Thermodynamics

Question 1

What do chemical reactions take place as a result of?

Answer 1

bond breaking and bond making reactions

Question 2

What kind of reaction is bond breaking?

Answer 2

endothermic

Question 3

What kind of reaction is bond making?

Answer 3

exothermic

Question 4

What is thermochemistry the study of?

Answer 4

the study of energy changes during chemical reactions, measured in kJ mol-1

Question 5

What are energy changes measured in terms of?

Answer 5

enthalpy changes (or changes in energy contents)

Question 6

What does a change in energy content result in?

Answer 6

a change in temperature of the surroundings

Question 7

What do total enthalpy changes always refer to?

Answer 7

the system and the surroundings

Question 8

What can the system be regarded as?

Answer 8

the part of the world we are investigating e.g. a chemical reaction

Question 9

What are the surroundings regarded as?

Answer 9

the rest of the world e.g. the air surroundings the reaction vessel, or water the reaction is taking place in.

Question 10

What is important to remember about the total amount of energy during a chemical reaction?

Answer 10

The total amount of energy is unchanged during a chemical reaction - energy may be exchanged between the materials and the surroundings, but the total amount of energy of the materials and surroundings remains constant

Question 11

What is the first law of thermodynamics?

Answer 11

energy can neither be created nor destroyed, but can be changed from one form to another

Question 12

State Hess’s law.

Answer 12

The enthalpy change for a reaction is the same regardless of the route taken

Question 13

What are the standard conditions for temp and pressure?

Answer 13

298K (25’c) and 1 atmosphere respectively (STP)

Question 14

What are substances said to be?

Answer 14

in their standard state, i.e. their normal physical states under standard conditions

Question 15

What concentration are solutions at in their standard state?

Answer 15

1 mol l-1

Question 16

What are enthalpy changes measured under standard conditions known as? What symbol is used?

Answer 16

a standard enthalpy change delta H’

Question 17

What is the enthalpy of formation?

Answer 17

the enthalpy change when 1 mole of a substance is formed for its elements under standard conditions.

Question 18

What is the standard enthalpy of formation of an element?

Answer 18

0

Question 19

How can the standard enthalpy of a reaction being calculated?

Answer 19

using standard enthalpies of formation

Question 20

Define entropy.

Answer 20

A term which refers to the chaos or disorder in a system, and measure of chaotic dispersal of energy and matter.

Question 21

The greater the disorder in a system, the…

Answer 21

greater the entropy

Question 22

Describe the effect of temperature on entropy.

Answer 22

Entropy increases as temperature increases

Question 23

What is the standard molar entropy, S*?

Answer 23

the entropy content of one mole of a substance, under standard conditions

Question 24

What are the units of entropy?

Answer 24

JK-1 mol-1

Question 25

What does the third law of thermodynamics state?

Answer 25

the entropy of a perfect crystal at 0K is zero

Question 26

What does the crystal lattice at absolute zero have?

Answer 26

perfect order and therefore no entropy i.e. the particles are not in motion

Question 27

What happens as a solid is heated?

Answer 27

the particles gain KE and entropy slowly increases

Question 28

Describe the entropy increase between solid, liquid and gas.

Answer 28

The entropy increase from liquid to gas is greater than that from solid to liquid

Question 29

What do changes in state involve?

Answer 29

changes in entropy

Question 30

For any spontaneous process describe what happens to the total entropy of the system and its surroundings.

Answer 30

increases

Question 31

What is a spontaneous process?

Answer 31

one that can take place on its own irrespective of how long it takes

Question 32

What are spontaneous exothermic reactions driven by?

Answer 32

the large increase in entropy they generate in the surroundings

Question 33

How is the entropy of surroundings calculated.

Answer 33

calculated from the standard enthalpy change for the reaction and temperature, T, in Kelvin.

Question 34

What are spontaneous endothermic reactions driven by?

Answer 34

the large increase entropy they generate in the system i.e. the reaction mixture itself

Question 35

For a spontaneous endothermic reaction explain what happens to the entropy of the surroundings.

Answer 35

Heat is absorbed from the surroundings - decreases the entropy of the surroundings

Question 36

What does the spontaneity of a reaction involve?

Answer 36

two thermodynamic quantities, enthalpy and entropy

Question 37

What are the favourable conditions for a spontaneous reaction?

Answer 37

/\H < 0 | /\S > 0

Question 38

What are the favourable conditions for a non-spontaneous reaction?

Answer 38

/\H > 0 | /\S < 0

Question 39

In order to determine whether a reaction is spontaneous what other thermodynamic quantity is used?

Answer 39

Gibbs Free Energy /\G

Question 40

What is the unit of /\G?

Answer 40

J mol-1

Question 41

For a spontaneous reaction, what will /\G always be?

Answer 41

negative

Question 42

What is the equation to calculate /\G? Include the units

Answer 42

``` /\G= /\H - T/\S /\G is Gibbs Free Energy in kJ /\H is the enthalpy change in kJ mol-1 T is the temperature in Kelvin (+273) /\S is the entropy change in JK-1 mol-1 ```

Question 43

For a non-spontaneous reaction, what will /\G always be?

Answer 43

positive

Question 44

What equation can be used to calculate standard free energy changes?

Answer 44

/\G= {sigma}/\Gproducts - {sigma}/\Greactants

Question 45

If /\G is negative, what will the reaction proceed at?

Answer 45

any temperature

Question 46

In terms of the reaction what will happen if /\G is positive?

Answer 46

the reaction will not proceed

Question 47

What is the temperature at which the reaction becomes feasible?

Answer 47

the CRITICAL TEMPERATURE

Question 48

At the critical temperature, what is the value of /\G?

Answer 48

0

Question 49

What equation is used to calculate the temp at which the reaction becomes feasible?

Answer 49

T= /\h//\S

Question 50

At chemical equilibrium what is the Gibbs free energy value?

Answer 50

At a minimum

Question 51

If /\G is <0 what does this suggest about the forward reaction?

Answer 51

it is feasible so [products]>[reactants] Equilibrium lies to the RHS and therefore K>1

Question 52

If /\G is >0 what does this suggest about the reverse reaction?

Answer 52

it is feasible so [reactant]>[products] | Equilibrium lies to the LHS and therefore K<1

Question 53

If /\G is equal to zero what is the value of K?

Answer 53

1

Question 54

What does the value of /\G for a reaction not predict?

Answer 54

the rate of the reaction

Question 55

What may a reaction have that prevents it from taking place?

Answer 55

A reaction may have a negative /\G but have a high activation energy

Question 56

In a closed system what must the /\Ssystem be?

Answer 56

positive

Question 57

If /\S is -ve what does this suggest about how disordered the reactants and products are?

Answer 57

[reactants]>[products]

Question 58

If /\S is +ve what does this suggest about how disordered the reactants and products are?

Answer 58

[products]>[reactants]