoic acid and sodium ethanoate.

Additional Information: Acidic buffers help maintain a stable pH in various chemical reactions.

52
Q

What is a Basic buffer?

A

Buffer solution consisting of a weak base and a strong salt of the base e.g. ammonia and ammonium chloride.

Additional Information: Basic buffers are essential in biological systems to regulate pH levels.

53
Q

How does an ethanoic acid-sodium ethanoate buffer solution work?

A

Addition of a small amount of hydrogen ions (from the acid) is controlled by ethanoate ions from the salt when the two react together forming molecules of ethanoic acid. Hence the pH is kept constant.

Addition of a small amount of hydroxide ions from an alkali is controlled by ethanoic acid when the two react to form water molecules. Therefore the pH is kept constant.

Example Sentence: The ethanoic acid-sodium ethanoate buffer solution maintained the pH level during the chemical reaction.

54
Q

How does an ammonia-ammonium chloride buffer solution work?

A

Addition of a small amount of hydrogen ions from an acid is controlled by ammonia when the two react to form ammonium ions.

Addition of a small amount of hydroxide ions from an alkali is controlled by ammonium ions from the salt when the two react together forming ammonia and water molecules. Hence pH kept constant.

Example Sentence: The ammonia-ammonium chloride buffer solution effectively regulated the pH in the lab experiment.

55
Q

State the applications of buffers

A
  1. Used in bacteriological research to maintain the pH of culture media used for growth of bacteria.
  2. The hydrogencarbonate – carbonic acid buffer system is used to maintain the pH of human blood.
  3. Buffer solutions are used to maintain the optimal pH for enzyme activity that take place in many organisms.
  4. Used in the fermentation process to maintain the optimal pH needed by the enzymes that cause fermentation.

Additional Information: Buffers play a crucial role in various scientific processes.

56
Q

Define salt hydrolysis

A

The reaction between a salt and water forming an acidic or alkaline solution.

Salt hydrolysis is the reverse of neutralization.

Example: Salt hydrolysis is a key concept in understanding the behavior of salts in solutions.

57
Q

What is the relationship between Kh, Ka and Kw

A

Kh × Ka = Kw and Kh = Kw / Ka.

Note: Kh = Kb of the salt (or conjugate base)

58
Q

What are Acid-base indicators?

A

Substances that change colour according to the hydrogen ion concentration (pH) of the solutions to which they are added.

Example: Acid-base indicators are commonly used in titration experiments.

59
Q

Give the uses of indicators

A

1) To test for acidity or alkalinity of solutions
2) To detect endpoints in acid-base titrations.

Additional Information: Indicators are valuable tools in analytical chemistry.

60
Q

What factors should influence choice of indicator?

A

a) The value of pKi should be as close as possible to the value of the pH of the solution at the endpoint.

This allows the colour change to occur as closely as possible to the equivalence point.

b) The indicator working pH range should lie on the vertical part of the pH curve.

Example: Selecting the right indicator is crucial for accurate titration results.

61
Q

Explain the graph obtained when titrating a strong acid with a strong alkali e.g. hydrochloric acid with sodium hydroxide

A

At point A (before addition of sodium hydroxide) the pH is very low because hydrochloric acid a strong acid which completely ionizes forming a high concentration of hydrogen ions.
Along AB there is a gradual increase in pH due to neutralization of the acid and increase in volume of the solution.
This causes a decrease in the hydrogen ion concentration of the solution and consequently increasing the pH.
At point B the neutralization is complete and the pH at the equivalence point is 7 because the salt formed (sodium chloride) is not hydrolysed since it is derived a strong acid and a strong base.
The sharp increase in the pH along BC, on addition of a small volume of sodium hydroxide, is because all the acid has been neutralized.
Along CD there is a gradual increase in pH due to excess sodium hydroxide solution added.
The final pH is high because the excess sodium hydroxide is a strong alkali which is completely ionised.

Example: The titration graph of hydrochloric acid with sodium hydroxide showed the expected pH changes during the process.

62
Q

Explain the shape of the graph obtained when titrating a strong alkali with a strong acid e.g. sodium hydroxide with hydrochloric acid

A

At point A (before addition of hydrochloric acid) the pH is very high (about 13) because sodium hydroxide is a strong alkali which completely ionizes forming a high concentration of hydroxide ions.
Along AB there is a gradual decrease in pH due to neutralization of some of the alkali and increase in volume of the solution.
This causes a decrease in the hydroxide ion concentration of the solution and consequently decreasing the pH.
At point B the neutralization is complete.
The pH at the equivalence point is 7 because the salt formed (sodium chloride) is not hydrolysed by water.
The sharp decrease in the pH along BC, on addition of a small volume of hydrochloric acid, is because all the alkali has been neutralized.
Along CD there is a gradual decrease in pH due to excess hydrochloric acid added.
The final pH is very low because the excess hydrochloric acid is

Example: The titration graph of sodium hydroxide with hydrochloric acid displayed the pH variations throughout the experiment.

63
Q

Explain the shape of the graph obtained when titrating a weak acid with a strong alkali e.g. ethanoic acid with sodium hydroxide

A

At point A (before addition sodium hydroxide solution) the pH is about three (3) because ethanoic acid is a weak acid which partially ionizes forming few hydrogen ions.

Example sentence: The pH at point A was measured to be approximately 3.

64
Q

Explain the shape of the graph obtained when titrating a weak acid with a strong alkali e.g. ethanoic acid with sodium hydroxide

A

Along AB the pH gradually increases as the acid is being neutralised by sodium hydroxide.

Example sentence: The pH increased steadily along the AB portion of the graph.

65
Q

Explain the shape of the graph obtained when titrating a weak acid with a strong alkali e.g. ethanoic acid with sodium hydroxide

A

The increase in pH is gradual due formation of ethanoic acid – sodium ethanoate buffer solution which resists change in pH.

Example sentence: The buffer solution helped maintain a stable pH during the titration.

66
Q

Explain the shape of the graph obtained when titrating a weak acid with a strong alkali e.g. ethanoic acid with sodium hydroxide

A

At point B the neutralization is complete.

Example sentence: The neutralization was confirmed to be complete at point B.

67
Q

Explain the shape of the graph obtained when titrating a weak acid with a strong alkali e.g. ethanoic acid with sodium hydroxide

A

The pH at the equivalence point is greater than seven (7) because the salt formed (sodium ethanoate) undergoes hydrolysis forming an alkali.

Example sentence: The pH at the equivalence point was found to be higher than 7.

68
Q

Explain the shape of the graph obtained when titrating a weak acid with a strong alkali e.g. ethanoic acid with sodium hydroxide

A

Along BC there is a sharp increase in pH on addition of a small volume of sodium hydroxide because all the acid has been neutralized.

Example sentence: The sharp increase in pH indicated complete neutralization of the acid.

69
Q

Explain the shape of the graph obtained when titrating a weak acid with a strong alkali e.g. ethanoic acid with sodium hydroxide

A

Along CD there is a gradual increase in pH due to excess sodium hydroxide solution added.

Example sentence: The gradual increase in pH along CD was due to the addition of excess sodium hydroxide.

70
Q

Explain the shape of the graph obtained when titrating a weak acid with a strong alkali e.g. ethanoic acid with sodium hydroxide

A

The final pH (at point D) is high because the excess sodium hydroxide is a strong alkali which is completely ionised.

Example sentence: The final pH reading at point D was indicative of the strong alkali presence.

71
Q

Explain the shape of the graph obtained when titrating a weak acid with weak alkali e.g. ammonia with ethanoic acid

A

At point A the pH is about 3 because ethanoic acid is a weak acid which partially ionized forming few hydrogen ions.

Example sentence: The pH at point A is approximately 3 due to the weak acid ethanoic acid.

72
Q

Explain the shape of the graph obtain when titrating a weak alkali with weak acid e.g. ethanoic acid with ammonia

A

At point A the pH is about 12 because ammonia is a weak alkali which partially ionized forming a low concentration of hydroxide ions.

Example sentence: The pH at point A is around 12 due to the weak alkali ammonia.

UACE Chemistry (27 decks)

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