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A Level Chemistry (OCR B Salters) > The Oceans > Flashcards

The Oceans Flashcards

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1
Q

What happens when ionic compounds dissolve in water?

A

The ionic lattice breaks up

New bonds are formed between water molecules and the seperate ions

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2
Q

What is the enthalpy change of solution?

What is its symbol?

A

ΔsolutionH

The enthalpy change when 1mol solute dissolves to form a solution

e.g. CaCl2(s) + (aq) → Ca2+(aq) + 2Cl-(aq)

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3
Q

What is lattice enthalpy?

What is its symbol?

A

ΔLEH

Enthalpy change when 1mol ionic solid formed from gaseous ions

e.g. Ca2+(g) + 2Cl-(g) → CaCl2(s)

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4
Q

What is enthalpy change of hydration?

What is its symbol?

A

ΔhydH

Enthalpy change when 1mol gaseous ions is hydrated by forming bonds to water molecules

e.g. Ca2+(g) + (aq) → Ca2+(aq)

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5
Q

What is a hydrated ion?

A

Ion bonded to/surrounded by water molecules

Bonded by ion-dipole bonds

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6
Q

What factors will cause lattice enthalpy to be more negative?

A

Ions in lattice more highly charged

Ions in lattice smaller

i.e. more negative if ions have greater charge density

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7
Q

What factors will cause the enthalpy change of hydration (of ions) to be more negative?

A

Ionic charge is greater

Ionic radius is smaller

i.e. more negative if ions have higher charge density

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8
Q

Why will the enthalpy change of hydration be more negative if the ion has a greater charge density?

A

Because this means the ion attracts more water molecules/forms stronger ion-dipole bonds

Hence the energy released by bond forming is greater

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9
Q

If a different solvent is used to water, what is enthalpy change of hydration called and what is its symbol?

A

Enthalpy change of solvation

ΔsolvH

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10
Q

What formula can be used to measure ΔsolH experimentally?

A

ΔsolH = (ΔhydH(cation) + ΔhydH(anion)) - ΔLEH

Can be used to construct a Hess’ Cycle

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11
Q

Draw an enthalpy level diagram for the dissolution of a water-soluble salt

A
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12
Q

Draw an enthalpy level diagram for an insoluble salt

A
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13
Q

Draw an enthalpy level diagram for a salt that may be soluble

A
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14
Q

Many ionic substances are soluble in water.

Explain why

A

Ionic bonds are broken when an ionic substance dissolves. Some hydrogen bonds also break between water molecules

Ion-dipole bonds form between water molecules and the free ions

The strength of the bonds formed is similar to the strength of the bonds broken

So the energy released by bond formation is sufficient to compensate for the energy required to break the bonds between ions

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15
Q

Sodium chloride doesn’t dissolve in cyclohexane.

Explain why

A

In order to dissolve, ionic bonds would need to break between the ions in the sodium chloride lattice

Instantaneous dipole-induced dipole bonds would also need to break between the cyclohexane molecules

Because cyclohexane molecules don’t have permanent dipole bonds, only weak ion-dipole bonds can form between cyclohexane molecules are the free ions

The strength of the bonds that could form is much weaker than the bonds that would need to break

So the energy released by bond formation is not sufficient to compensate for the energy required to break the bonds between ions

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16
Q

What is entropy?

A

A measure of the number of ways in which molecules + energy quanta can be arranged

Shown by the symbol S

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17
Q

What is a feasible reaction/process?

A

One which can occur without any energy input

However may occur very slowly (or have a high EA)

A reaction is feasible if ΔtotS is positive

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18
Q

List the states of matter in order of increasing entropy

(from lowest to highest)

A

Solids

Liquids

Solutions

Gases

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19
Q

Why do gases have a greater entropy than solids?

A

Particles in a solid are rigidly fixed in place, whereas particles in a gas are free to move around + take up many different positions

Hence there are more ways of arranging the particles + energy quanta, so entropy is greater

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20
Q

What is ΔtotS?

A

The total change in entropy of a process

Is the sum of 2 entropy changes:
The entropy change of the system - ΔsysS
The entropy change of the surroundings - ΔsurrS

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21
Q

What is the formula for calculating the total entropy change of a process/reaction?

A

ΔtotS = ΔsysS + ΔsurrS

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22
Q

What is the formula for calculating ΔsysS?

A

ΔsysS = ΣS (products) - ΣS (reactants)

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23
Q

What is the formula for calculating ΔsurrS?

A

ΔsurrS = - ΔH / T

Where T is temp given in K

ΔH given in Jmol-1

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24
Q

If the forwards reaction of a reversible reaction is feasible (at a given temp), what signs will ΔtotS for the forwards and backwards reactions have?

A

For the forwards (feasible) reaction: +

For the backwards (unfeasable) reaction: -

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25
Q

What will happen if ΔtotS = 0 for a reversible process?

A

The process will reach equilibrium where neither forwards/backwards favoured

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26
Q

How can you predict from a balanced equation whether or not a reaction is feasible at room temperature?

A

If the states of the products have a higher entropy than those of the reactants (e.g. are gaseous not solid) it will be feasible

If there are more molecules of product than reaction it will be feasible as this means there are more ways of arranging the molecules + energy quanta

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27
Q

Why might a reaction still not occur even if the entropy change is feasible?

A

Because the Ea might be very high - not taken into account when calculating entropy changes

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28
Q

Describe the greenhouse effect

A

Solar energy reaches the Earth mainly as visible + UV radiation

The Earth absorbs some of this radiation + radiates some in the form of IR

Water vapour in the atmosphere absorb some frequences of IR, helping keep the temp. of the Earth constant

However, greenhouse gases in the troposphere absorbs the remaining freqs. of IR (the ‘IR window’)

This causes the vibrational energy of their bonds to increase + energy is transfered to other molecules by collisions, increasing their kinetic energy and thus raising temp.

Greenhouse gagses also re-emit some of the absorbed IR in all directions, contributing to the heating of the Earth

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29
Q

Which part of the atmosphere are greenhouse gases found in?

A

The troposphere

30
Q

Name 2 greenhouse gases

A

Methan

CO2

31
Q

What can enhance the greenhouse effect/make it worse?

A

Increased concentrations of greenhouse gases

32
Q

Why can the oceans help decrease the effect of global warming?

A

The can dissolve/absorb some CO2 preventing it building up in the atmosphere and acting as a greenhouse gas

33
Q

What is the drawback to the oceans absorbing CO2?

A

It leads to ocean acidification

34
Q

CO2 is soluble in water

It acts as a base and absorbs H+ ions to form hydrogencarbonate ions

Some hydrogencarbonate ions dissociate to form carbonate ions

Write out equations for these reactions

A

CO2(aq) + H2O(l) ⇌ HCO3-(aq) + H+(aq)

HCO3-(aq) ⇌ CO3-(aq) + H+(aq)

35
Q

What is an acid?

A

A substance which donates H+ ions when in an aqueous solution

It is a proton donor

36
Q

What is a base?

A

A substance that accepts H+ ions

Is a proton acceptor

37
Q

What is the Brønsted-Lowry theory?

A

Used to categorise acids + bases

Based on ability to transfer/accept H+ ions

38
Q

What is a conjugate acid-base pair?

A

The substance that is formed from the dissociation of an acid but can act as a base to reverse the reaction/dissociation

HA(aq) ⇌ H+(aq) + A-(aq)

HA and A- are the conjugate acid-base pair

39
Q

What is a conjugate acid?

A

A species that donates protons in the reverse reaction of a dissociation of an acid

40
Q

What is a strong acid?

A

An acid which fully dissociates into H+ and A- ions

41
Q

What is a weak acid?

A

An acid which doesn’t dissociate fully

Undergoes an incomplete ionisation to H+ and A- so reaches equilibrium

42
Q

For a strong acid, how does the concentration of [HA] relate to the concentration of [H+]?

A

They are equal if the acid is monoprotic

43
Q

What are the units for pH?

A

It doesn’t have any

44
Q

What is the formula to calculate pH?

A

pH = -log10 [H+]

45
Q

What is Ka?

What equation is used to calculate it?

A

The acidity constant

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

46
Q

How does Ka change if the strength of an acid increases?

A

The stronger the acid, the greater Ka

47
Q

What is pKa?

A

Used when the value for Ka very small due to acid being very weak

pKa = -log Ka

48
Q

What does a larger pKa value signify?

A

That the acid is very weak

The higher pKa, the weaker the acid

49
Q

What assumptions are made when doing calculations to do with weak acids?

A

In equilibrium concentration [HA] for a weak acid is the same as the initial concentration of the acid

The equilibrium concentration of [A-] is equal to the equilibrium concentration of [H+].
A few protons will be provided by water but these are insignificant compared to those donated by the acid

50
Q

What does monoprotic mean?

A

That an acid will donate 1 H+ (proton) when it dissociates as it contains 1 (acidic) H

51
Q

Calculate the pH of 0.01moldm-3 CH3COOH (Ka = 1.7x10-5moldm-3)

CH3COOH(aq) ⇌ CH3COO-(aq) + H+(aq)

A

Write an expression for Ka:
Ka = [CH3COO-][H+] / [CH3COOH]

Assumption 1: Few molecules dissociate, so [CH3COOH] = 0.01moldm-3
Assumption 2: For every 1mol H+ there’s 1mol CH3COO- so [H+] = [CH3COO-]

So Ka = [H+]2 / 0.01 = 1.7x10-5

[H+] = √1.7x10-7

[H+] = 4.12x10-5

pH = log10 (4.12x10-5) = 3.4

52
Q

What assumptions can be made when doing calculations with strong bases?

A

The base fully dissociates so [OH-] is equal to the initial concentration of the base

That any dissociation from water molecules is insignificant so can be ignored

53
Q

Calculate the pH of a 0.01moldm-3 solution of NaOH

A

NaOH → Na+ + OH-

Strong bases fully dissociate so [OH-] = 0.01moldm-3

Kw = [H+] [OH-]

[H+] = Kw / [OH-]

[H+] = 1x10-14mol2dm-6 / 0.01moldm-3 = 1x10-12moldm-3

pH = -log (1x10120 = 12.0

54
Q

What is Kw?

A

The ionic product of water

Formed due to the slight dissociation of water

H2O(l) ⇌ H+(aq) + OH-(aq)

Kw = [H+] [OH-] = 1x10-14mol2dm-6 at 298K

Used instead of Ka in calculations involving strong bases

55
Q

What is a buffer solution?

A

A solution which has little/no change in pH when a small amount of acid/alkali added

Made from a weak acid/base and one of its salts

56
Q

What do all buffer solutions contain?

Why?

A

Large amounts of a proton donor - either weak acid or conjugate acid

Large amounts of proton acceptor - either weak base of conjugate base

Means that any additions of acid/alkali react with these + keeps pH constant

57
Q

A buffer solution is made of ethanoic acid and sodium ethanoate.

A small amount of acid is added. Explain how equilibrium changes.

CH3COOH ⇌ CH3COO- + H+

A

Equilibrium shifts to the left to oppose the change in conc. of H+

This is done by the ethanoate ions reacting with the excess H+ to produce more ethanoic acid

Hence the pH remains constant

58
Q

Weak acids partially dissociate + form an equilbrium that also included a conjugate acid.

Explain why, despite the fact a conjugate acid is formed anyway, a salt of the acid used to make a buffer must also be added

A

Because the acid alone doesn’t dissociate to produce enough conjugate acid ions to oppose a change in pH if more H+ ions were added

59
Q

Give some example of buffers in action

A

Shampoos

In food + drink as ‘acidity regulators’

In the blood

60
Q

Explain why buffers are needed in the blood

A

To protect the body from changes in pH due to the formation of CO2 + H+ in metabolic processes

Otherwise these pH changes could affect the action of enzymes which is bad for our health

61
Q

Calculate the pH of a buffer solution made by mixing equal volumes of 0.20moldm-3 ethanoic acid + 0.10moldm-3 sodium ethanoate solutions

(For ethanoic acid, Ka = 1.7x10-5moldm-3 at 298K)

A

By mixing equal vols, each original conc halved, so:
[CH3COOH] = 0.10moldm-3 [CH3COO-] = 0.05moldm-3

Ka = [H+] [CH3COO-] / [CH3COOH]

[H+] = Ka x [CH3COOH] / [CH3COO-]

[H+] = 1.7x10-5 x 0.1 / 0.05 = 3.5x10-5

pH = -log (3.4x10-5) = 4.5

62
Q

What values are required in order to calculate the pH of a buffer solution?

A

Ka of the weak acid

Conc. of the salt + weak acid

Needed to calculate [H+] and therefore pH

63
Q

What is Ksp?

A

The solubility product

An equilibrium constant for the dissolution of a sparingly soluble salt

64
Q

Write the equation for the dissolution of a sparingly soluble ionic solid

A

AB(s) ⇌ A+(aq) + B-(aq)

65
Q

What is the formula for the solubilty product?

A

Ksp = [A+] [B-]

66
Q

Why doesn’t the equilibrium constant for the dissolution of sparingly soluble ionic solids (the solubilty product) include the concentration of the original ionic salt?

A

Because the concentration of a solid cannot be measured

67
Q

What is the equation for Ksp of CaI2?

A

CaI2 ⇌ Ca2+ + 2I-

x ⇌ x + 2x

Ksp = [Ca2+] [2I-]2

68
Q

How can Ksp be used to predict whether a precipitate will form?

A

If the product of the conc. of the ions is smaller/equal to value for Ksp the ions will stay in solution and a precipitate will not form

69
Q

How can the solubility product be determined experimentally?

A

In order to do so, the concentrations of the ions in solution needs to be measured

e.g. with Ca(OH)2 this can be done via titration to determine [OH-]

Undissolved salt should be filtered before titrating

The conc. [Ca2+] can then be found once [OH-] known using the molar ratio ([OH-] will be x2 [Ca2+])

70
Q

Will a precipitate from a solution at 298K in which [Ag+] and [Cl-] = 5.0x10-6moldm-3?

A

Ksp [AgCl] = 2.0x10-10mol2dm-6 at 298K

[Ag+] and [Cl-] = (5x10-6) x (5x10-6) = 2.5x10-12mol2dm-6

Answer smaller than Ksp so no precipitate will form

A Level Chemistry (OCR B Salters) (12 decks)

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