Thermodynamics

Question 1

Give the definition for Standard conditions

Answer 1

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

Question 2

Give the definition for Standard enthalpy of reaction ΔrHθ

Answer 2

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

Question 3

Give the definition for Standard enthalpy of formation ΔfHθ

Answer 3

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.

Question 4

Give the definition for Standard enthalpy of combustion ΔcHθ

Answer 4

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.

Question 5

Give the definition for Mean bond energy

Answer 5

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

Question 6

Give the definition for Enthalpy of lattice dissociation ΔLHθ

Answer 6

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

Question 7

Give the definition for Enthalpy of lattice formation ΔfHθ

Answer 7

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.

Question 8

Give the definition for Enthalpy of atomisation ΔatHθ

Answer 8

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

Question 9

Give the definition for Bond dissociation energy

Answer 9

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

Question 10

Give the definition for First ionisation energy ΔIE1Hθ.

Answer 10

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.

Question 11

Give the definition for Second ionisation energy ΔIE2Hθ

Answer 11

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.

Question 12

Give the definition for Third ionisation energy ΔIE3Hθ

Answer 12

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.

Question 13

Give the definition for First Electron affinity ΔEA1Hθ

Answer 13

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

Question 14

Give the definition for Second electron affinity ΔEA2Hθ

Answer 14

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.

Question 15

Give the definition for Enthalpy of solution ΔsolHθ

Answer 15

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

Question 16

Give the definition for Enthalpy of hydration ΔhydHθ –.

Answer 16

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

Question 17

Give the definition for Entropy change Sθ

Answer 17

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

Question 18

Give the definition for Enthalpy

Answer 18

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

Question 19

Give the definition for ΔH

Answer 19

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

Question 20

What happens to the enthalpy of atomisation for diatomic molecules?

Answer 20

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

Question 21

What does the value of the lattice enthalpy tell you?

Answer 21

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

Question 22

What 2 factors affect the value of the lattice enthalpy?

Answer 22

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

Question 23

What combination of charge and size gives stronger ionic bonds?

Answer 23

Smaller ions and higher charges give stronger ionic bonds.

Question 24

Why do smaller ions give a higher bond energy?

Answer 24

Smaller ions are closer so are more attracted.

Question 25

Why do higher charges give a higher bond energy?

Answer 25

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

Question 26

Are all compounds either ionic or covalent?

Answer 26

Not all compounds are 100% ionic or covalent.

Question 27

When are covalent compounds 100% covalent?

Answer 27

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

Question 28

What happens to polar covalent compounds?

Answer 28

Polar covalent compounds have some ionic character.

Question 29

When are ionic compounds covalent?

Answer 29

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

Question 30

What does the perfect ionic model assume?

Answer 30

The perfect ionic model assumes:

Question 31

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

Answer 31

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

Question 32

Are values for hydration exothermic or endothermic and why?

Answer 32

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

Question 33

Do smaller ions attract water molecules more or less strongly?

Answer 33

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

Question 34

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

Answer 34

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

Question 35

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

Answer 35

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

Question 36

How does the group number affect the enthalpy of hydration?

Answer 36

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

Question 37

What happens to ionic compounds when they dissolve in water?

Answer 37

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

Question 38

How do you calculate the enthalpy of solution?

Answer 38

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

Question 39

Which process in dissolving are exothermic and which are endothermic?

Answer 39

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

Question 40

What does the feasibility of a reaction depend on?

Answer 40

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

Question 41

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

Answer 41

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. 

Question 42

Give the equation for Gibbs free energy.

Answer 42

ΔGθ = ΔHθ - TΔSθ

Question 43

How do you calculate the temperature that something becomes feasible

Answer 43

To calculate the temperature that something becomes feasible use:

Question 44

What are the units for ΔGθ and ΔHθ

Answer 44

ΔGθ and ΔHθ have units of KJ mol-1

Question 45

What are the units for ΔSθ

Answer 45

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

Question 46

What does Entropy measure?

Answer 46

Entropy is the measure of disorder.  

Question 47

What are the units of entropy?

Answer 47

It is measured in JK-1mol-1

Question 48

What is the entropy value at 0K?

Answer 48

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

Question 49

What does a higher entropy value tell you?

Answer 49

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

Question 50

Which are more order, solids, liquids or gases?

Answer 50

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

Question 51

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

Answer 51

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

Question 52

What is the link between Mr and entropy?

Answer 52

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

Question 53

How do you calculate entropy?

Answer 53

 ΔSθ= Σsθproducts – Σsθreactants

Question 54

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

Answer 54

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.  

Question 55

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

Answer 55

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

Question 56

What is a spontaneous reaction?

Answer 56

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

Question 57

What must happen for a reaction to be spontaneous?

Answer 57

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

Question 58

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

Answer 58

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

Question 59

During a change of state what does ΔGθ =?

Answer 59

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

Question 60

ΔGθ is positive for temperatures below the melting points?

Answer 60

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

Question 61

ΔGθ is negative for temperatures above the melting points

Answer 61

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

Question 62

Is the same true for boiling points?

Answer 62

The same is true for the boiling points.

Question 63

What value does ΔGθ when a system is at equilibrium?

Answer 63

When ΔGθ =0, the system is at equilibrium.  

Question 64

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

Answer 64

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

Question 65

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

Answer 65

ΔGθ = ΔHθ - TΔSθ

Question 66

How do you plot the line graph?

Answer 66

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

Question 67

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

Answer 67

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