Enthaloy & entropy

Question 1

What is the law of conservation of energy (first law of thermodynamics)?

Answer 1

Energy cannot be created nor destroyed, merely converted from one form into another

Question 2

Describe enthalpy

Answer 2

the energy of a substance under constant atmospheric pressure

Question 3

What does a change in enthalpy mean?

Answer 3

the amount of heat energy absorbed/produced by a chemical/physical change carried out at constant pressure

Question 4

What is the equation for enthalpy change?

Answer 4

= (sum of enthalpies of products) - (sum of enthalpies of reactants)

Question 5

What is an exothermic reaction?

Answer 5

a reaction that gives out heat during a reaction

Question 6

Describe the difference in energy levels for reactants and products in an exothermic reaction?

Answer 6

Reactants are at a higher energy level than products

Question 7

Describe the difference in energy levels for reactants and products in an endothermic reaction?

Answer 7

Reactants are at a lower energy level than products

Question 8

Describe the difference in enthalpy change between exo and endothermic reactions?

Answer 8

enthalpy change is negative in exothermic and positive in endothermic

Question 9

What is a thermochemical equation?

Answer 9

the combination of a chemical equation and its enthalpy change value

Question 10

What is the standard enthalpy change of a reaction ?

Answer 10

the difference between the sum of the enthalpies of the products of the reaction and the sum of the enthalpies of the starting materials
( Energy Change accompanying a chemical reaction )

Question 11

What is thermodynamic spontaneity?

Answer 11

tendency to change irrespective of the rate at which it changes

Question 12

What is a standard state?

Answer 12

A substance is in standard state when it is in its pure form at an atmospheric pressure of 1atm

Question 13

What is entropy?

Answer 13

The measure of disorder used in thermodynamics

Question 14

In chemical reactions what does an increase in entropy mean?

Answer 14

A dispersal of energy (heat) or of matter (stuff)

Question 15

What does it mean if the entropy change is positive?

Answer 15

• an increase in disorder,
• heat released into surroundings
• its an exothermic reaction

Question 16

What does it mean that the enthalpy change of a reaction is an extensive property?

Answer 16

it means the enthalpy change depends upon the amount of reactants consumed

Question 17

What is the second law of thermodynamics?

Answer 17

• The Entropy of an isolated system tends to increase

- The Entropy of the Universe increases in a spontaneous process

Question 18

What does Gibbs energy allows us to do?

Answer 18

to evaluate the entropy change of the universe, in terms of properties of the system only.

Question 19

If a system moves to a higher disorder what does it mean?

Answer 19

Entropy change is positive

Question 20

If a system moves to a lower energy what does it mean?

Answer 20

Enthalpy change is negative

Question 21

Whats the difference between entropy and enthalpy?

Answer 21

• Enthalpy (H) ; energy of a substance/thermodynamic system under constant pressure
• Entropy (S) ; measure of disorder used in thermodynamic system

Question 22

What are the units for entropy change?

Answer 22

J K-1 mol-1

Question 23

What are the units for enthalpy change and gibbs energy change?

Answer 23

J mol-1

Question 24

Describe the behaviour of system when

• enthalpy is negative
• Entropy is positive

Answer 24

exothermic + entropy increase

always spontaneous

Question 25

Describe the behaviour of system when

• enthalpy is negative
• Entropy is negative

Answer 25

exothermic + entropy decreases

becomes spontaneous below certain temperature

Question 26

Describe the behaviour of system when

• enthalpy is positive
• Entropy is positive

Answer 26

endothermic + entropy increases

becomes spontaneous above certain temperature

Question 27

Describe the behaviour of system when

• enthalpy is positive
• Entropy is negative

Answer 27

endothermic + entropy decreases

never spontaneous

Question 28

what does spontaneous mean in this context?

Answer 28

spontaneous process means it can proceed without any outside intervention (‘thermodynamically favourable’)

Question 29

What are standard conditions?

Answer 29

• 298 K
• a pressure of 1 bar
• where solutions are involved, a concentration of 1 bar
• where solutions are involved, a concentration of 1 mol dm-3

Question 30

What is the standard enthalpy change of formation?

Answer 30

the enthalpy change which occurs when one mole of the compound is formed from its elements under standard conditions, and with everything in its standard state.

Question 31

What happens to the chemical reaction if the gibbs energy change is negative?

Answer 31

the process/chemical reaction proceeds spontaneously in the forward direction

Question 32

What happens to the chemical reaction if the gibbs energy change is negative?

Answer 32

the process/chemical reaction proceeds spontaneously in reverse

Question 33

What happens to the chemical reaction if the gibbs energy change is zero

Answer 33

the process is already in equilibrium, no net change taking place over time

Question 34

what three factors affects gibbs free energy?

Answer 34

• entropy change
• enthalpy change
• temperature

Question 35

What is the equation that links gibbs energy to entropy, enthalpy and temp?

Answer 35

G = H - TS

Question 36

If gibbs energy < 0 what happens?

Answer 36

It will occur spontaneously, exergonic reaction

Question 37

If gibbs energy > 0 what happens?

Answer 37

A reaction will not occur spontaneously, endergonic reaction

Question 38

Give an example of an exothermic reaction

Answer 38

combustion

Question 39

Give an example of an endothermic rection

Answer 39

melting if ice

Question 40

What happens to the gibbs energy in a spontaneous change at constant temp and pressure?

Answer 40

gibbs energy decreases