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A2 Chemistry Unit 1 > 2.1 Rates, Equilibrium And pH > Flashcards

2.1 Rates, Equilibrium And pH Flashcards

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0
Q

What units is the rate of reaction measured in?

A

Mol dm^-3 s^-1

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1
Q

How do you work out the rate of a reaction?

A

(Change in conc of reactant or product)/(time for the change to take place)
Usually plot a graph of concentration against time, then work out the gradient of a tangent to the curve

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2
Q

What is a shorthand way of writing ‘concentration of product in mol dm^-3)?

A

[product]

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3
Q

What happens to the conc of the reactants as a reaction proceeds? Why?

A

The concs decrease:

  • Fewer collisions take place per second between reactant particles
  • The rate slows down
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4
Q

What happens to the concentration of a product as a reaction proceeds?

A

The concentration increases

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5
Q

When is the rate of reaction fastest?

A

At the beginning of the reaction

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6
Q

For reactions involving acids or bases, what can you measure in order to monitor the rate of reaction?

A
  • pH changes by carrying out titrations

- pH changes by using a pH meter

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7
Q

For reactions that produce gases, what can you measure in order to monitor the rate of reaction?

A
  • The change in volume or pressure

- The loss of mass of reactants

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8
Q

For reactions that produce visual changes, what can you observe in order to monitor the rate of reaction?

A
  • The formation of a precipitate

- A colour change (by using a colorimeter)

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9
Q

What is the order with respect to a reactant?

A

The power to which the concentration of the reactant is raised in the rate equation

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10
Q

What is the rate equation?

A

Take the reaction A + B + C —> products
Let the orders with respect to each reactant be A=0, B=1, C=2
Rate α [A]^0, rate α [B]^1, rate α [C]^2
So rate α [A]^0[B]^1[C]^2
So the rate equation is: rate = k [A]^0[B]^1[C]^2
Anything to the power of 0 is 1, so rate = k [B]^1[C]^2

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12
Q

What is the overall order of a reaction?

A

The sum of the individual orders

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13
Q

Definition of half-life of a reactant

A

The time taken for the concentration of the reactant to reduce by half

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14
Q

What happens to the concentration and half-life of a reactant with zero order?

A

Concentration decreases at a constant rate

Half-life decreases with time

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15
Q

What happens to the concentration and half-life of a reactant with first order?

A

Concentration halves in equal time intervals

Half-life is constant

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16
Q

What happens to the concentration and half-life of a reactant with second order?

A

Concentration decreases rapidly, but the rate of decrease then slows down
Half-life increases with time

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17
Q

What does the reaction rate depend on?

A

The rate constant and the concs of the reactants present in the rate equation

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18
Q

What is the key factor affecting the reaction rate?

A

The number of collisions that exceed the activation energy

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19
Q

What happens if the temp of a reaction is increased but the concs of the reactants stay the same?

A

The rate of reaction increases

The rate constant increases

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20
Q

What is the rate-determining step?

A

The slowest step in the reaction mechanism of a multi-step reaction
- The overall reaction can be no faster than the slowest step

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21
Q

How can you determine which reactants, and how many particles of each reactant, are involved in the rate-determining step?

A
  • If a reactant appears in the rate equation, that reactant is involved in the rate-determining step
  • The order with respect to the reactant tells you how many particles of the reactant are involved in the rate-determining step
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22
Q

What is an intermediate?

A

A species formed in 1 step of a multi-step reaction that is used up in a subsequent step, and is not seen as either a reactant or a product of the overall equation

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23
Q

How can you work out the equilibrium constant, (Kc), for the equation aA + bB cC + dD

A

Kc = ([C]^c x [D]^d)/([A]^a x [B]^b)

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24
Q

What are the units for Kc?

A

Depends on the Kc expression:
Unit for concs is mol dm^-3
These will cancel

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25
Q

If the value of Kc is 1, what does that mean?

A

That the position of equilibrium is halfway between reactants and products

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26
Q

If the value of Kc is greater than 1, what does that mean?

A

The reaction is product-favoured

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27
Q

If the value of Kc is less than 1, what does that mean?

A

The reaction is reactant-favoured

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28
Q

How does an increase in temperature affect the position of equilibrium?

A

It shifts it in the endothermic direction
If the forward reaction is endothermic: Kc increases
If the forward reaction is exothermic: Kc decreases

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29
Q

How does an decrease in temperature affect the position of equilibrium?

A

It shifts it in the exothermic direction

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30
Q

How does an increase in reactant concentration affect the position of equilibrium?

A

The equilibrium position shifts from left to right

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31
Q

How does an increase in pressure affect the position of equilibrium?

A

The equilibrium position shifts towards the side with fewer gaseous moles

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32
Q

How does the presence of a catalyst affect Kc?

A

It doesn’t: catalysts speed up both the forward and reverse reactions by the same factor

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33
Q

If a reaction is at a low temperature, how big are Kc and k (rate constant)?
Assume forward reaction is exothermic

A

Kc is large, but k is small

So high yield but slow reaction

34
Q

If a reaction is at a high temperature, how big are Kc and k (rate constant)?
Assume forward reaction is exothermic

A

Kc is small but k is large

Low yield but quick reaction

35
Q

What is oxidation?

A
  • The loss of electrons

- An increase in oxidation number

36
Q

What is reduction?

A
  • The gain of electrons

- A decrease in oxidation number

37
Q

What happens when an acid is added to water?

A

The acid dissociates, releasing protons into solution

38
Q

What is a monobasic acid?

A

An acid that releases 1 proton per molecule

39
Q

What is a dibasic acid?

A

An acid that releases 2 protons per molecule

- Done in 2 stages

40
Q

What is a tribasic acid?

A

An acid that releases 3 protons per molecule

- Done in 3 stages

41
Q

What is neutralisation?

A

A chemical reaction in which an acid and a base react together to produce a salt and water

42
Q

What are hydronium ions?

A

H3O^+

  • A hydrogen ion that is dative covalently bonded to a water molecule
  • Represents protons in aqueous solution
43
Q

What is an acid-base pair?

A

A set of 2 species that transform into each other by gain or loss of a proton
E.g. HNO2 + H2O >< H3O^+ NO2^-

51
Q

What relationship is there between pH and [H^+] (hydrogen ion concentration)?

A 
pH = -log[H^+]
[H^+] = 10^(-pH)
52
Q

What does a low pH value mean!

A

A large [H^+]

53
Q

If the acid is strong, how much of it will dissociate in aqueous solution?

A

100%

54
Q

What is a weak acid?

A

An acid that only partially dissociates in solution

55
Q

What is the acid dissociation constant?

A
  • An equilibrium constant that measures the actual extent of acid dissociation
  • Ka = ([H^+][A^-])/[HA] for HA >< H^+ +A^-
  • Units = mol dm^(-3)
56
Q

What does a large Ka value mean?

A

There is a large extent of dissociation, so the acid is strong

57
Q

What does a small Ka value indicate?

A

There is a small extent of dissociation, so the acid is weak

58
Q

What are the pKa and Ka conversions?

A 
pKa = -log(Ka)
Ka = 10^(-pKa)
59
Q

Which acids completely dissociate in water?

A

HCl
HNO3
H2SO4

60
Q

How can you work out [H^+] for weak acids?

A

[H^+] = square root(Ka x [HA])

61
Q

In water and neutral solutions, will the conc of H+ ions be greater than, less than or equal to the conc of OH- ions?

A

Equal to

62
Q

In acidic solutions, will the conc of H+ ions be greater than, less than or equal to the conc of OH- ions?

A

Greater than

63
Q

In alkaline solutions, will the conc of H+ ions be greater than, less than or equal to the conc of OH- ions?

A

Less than

64
Q

If you know the conc of H+ ions in a solution at 25°C, how can you work out the conc of OH- ions?

A

Kw = [H+] x [OH-]

But we also know that Kw always = 1.00 x 10^-14 mol^2 dm^-6

65
Q

How is the strength of a base measured?

A

By its dissociation in solution to generate OH- ions

- Strong bases (alkalis) dissociate more than weak bases

66
Q

What are alkalis?

A

Bases that dissociate in water to release hydroxide ions

67
Q

What is a buffer solution?

A

A mixture that minimises pH changes in addition of small amounts of acid or base

68
Q

What is a buffer solution a mixture of?

A
  • A weak acid, HA

- Its conjugate base, A^-

69
Q

How can buffer solutions be made?

A
  • From a weak acid and a salt of the weak acid
  • From partial neutralisation (using an aqueous alkali) of a weak acid to give a solution containing a mixture of the salt and the excess of weak acid
70
Q

How does a buffer solution work?

A
  • The weak acid partially dissociates, forming its conjugate base and H^+ ions (H+ and A-)
  • The salt of the weak acid completely dissociates, forming more if the conjugate base
  • This means that there is a large reservoir of weak acid, HA, and a large reservoir of conjugate base, A-. The concentration of H+ ions is very small
71
Q

How does a buffer work?

A

It minimises the pH by using the equilibrium:
HA H+ + A-
- The weak acid, HA, removes added alkali
- The conjugate base, A-, removes added acid

72
Q

What happens when an acid, H+, is added to a buffer solution?

A
  • [H+] is increased
  • The conjugate base, A-, reacts with H+ ions
  • The equilibrium shifts to the left, removing most of the added H+ ions
73
Q

What happens when an alkali, OH-, is added to a buffer solution?

A
  • [OH-] is increased
  • The small conc of H+ ions react with the OH- ions, producing H2O
  • HA dissociates, shifting the equilibrium to the right to restore most of the H+ ions that have reacted
74
Q

How can you calculate the pH of a buffer solution?

Given conc. of HA, conc of salt of HA and Ka

A
  • Only a small proportion of HA dissociates, so we can assume that HA = HA
  • The salt of HA dissociates completely, so [salt] = [A-]
    So [H+] = Ka x [HA]/[A-]
    Then pH = -log[H+]
75
Q

What is the most important buffer system for controlling the pH of blood?

A

The carbonic acid-hydrogencarbonate ion buffer

  • Carbonic acid (H2CO3) acts as the weak acid
  • HCO3- acts as the conjugate base
76
Q

What is the equivalence point?

A

The point in a titration at which the volume of 1 solution has reacted exactly with the volume of the 2nd solution

77
Q

Describe the key features of an acid-base titration curve

A
  • When the base is first added, the pH increases very slightly - the acid is in great excess
  • Within 1-2 cm^3 of the equivalence point, the pH starts to increase more quickly. There is now only a small excess of acid present
  • Eventually, there is a very sharp increase in pH brought about by a very small addition of base (usually just 1 drop)
  • This shows on the titration curve as a vertical section
  • The equivalence point is at the centre of this vertical section
  • As further base is added, there is little additional change in pH - the base is now in great excess
78
Q

How can a titration curve be made?

A
  • The pH of an acid-base titration can be data logged continuously
  • A titration curve can be plotted by a computer
79
Q

Which indicators are suitable for a strong acid-strong base titration?

A

Phenolphthalein

Methyl orange

80
Q

Which indicators are suitable for a strong acid-weak base titration?

A

Methyl orange

81
Q

Which indicators are suitable for a weak acid-strong base titration?

A

Phenolphthalein

82
Q

How is an indicator chosen for a titration?

A
  • A weak acid is used
  • Has 1 colour in its acid form and a different colour in its conjugate base form (the colour at the end point is midway between)
  • The pH value of the end point of the indicator is as close as possible to the pH value of the titration’s equivalence
  • A suitable indicator changes colour within the pH range in the vital section of the titration curve
83
Q

Define ‘end point’

A

The point in a titration at which there are equal concentrations of the weak acid and conjugate base forms of the indicator

84
Q

How can an acid-base indicator be represented?

A

HIn

  • Acid form = HIn
  • Conjugate base form = In-
85
Q

Which indicators are suitable for a weak acid-weak base titration?

A

No indicators are really suitable

  • As the base is added, the pH changes slowly through the equivalence point and there is no vertical section to the titration curve
  • An indicator would change colour gradually over a few cm^3
86
Q

What is the standard enthalpy change of neutralisation?

A

The energy change that accompanies the neutralisation of an aqueous acid by an aqueous base to form 1 mole of H2O under standard conditions

87
Q

How can you determine the enthalpy change of neutralisation by an experiment?

A
  • Measure the temperature change
  • Work out the heat gained by surroundings, using Q = mcΔT
  • Hence find the heat lost by the chemical system (change sign)
  • Work out the amount, in mol, that reacted (n = c x v)
  • Then scale the quantities to match the molar quantities needed to form 1 mole of H2O
88
Q

Why do we get the same value for enthalpy change of neutralisation for different strong acids?

A
  • Strong acids are completely dissociated
  • The actual reaction taking place has the same ionic equation:
    H^+ + OH^- —-> H2O
  • The other ions are spectator ions that do not take part in the reaction
89
Q

Why do weak acids give a less exothermic value for the enthalpy change of neutralisation using strong acids?

A
  • Weak acids are only partially dissociated
  • Although the same neutralisation reaction takes place, most of the weak acid molecules must first dissociate to release H^+
  • This requires energy, so the enthalpy change is slightly less exothermic

A2 Chemistry Unit 1 (3 decks)

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