Energy and Entropy

Question 1

Spontaneous process?

Answer 1

Those that are started and continue without any outside intervention, move towards their equilibrium state without being driven on by an external influence

Question 2

Entropy?

Answer 2

Amount of disorder or randomness in a system reflected in the number of ways that the molecules and energy in a system can be arranged

Question 3

Change in entropy?

Answer 3

Given by the difference between the final and the initial states of the system

Question 4

Second law of thermodynamics?

Answer 4

Spontaneous processes are those that increase the total entropy of the universe

Question 5

Total entropy change of a system?

Answer 5

Change of entropy in system + change in entropy of surroundings

Question 6

When is a process spontaneous?

Answer 6

When the total entropy is greater than 0

Question 7

When is a process non spontaneous?

Answer 7

When the total entropy is less than 0

Question 8

When is a process at equilibrium?

Answer 8

When the total entropy is equal to 0

Question 9

Adding heat reversibly?

Answer 9

Heat is added very slowly so that at any stage the temperature difference between the system and its surroundings is infinitesimally small and so is always close to thermal equilibrium, molecules move around faster chemical bonds vibrate more and higher energy levels are populated, this leads to the system having greater disorder and hence higher energy, adding the same amount of heat has a greater effect at lower temperature than at higher temperature

Question 10

Entropy changes during changes of phase?

Answer 10

A change of phase is reversible since it takes place at equibrlium at the normal temperature of the phase change the reversible heat change at constant pressure is the enthalpy change so that qrev = enthalpy change

Question 11

Entropy changes for vaporisation and fusion?

Answer 11

entropy change of vaporisation = entropy of vapour - entropy of liquid
entropy change of fusion = entropy of liquid - entropy solid
They have positive values consistent with idea of entropy and measure of disorder

Question 12

Entropy changes for reverse process of vaporisation and fusion?

Answer 12

They have the opposite sign but the same value

Question 13

Entropy and temperature?

Answer 13

As the temperature increases matter generally becomes more disordered, therefore entropy of system increases with increasing temperature.

Question 14

What does the derivation of the equation for change of entropy with change of temperature assume?

Answer 14

That no phase change occurs within the temperature range so the compound remains in the same state between the two temperatures, equation gives sufficient accuracy over small temperature ranges where cp is the mean value over the range, for greater accuracy the temperature variation of cp must be taken into account

Question 15

Third law of thermodynamics?

Answer 15

The entropy of a perfect crystal at zoo kelvin is zero, a perfectly ordered system has no disorder or motion of any type so must have zero entropy and since at 0K all molecules are in their ground state there is only one way of arranging the molecules so entropy is zero

Question 16

Perfect crystal?

Answer 16

One with all the atoms or ions or molecules aligned perfectly and with no defects - in reality this is impossible to achieve since all crystals contain defects

Question 17

Standard enthalpy change?

Answer 17

Entropy changes on heating 1 mol of a substance from 0K to 298K taking into account any phase changes that occur, sometimes called absolute or third law entropies, provide a convenient way to calculate entropy changes during chemical reactions

Question 18

To find standard entropy?

Answer 18

Calculate each of the entropy changes at 1 bar in taking 1 mole of the substance through the phase changes, it can also be found using a graphical method with experimental data for Cp and T, finding the area under the curves is equivalent to performing the integrals

Question 19

How to find standard entropy of reaction?

Answer 19

Sum of entropy of products - sum of entropy of reactants

Question 20

Spontaneous process and sign of Gibbs energy?

Answer 20

A spontaneous process at constant temperature and pressure has a negative value for the change in Gibbs energy

Question 21

How can change in Gibbs energy be used to predict spontaneity?

Answer 21

delta G < 0 the reaction is spontaneous
delta G > 0 the reaction is non spontaneous
delta G = 0 the reaction is at equilibrium

Question 22

Why are processes that have delta G < 0 spontaneous?

Answer 22

They increase the entropy of the universe and thus obey the second law, Gibbs energy gives a convenient way of applying the second law while considering properties only of the system, if a process that is non spontaneous is the forward reaction it will be spontaneous in the reverse reaction

Question 23

For an endothermic reaction what is the value of enthalpy change, entropy change, change in Gibbs energy at different temperatures and the spontaneity?

Answer 23

change in enthalpy Positive > 0
change in entropy negative < 0
value of Gibbs energy POSITIVE for all T
Never spontaneous

change in enthalpy Positive > 0 
change in entropy positive > 0
value of Gibbs energy POSITIVE for LOW T 
NEGATIVE at HIGH T
Becomes spontaneous on heating

Question 24

For an exothermic reaction what is the value of enthalpy change, entropy change, change in Gibbs energy at different temperatures and the spontaneity?

Answer 24

change in enthalpy negative < 0
change in entropy positive > 0
value of Gibbs energy NEGATIVE for all T
Always spontaneous

change in enthalpy negative < 0 
change in entropy negative < 0
value of Gibbs energy POSITIVE for HIGH T 
NEGATIVE at LOW T 
Becomes spontaneous on cooling

Question 25

Lowering of Gibbs energy?

Answer 25

If a process or reaction results in a lowering of the Gibbs energy it is spontaneous it is favourable and can happen however it does not mean that it will happen, a reaction may be kinetically unfavourable as it has a high activation energy, however once started the reaction will readily go to completion, the Gibbs energy change is useful since if the value of delta G can be calculated for a reaction or process the spontaneity can be predicted

Question 26

Standard Gibbs energy change of formation?

Answer 26

The change in Gibbs energy when 1 mole of a compound is formed at 1 bar and 298K from its elements in their standard states, compounds have lower Gibbs energies (negative values) than their constituent elements and are therefore more stable with respect to their elements

Question 27

Gibbs energy change of reaction?

Answer 27

Gibbs energy changes of formation can be used to calculate the change in Gibbs energy driving the reaction delta G of reaction = sum of Gibbs energy changes of formation of products - sum of Gibbs energy changes of formation of reactants

Question 28

How can Gibbs energy changes be used for coupled reactions?

Answer 28

Even if a reaction has a positive value of Gibbs energy change of reaction that does not necessarily mean it cannot be used, the reaction in isolation will not be spontaneous but it can be made to occur if it is couple to another reaction with a larger Gibbs energy change of reaction, such reactions are sometimes called tandem reactions

Question 29

Gibbs energy changes and maximum work?

Answer 29

Gibbs energy changes value is equivalent to the maximum non expansion work that can be obtained from a change, this is the maximum amount of work that the change can do after any work due to change in valued has been accounted for

Question 30

What is thermodynamic activity used for?

Answer 30

Most chemical reactions occur under non standard conditions at temperatures other than 298K and do not necessarily go to completion. To describe the effects of concentration and pressure a term called the thermodynamic activity is used

Question 31

What is thermodynamic activity?

Answer 31

Sometimes called the effective concentration of the component and is the ratio of the pressure or concentration to a standard value

Question 32

What are the activities of ideal gases?

Answer 32

Defined in terms of pressures using a standard pressure of 1 bar

Question 33

What are the activities of substance solution?

Answer 33

Defined in terms of their molar concentrations using a standard concentration of 1moldm-3

Question 34

Activities of pure liquids and solids?

Answer 34

Have fixed composition and their activity is defined as 1

Question 35

Activity of an ideal gas?

Answer 35

For an ideal gas activity is given by the ratio of the gas pressure measured in bar to the standard pressure 1 bar it is numerically equal to the pressure in bar

Question 36

Activity of an ideal solution?

Answer 36

For an ideal solution activity is numerically equal to the concentration in moldm-3

Question 37

Activity units?

Answer 37

Because the activity is a ratio of concentration or partial pressure it does not have units

Question 38

Plot of Gm against temperature?

Answer 38

Will give a straight line with -Sm as the gradient, in most systems Hm and Sm do depend slightly on temperature so the plots won't be perfectly linear except over small temperature intervals, substance with a large molar entropy Sm such as gases show the greatest variation of Gm with temperature (plot for gases has the steepest slope). For solids and liquids Gm does not vary greatly with temperature since their entropy is low