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as chemistry > Unit 1-7 > Flashcards

Unit 1-7 Flashcards

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1
Q

What are reversible reactions?

A

Continue indefinitely
Concentrations of reactants and products remain constant

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2
Q

What is dynamic equillibrium?

A

Rate of backwards and forward reactions are equal
There is no further observable change

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3
Q

What are the steps leading to dynamic equillibrium?

A
  1. Forward reaction is fastest at the start
  2. Forward reaction slows down as reactants are used up and reverse reactions start to increase
  3. Dynamic equilibrium is reached
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4
Q

What is Le Chatelier’s principle?

A

If a system in dynamic equilibrium is subjected to a change, the position of equilibrium will shift to minimise that change

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5
Q

What is the effect of concentration on equillibrium?

A

A higher concentration of reactants will favour the forward reaction
Equilibrium moves to the right
A higher concentration of products will favour the reverse reaction
Equilibrium moves to the left

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6
Q

What is the effect of pressure on equillibrium?

A

Increasing the pressure will favour the side with fewer gaseous molecules
Equilibrium shifts to the side with fewer gaseous molecules
Number of molecules is decreased
Decreasing the pressure will favour the side with more gaseous molecules
Equilibrium shifts to the side with more gaseous molecules
Number of molecules is increased

No change when equal numbers of gaseous molecules appear on both sides

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7
Q

What is the effect of temperature on equillibrium?

A

Increase in an exothermic (-ΔH) reaction will move equilibrium to the left
Increase in an endothermic (+ΔH) reaction will move equilibrium to the right

The reverse reaction is always the opposite enthalpy (ΔH)

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8
Q

What is the effect of a catalyst on equillibrium?

A

Don’t affect the position of equilibrium
Cause equilibrium to be achieved faster

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9
Q

What is the equillibrium constant Kc?

A

A constant for concentration (moldm-3)
Given by the ratio between products and reactants
aA+bB→cC+dD
Kc=[C]c[D]d/[A]a[B]b

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10
Q

What is the effect of concentration on equillibrium constant (Kc)?

A

No effect

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11
Q

What is the effect of pressure on equillibrium constant (Kc)?

A

No effect

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12
Q

What is the effect of temperature on equillibrium constant (Kc)?

A

Kc is larger when the forward reaction is favoured

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13
Q

What do square brackets [] denote?

A

Concentration

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14
Q

What does a large value for Kc indicate?

A

The reaction is mostly on RHS and favours the products

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15
Q

What does a small value for Kc indicate?

A

The reaction is mostly on LHS and favours the reactants

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16
Q

What does a medium value for Kc indicate?

A

“Equalish” reactants and products

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17
Q

What is the unit for Kc?

A

Varies between reactions
[H2]x[Br2]/[HBr2]2
[moldm-3]x[moldm-3]/[moldm-3]2
No unit
[Cu2+]x[NH3]4/[Cu(NH3)42+]
[moldm-3]4
mol4dm-12

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18
Q

What is heterogenous equillibria?

A

A system consisting of more than one phase (state of matter)

19
Q

What is the Haber process?

A

Nitrogen is obtained from the atmosphere
Hydrogen is obtained from methane
Equation for equilibrium N2(g)+ 3H2(g) ⇄ 2NH3(g)

To separate ammonia from the reaction mixture ammonia is cooled under pressure until it condenses into other liquids
Nitric acid (NH3) is produced from the oxidation of ammonia

20
Q

What are the conditions for the Haber process?

A

400-450C°
200atm
Catalyst
Is poisoned over time and must be replaced (deactivation)

21
Q

Why is 400 - 450C° used for the Haber process?

A

Low temperature favours the forward exothermic reaction
Rate is too slow so temperature is increased

22
Q

Why is 200atm of pressure used for the Haber process?

A

High pressure is optimal, equilibrium shifts to the side with less gaseous molecules
Very high pressures are to expensive and dangerous

23
Q

What is the conversion of the Haber process?

A

Conversion of 15%

24
Q

During the Haber process, what happens to the hydrogen and nitrogen that isn’t converted?

A

Recycled back into the reaction

25
Q

What is the contact process?

A

Sulphuric acid is produced
1. S(s) + O2(g) → SO2(g)
Sulphur is oxidised to form sulphur dioxide
2. 2SO2(g) + O2(g) ⇄ 2SO3(g) ΔH-
Sulphur dioxide is further oxidised to form sulphur trioxide
3. H2SO4(l) + SO3(g) → H2S2O7(l)
SO3 is dissolved into concentrated H2SO4 to form H2S2O7
4. H2S2O7(l) + H2O → 2H2SO4
H2SO4 if formed by reacting H2S2O7 with water

26
Q

What are the conditions for the 2nd step (2SO2(g) + O2(g) ⇄ 2SO3(g) ΔH-) of the contact process?

A

400-420C°
Just above atmospheric pressure
Vanadium oxide (V2O5) is used as a catalyst

27
Q

Why is 400-420C° used for the 2nd step (2SO2(g) + O2(g) ⇄ 2SO3(g) ΔH-) of the contact process?

A

Lower temperatures favour the forward exothermic reaction
Rate is too slow so temperature is increased

28
Q

Why are pressures above atmospheric used for the 2nd step (2SO2(g) + O2(g) ⇄ 2SO3(g) ΔH-) of the contact process?

A

High pressures favour the forward reaction
Percentage yield is high anyways (99.5%)

29
Q

Describe the alternative reaction to step 3 and 4 of the contact process

A

SO3(g) + H2O(l) → H2SO4(l)
Water is chemically added to produce sulphuric acid
Can’t be done because the reaction is uncontrollable and creates a fog of H2SO4

30
Q

What is the Lowri-Brønsted theory?

A

Any compound that can transfer a proton to any other compound is an acid, and the compound that accepts the proton is a base

31
Q

What is a conjugate acid-base pairs?

A

Compounds that differ by a proton
A proton is added to get the conjugate acid
A proton is removed to get the conjugate base

H2O + H3O+

32
Q

If one species acts as an acid in any equilibrium then what does the other acts as?

A

A base
CH3COOH + H2O ⇄ CH3COO- + H3O+
Acid1 + Base2 ⇄ Base1 + Acid2

33
Q

What is an amphoteric species?

A

Can act as acids and bases
H2O → OH- + H+ or H2O + H+ → H3O+

34
Q

What is hydroxonium ion?

A

A water molecule hydrated by a proton (H+)
H3O+

35
Q

What is a strong acid?

A

Completely dissociates in aqueous solution
Every mole of H+ from HCl is transferred
Low pH around 1
HCl + H2O → Cl- + H3O+

36
Q

What is a weak acid?

A

Partially dissociates in aqueous solutio
Not every mole of H+ from CH3COOH is transferred
High pH around 4
CH3COOH + H2O ⇄ CH3COO- + H3O+

37
Q

What is a monoprotic acid?

A

Dissociation of 1 mole gives 1 mole of H+ ions
HCl → H+ + Cl-

38
Q

What is a diprotic acid?

A

Dissociation of 1 mole gives 2 moles of H+ ions
H2SO4 → 2H+ + SO42-

39
Q

How is the pH scale derived?

A

Most [H+] fall between 10^0 and 10^-14 moldm-3
pH of 1 has a [H+] of 10^-1

40
Q

How is pH calculated?

A

pH = -log10[H+]

41
Q

How is proton concentration calculated?

A

For strong acids H+ concentration can be found from the concentration of the acid (x2 for diprotic acids)
Impossible for weak acids (if pH is not known) since they do not fully dissociate
Also [H+] = 10^-pH

42
Q

What is a concentrated acid?

A

An acid which does not contain much water

43
Q

What is a dilute acid?

A

An acid which does contain much water

44
Q

How is carbon dioxide acidic?

A

H+ is produced upon reacting with H2O

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