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Chemistry A2 > Thermodynamics > Flashcards

Thermodynamics Flashcards

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1
Q

Standard Enthalpy Change

A

Change in heat energy under standard conditions, i.e. 298K and 100kPa

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2
Q

Standard Enthalpy of Formation

A

Enthalpy change when one mole of a compound is formed from its constituent elements all in their standard states under standard conditions

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3
Q

Standard Enthalpy of Combustion

A

Enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions

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4
Q

Enthalpy of Atomisation

A

Enthalpy change when one mole of gaseous atoms is formed from an element in its standard state

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5
Q

First Ionisation Energy

A

Enthalpy change when one mole of gaseous atoms each lose an electron to form one mole of gaseous ions with a single positive charge

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6
Q

First Electron Affinity

A

Enthalpy change when one mole of gaseous atoms each acquire an electron to form one mole of gaseous ions with a single negative charge

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7
Q

Lattice Enthalpy of Formation

A

Enthalpy change when one mole of solid ionic compound is formed from its gaseous ions infinitely far apart

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8
Q

Lattice Enthalpy of Dissociation

A

Enthalpy change to separate one mole of solid ionic compound into its gaseous ions infinitely far apart

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9
Q

Enthalpy of Hydration

A

Enthalpy change when one mole of gaseous ions form aqueous ions

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10
Q

Enthalpy of Solution

A

Enthalpy change when one mole of an ionic compound completely dissolves in sufficient water to form an infinitely dilute solution

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11
Q

Mean Bond Enthalpy

A

Enthalpy change when breaking one mole of a given bond in a molecule of a gaseous species (averaged over different molecules)

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12
Q

Entropy

A

A measure of disorder of a system

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13
Q

Feasible/Spontaneous

A
  • reactions that occur on their own accord

- free energy is zero or negative

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14
Q

Gibbs Free Energy

A

Thermodynamic quantity that combines enthalpy and entropy under a constant temperature and pressure to determine the spontaneity of a reaction (direction it takes)

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15
Q

First Law of Thermodynamics

A

Energy can be neither created nor destroyed, only converted into different forms

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16
Q

Second Law of Thermodynamics

A

Entropy of an isolated system not at equilibrium will tend to increase over time

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17
Q

Third Law of Thermodynamics

A

Entropy of a perfect crystal at absolute zero is zero

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18
Q

Suggest why ionic model and actual lattice enthalpies are different

A
  • Ionic model assumes ions are spherical and arranged in a lattice
  • Calculated value is more negative suggesting stronger attraction
  • Compound has covalent character
19
Q

Suggest what can be deduced from comparisons between theoretical and actual lattice enthalpies

A 
Similar = Purely Ionic 
Different = Covalent Character
20
Q

Which types of ionic compounds have covalent character

A
  • high charge
  • small positive ion
  • large negative ion
    = negative ion polarised > more sharing > covalent character
21
Q

Suggest why successive ionisation energies get larger

A
  • negative electron is removed from a positive ion
  • requires more energy than removing from an atom
  • removing electrons from orbitals closer to the nucleus require much more energy
22
Q

Suggest why first electron affinity is negative but second is not

A
  • first electron affinity is exothermic since energy is released when an electron is added to an atom due to attraction
  • second electron affinity is endothermic since energy is required to overcome repulsion from negative ion
23
Q

How to determine Enthalpy of Solution

A
  • difficult to measure directly

- calculated using lattice dissociation enthalpy and enthalpy of hydration

24
Q

Suggest why Enthalpy of Hydration is always negative

A
  • (hydrogen) bonds are always formed between ions and water

- bond making is exothermic

25
Q

Suggest what factors result in the most negative Enthalpy of Hydration

A
  • highly charged
  • small ions
    (high charge density)
26
Q

Suggest why calcium fluoride is most likely to be found as CaF2 rather than CaF or CaF3

A
  • Ca2+ has a higher charge density than Ca+ so formation of CaF2 is more exothermic so more favourable
  • Third ionisation energy of Ca is much higher than first and second so formation of CaF3 is endothermic so less favourable than CaF2
27
Q

Why is Gibbs free energy zero at equilibrium

A

Forward and backward reaction occur at the same rate at the same time

28
Q

What is the value for Gibbs free energy when a reaction is feasible

A

zero or negative

29
Q

When is a reaction first feasible

A

When Gibbs free energy is zero

30
Q

When does entropy increase

A
  • moles of gas increase
  • complexity increases
  • more product particles
31
Q

Suggest why calculating the lattice enthalpy of sodium carbonate would be more difficult than sodium oxide

A

Carbonate ion is complex and calculating lattice enthalpy would require gaseous ions

32
Q

Suggest the purpose of constructing a Born Haber cycle

A

Calculating enthalpy changes that cannot be measured directly, e.g. Lattice Enthalpy or Enthalpy of Solution

33
Q

Born Haber Method for Lattice Enthalpy

A

Start with Enthalpy of Formation

34
Q

Born Haber Method for Enthalpy of Solution

A

Start with Lattice Enthalpy

35
Q

Explain why the entropy is zero at 0K

A
  • absolute zero = minimum energy
  • no molecular motion
  • no entropy / randomness
36
Q

Suggest why the gradient of a free energy-temperature graph changes below a certain temperature

A

Compound changes state (condenses/freezes)

37
Q

Suggest why magnesium is stable in air at room temperature despite the formation of MgO having a negative free energy value

A
  • protective layer of MgO prevents further attack on Mg
  • slow reaction
  • high activation energy
38
Q

Partial Pressure

A

Pressure that would be exerted by one of the gases in a mixture if it occupied the same volume on its own

= mole fraction x total pressure

39
Q

Mole Fraction

A

number of moles of gas/ total moles of gases

40
Q

Suggest why free energy is zero during a change in state

A
  • forward and backward reaction occur at same rate

- at equilibrium

41
Q

Suggest the condition under which diamond would have an entropy of zero

A

absolute zero/0K

42
Q

Suggest why a feasible reaction does not take place in the absence of a catalyst

A
  • high activation energy

- catalyst lowers activation energy by providing an alternative route for reaction

43
Q

Suggest what is meant by perfect ionic model

A
  • ions are spherical and arranged in lattice

- only electrostatic attraction between ions/ no covalent character

44
Q

Describe what occurs when free energy is positive for a reversible reaction

A

reaction proceeds in reverse direction

Chemistry A2 (12 decks)

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