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AQA Chemistry > Thermodynamics > Flashcards

Thermodynamics Flashcards

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1
Q

Give the definition for Standard conditions

A

Standard conditions – 298K, 100kPa, solutions of 1 mol dm-3

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2
Q

Give the definition for Standard enthalpy of reaction ΔrHθ

A

Standard enthalpy of reaction ΔrHθ – The enthalpy change when substances react under standard conditions given by the equation for the reaction.

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3
Q

Give the definition for Standard enthalpy of formation ΔfHθ

A

Standard enthalpy of formation ΔfHθ – The enthalpy change when 1 mole of a compound is formed from its constituent elements with all reactants and products in standard states under standard conditions.

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4
Q

Give the definition for Standard enthalpy of combustion ΔcHθ

A

Standard enthalpy of combustion ΔcHθ – The enthalpy change when 1 mole of a substance is burned completely in excess oxygen with all reactants and products in their standard states under standard conditions.

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5
Q

Give the definition for Mean bond energy

A

Mean bond energy – the energy required to break one mole of a covalent bond averaged across compounds containing that bond.

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6
Q

Give the definition for Enthalpy of lattice dissociation ΔLHθ

A

Enthalpy of lattice dissociation ΔLHθ– The enthalpy change when one mole of an ionic compound is separated into its component gaseous ions.

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7
Q

Give the definition for Enthalpy of lattice formation ΔfHθ

A

Enthalpy of lattice formation ΔfHθ – The enthalpy change when one mole of an ionic compound is formed from its constituent ions in the gaseous state.

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8
Q

Give the definition for Enthalpy of atomisation ΔatHθ

A

Enthalpy of atomisation ΔatHθ – The enthalpy change for the formation of one mole of gaseous atoms from the element in its standard state.

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9
Q

Give the definition for Bond dissociation energy

A

Bond dissociation energy– The enthalpy change when one mole of a covalent bond is broken under standard conditions in gaseous state.

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10
Q

Give the definition for First ionisation energy ΔIE1Hθ.

A

First ionisation energy ΔIE1Hθ - The enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form one mole of gaseous ions with a single positive charge.

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11
Q

Give the definition for Second ionisation energy ΔIE2Hθ

A

Second ionisation energy ΔIE2Hθ– The enthalpy change when one mole of electrons is removed from one mole of gaseous ions with a single positive charge to form one mole of gaseous ions with a 2+ charge.

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12
Q

Give the definition for Third ionisation energy ΔIE3Hθ

A

Third ionisation energy ΔIE3Hθ - The enthalpy change when one mole of electrons is removed from one mole of gaseous ions with a 2+ charge to form one mole of gaseous ions with a 3+ charge.

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13
Q

Give the definition for First Electron affinity ΔEA1Hθ

A

First Electron affinity ΔEA1Hθ– The enthalpy change when one mole of gaseous atoms forms one mole of negative ions with a single negative charge.

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14
Q

Give the definition for Second electron affinity ΔEA2Hθ

A

Second electron affinity ΔEA2Hθ– The enthalpy change when one mole of gaseous atoms with a single negative charge forms one mole of negative ions with a double negative charge.

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15
Q

Give the definition for Enthalpy of solution ΔsolHθ

A

Enthalpy of solution ΔsolHθ - The enthalpy change when one mole of a solute dissolves in water.

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16
Q

Give the definition for Enthalpy of hydration ΔhydHθ –.

A

Enthalpy of hydration ΔhydHθ – The enthalpy change when one mole of gaseous ions is converted into one mole of aqueous ions.

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17
Q

Give the definition for Entropy change Sθ

A

Entropy change Sθ– the difference between the number of reacting particles in solution and the number of product particles in solution.

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18
Q

Give the definition for Enthalpy

A

Enthalpy – A thermodynamic property of a system linked to the internal energy. Represented by H.

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19
Q

Give the definition for ΔH

A

ΔH – change in enthalpy. Negative for an exothermic reaction, positive for an endothermic reaction.

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20
Q

What happens to the enthalpy of atomisation for diatomic molecules?

A

For gaseous diatomic elements (e.g. Cl2, O2 etc) the bond dissociation energy is twice the enthalpy of atomisation.

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21
Q

What does the value of the lattice enthalpy tell you?

A

The value of the lattice enthalpy tells you how strong the ionic bond is.

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22
Q

What 2 factors affect the value of the lattice enthalpy?

A

This depends on the charge on the ion and the size.

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23
Q

What combination of charge and size gives stronger ionic bonds?

A

Smaller ions and higher charges give stronger ionic bonds.

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24
Q

Why do smaller ions give a higher bond energy?

A

Smaller ions are closer so are more attracted.

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25
Q

Why do higher charges give a higher bond energy?

A

Higher charges are more attracted. They have stronger the electrostatic attraction.

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26
Q

Are all compounds either ionic or covalent?

A

Not all compounds are 100% ionic or covalent.

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27
Q

When are covalent compounds 100% covalent?

A

If covalent compounds are non polar they are 100% covalent.

28
Q

What happens to polar covalent compounds?

A

Polar covalent compounds have some ionic character.

29
Q

When are ionic compounds covalent?

A

A small highly charged cation can distort the anion giving some covalent character (Al2O3). The ions are distorted and not spherical.

30
Q

What does the perfect ionic model assume?

A

The perfect ionic model assumes:

31
Q

If the bond energy values are different from the calculated values what does it show?

A

If the values are different then the compound has covalent character. The greater the difference the more covalent it is.

32
Q

Are values for hydration exothermic or endothermic and why?

A

Values for hydration are exothermic as energy is released when the ions are attracted to the polar water molecules.

33
Q

Do smaller ions attract water molecules more or less strongly?

A

Smaller anions ions attract the δ+ of the water molecule more strongly.

34
Q

What effect does the charge have on the attraction to water molecules?

A

The more charge it has the more it attracts the water molecule.

35
Q

What part of the water molecule does the cation attract? What can make this attraction greater?

A

Cations attract the δ- of the oxygen on the water molecule. The smallerand more charged it is the more it attracts.

36
Q

How does the group number affect the enthalpy of hydration?

A

Group 2 (2+ oxidation state) need 2x the enthalpy of hydration for the anion. e.g. 2 x Cl in CaCl2.

37
Q

What happens to ionic compounds when they dissolve in water?

A

Ionic compounds dissolve when the ionic lattice breaks up and the polar water molecules form bonds with the ions.

38
Q

How do you calculate the enthalpy of solution?

A

Enthalpy of solution = Enthalpy of lattice dissociation + Enthalpy of hydration

39
Q

Which process in dissolving are exothermic and which are endothermic?

A

Breaking up the ionic lattice is endothermic. Forming bonds with the water is exothermic. The balance of these gives the enthalpy of solution.

40
Q

What does the feasibility of a reaction depend on?

A

The feasibility of a reaction depends upon the enthalpy change and the entropy change and the temperature.

41
Q

What does Gibbs free energy tell you and what value does it need to have?

A

Gibbs free energy ΔGθ is used to predict the feasibility of a reaction. For it to be feasible it must be equal or less than 0.

42
Q

Give the equation for Gibbs free energy.

A

ΔGθ = ΔHθ - TΔSθ

43
Q

How do you calculate the temperature that something becomes feasible

A

To calculate the temperature that something becomes feasible use:

44
Q

What are the units for ΔGθ and ΔHθ

A

ΔGθ and ΔHθ have units of KJ mol-1

45
Q

What are the units for ΔSθ

A

ΔSθ has units of JK-1mol-1 remember to divide by 1000 before using!

46
Q

What does Entropy measure?

A

Entropy is the measure of disorder.

47
Q

What are the units of entropy?

A

It is measured in JK-1mol-1

48
Q

What is the entropy value at 0K?

A

Absolute values are for one mole based on a scale where the substance has an entropy value of zero at 0K.

49
Q

What does a higher entropy value tell you?

A

The higher the absolute entropy the higher the degree of disorder.

50
Q

Which are more order, solids, liquids or gases?

A

Solids are more ordered than liquids which are more ordered than gases.

51
Q

What happens to entropy as you turn solids to liquids and liquids to gases?

A

There is an increase in entropy from solid to liquid and an even greater increase from liquid to gas.

52
Q

What is the link between Mr and entropy?

A

The higher the Mr value of a substance in the same state the higher the entropy value.

53
Q

How do you calculate entropy?

A

ΔSθ= Σsθproducts – Σsθreactants

54
Q

From the equation for a reaction how can you tell if ΔSθ is negative or positive?

A

From the equation for a reaction you can tell if ΔSθ is negative or positive by looking at the states and number of moles of the reactants and products.

55
Q

Why may a feasible reaction not happen at a particular temperature?

A

A feasible reaction may not occur at a particular temperature as the activation energy may be too high for it to proceed.

56
Q

What is a spontaneous reaction?

A

A spontaneous reaction is one that is feasible and occurs under standard conditions.

57
Q

What must happen for a reaction to be spontaneous?

A

To be spontaneous ΔGθ must be less than 0 and the activation energy must be low enough for it to proceed.

58
Q

How do you determine the temperature at which a reaction changes from not feasible to feasible

A

To determine the temperature at which a reaction changes from not feasible to feasible divide ΔHθ by ΔSθ (remember to change the units).

59
Q

During a change of state what does ΔGθ =?

A

During a change of state ΔGθ = 0 kJ mol -1.

60
Q

ΔGθ is positive for temperatures below the melting points?

A

ΔGθ is positive for temperatures below the melting points as it is not feasible.

61
Q

ΔGθ is negative for temperatures above the melting points

A

ΔGθ is negative for temperatures above the melting points as it is feasible.

62
Q

Is the same true for boiling points?

A

The same is true for the boiling points.

63
Q

What value does ΔGθ when a system is at equilibrium?

A

When ΔGθ =0, the system is at equilibrium.

64
Q

How can you plot the equation ΔGθ = ΔHθ - T ΔSθ

A

The equation ΔGθ = ΔHθ - T ΔSθ can be plotted as a straight line graph.

65
Q

How does the equation ΔGθ = ΔHθ - T ΔSθ link to a line graph?

A

ΔGθ = ΔHθ - TΔSθ

66
Q

How do you plot the line graph?

A

ΔGθ is plotted against temperature in K (add 273 to oC).

67
Q

What are the units for ΔSθ when you are plotting the graph?

A

ΔSθ is in kJ K-1 mol-1. Multiply by 1000 to get ΔSθ.

AQA Chemistry (13 decks)

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