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Year 13 Chemistry > 5.2 - Energy > Flashcards

5.2 - Energy Flashcards

1
Q

Bond Enthalpy def

A

Average energy required to break one mole of bonds in one mole of gaseous species

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2
Q

Enthalpy change of combustion def

A

The enthalpy change when 1 mole of substance is burnt completely, in excess oxygen

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3
Q

Enthalpy change of reaction def

A

Energy change when the amount in moles of the substances as written react

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4
Q

Enthalpy change of formation def

A

Enthalpy change when 1 mole of substances is formed from its constituent elements in their standard states

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5
Q

Enthalpy change of neutralisation

A

Enthalpy change when 1 mole of water is formed in a reaction between an acid and a base

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6
Q

Lattice Enthalpy def

A

The enthalpy change that accompanies the formation of 1 mole of solid ionic lattice/compound from its constituent gaseous ions

K+(g) + Cl­-(g) → KCl(g)
Delta(LE)H = ­711 kJmol­1

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7
Q

Method to determine lattice enthalpies

A

Born-Haber cycle

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8
Q

Why is lattice Enthalpy negative?

A

Bonds are being made - BENDOMEX

- reaction is exothermic

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9
Q

Atomisation def

A

Formation of one mole of gaseous atoms from constituent elements in standard states

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10
Q

Enthalpy change of 1st electron affinity def

A

When 1 mole of gaseous 1- ions is formed from 1 mole of gaseous atoms

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11
Q

The student looked in a text book and found that the actual value for the standard enthalpy change of combustion of propan­1­ol was more exothermic than the experimental value. Suggest two reasons for the difference between this value and the one he obtained experimentally.
(2 Marks)

A
  • heat losses to the surroundings
  • Incomplete combustion
  • alcohol evaporated off
  • non-standard conditions (not very usual on mark schemes anymore)
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12
Q

Enthalpy change of hydration def

A

Enthalpy change when one mole of gaseous ions dissolve in water to form an infinitely dilute solution (under standard conditions and in standard states)

K+(g) + aq —> K+(aq)

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13
Q

Enthalpy change of solution def

A

Enthalpy change when one mole of solid dissolves in a solvent to form an infinitely dilute solution (under standard conditions and in standard)

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14
Q

When do substances tend to dissolve?

In terms of enthalpy changes - exo and endo

A

When enthalpy change of solution is = 0 or -ve (exothermic)

This is only usually occuring though - some ammonia salts can dissolve during endothermic reactions

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15
Q

What factors affect enthalpy of hydration:

A

Size:

  • smaller size of ions - more exothermic
  • larger charge density ratio: form more bonds

Charge:

  • greater charge = more exothermic
  • greater charge density so can also form more bonds
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16
Q

What is entropy?

A
  • A measure of how disordered a system is
  • the number of ways that particles can be arranged
  • the dispersion of energy
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17
Q

What causes an increase in entropy?

A

The more disordered the system is or the more disordered the particles are - the higher the entropy of the system is

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18
Q

Entropies of each state/condition

A

Solid - no disorder - low entropy
Liquid - some disorder - higher entropy
Gas - lots of disorder - highest entropy

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19
Q

Effect of more particles on entropy

A

More particles = more disorder in universe = higher entropy

20
Q

Effect on arrangements of particles/compounds on entropy

A

More arrangement = less disorder = decreased entropy

21
Q

1st law of thermodynamics

A

Energy is conserved

22
Q

2nd law of thermodynamics

A

Entropy after a reaction of the universe is greater than entropy before of the products

23
Q

Entropy change of a reaction calcualtion

Is the value postive or negative?

A

Entropy change of reaction = entropy of products - entropy of reactants
Always gives a positive value

24
Q

Symbol for entropy

A

S

25
Q

Free Energy (Gibbs free energy) equation

A

Delta(G) = Delta(H) - T x Delta(S)
Rearranged - Enthalpy change = free energy + (Temp x change in entropy)
Free energy is the accessible energy that can do work

G -
H - kJ mol-1
S - J K-1 mol-1

26
Q

What must happen for a reaction to be (thermodynamically) feasible?

A

When free energy equation is rearranged:

Free energy must have a negative value

27
Q

Process of dissolving a compound - enthalpy of solution

A
  • Ionic lattice must be broken down - lattice enthalpy

- Hydration of ions - gaseous ions must bond with water molecules - enthalpy of hydration

28
Q

Relationship between enthalpy of hydration and lattice enthalpy

A

energy involved in breaking the lattice is the opposite of lattice enthalpy

29
Q

Do you expect enthalpy of hydration to be exo or endothermic?

A

Exothermic

- as bonds are being made with water and the ions involved

30
Q

Can enthalpy of solution be endo or exothermic?

A

Can be exo or endothermic

- depends on the balance between the magnitude of the lattice enthalpy and the enthalpy of hydration

31
Q

Describe how, and explain why, the enthalpy change of hydration of sodium ions differ from that of rubidium ions.
(4 Marks)

A
  • Rubidium has a larger ionic radius than Sodium
  • but has the same charge
  • so has a lower charge density
  • Rb forms a weaker attaction to the water molecules
  • Rb has a less exothermic enthalpy change of hydration
32
Q

What does the universe tend to more often in terms of entropy?

A

There is a tendency towards greater entropy

- the more ‘disordered’ a chemical system, the more energetically favourable

33
Q

Standard entropy def

A

Entropy content of one mole of substance under standard condition (in standard states)

34
Q

Standard entropy calculation

A

Entropy of products - Entropy of reactants

35
Q

When is a reaction feasible?

A

If overall energy of products is lower than overall energy of reactants

36
Q

Free energy change equation

Units?

A

DeltaG = Delta(H) - T x Delta(S)

Free energy = Enthalpy change - (Temp. x Standard Entropy)

Standard entropy - JK-1 mol-1
Enthalpy change - kJmol-1
Temperature- Kelvin

37
Q

Using the Free Energy equation, when is a reaction feasible?

A

Enthalpy change < 0

Standard enthalpy > 0

  • When one of these is within the stated bounds, the reaction is always possible depending on Temperature
  • when both are within bounds, reaction is always feasible
  • when neither are within bounds, reaction is never feasible
38
Q

Suggest why the entropy of water is zero at 0 K.

1 Mark

A

The particles are in the maximum state of order

39
Q

The enthalpy change of solution or the magnesium halides shows a trend from MgF2 to MgI2.

Why is it difficult to predict whether the enthalpy change of solution becomes more exothermic or less exothermic down the group from MgF2 to MgI2.
(4 Marks)

A

Ionic radius increases down the group (halides)
F2 has the same charge as I2
So F2 has a higher charge density
MgF2 has a stronger bond than MgI2
So lattice enthalpy higher in MgF2
So down the group, lattice enthalpy less exothermic
Enthalpy change of hydration - less exothermic
Enthalpy change of solution is made up of enthalpy change of hydration and lattice enthalpy of compound
So is difficult to predict enthalpy change of sol.
As unsure which enthalpy change (hydration or lattice enthalpy) has a larger effect

40
Q

Suggest why the second electron affinity of oxygen is positive.
(2 Marks)

A
  • Oxide/O- ion and electron (e-) are both negative
  • both have same charge so repel each other
  • energy is required to overcome the repulsion between the two
41
Q

Explain why the first ionisation energy of calcium is endothermic
(1 Mark)

A

Energy required to overcome the electrostatic forces of attraction between outer shell electrons and the nucleus

42
Q

Explain why the first electron affinity for oxygen is exothermic.
(1 Mark)

A

Electron affinity involves an electron experiencing attraction to the nucleus
Forming electrostatic forces of attractions/bonds, so is exothermic

43
Q

Explain why the second ionisation energy of calcium is more endothermic than the first ionisation energy of calcium.
(2 Marks)

A

Ca+ ion has a smaller ionic radius than Ca atoms
- less shielding from outer shell electrons
- outer shell electrons experience more nuclear attraction
More energy required to remove outer shell electrons

44
Q

What value of Gibbs free energy must be used for a reaction to be feasible?

A

When Delta(G) is negative

45
Q

Table of feasibility for Gibbs free energy

A
46
Q

What is a feasible reaction?

A 
When Delta(G)/Gibbs Free energy =< 0
Means a reaction is able to take place spontaneously
47
Q

How to work out enthalpy change of hydration via born-Haber cycles
- image of calculations

A

Year 13 Chemistry (20 decks)

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