KA 2: reaction feasibility

Question 1

what is the standard enthalpy of formation (ΔH°f)

Answer 1

the enthalpy change when one mole of a substance is formed from its elements in their standard states

Question 2

what is the standard state of a substance?

Answer 2

its most stable state at a pressure of 1 atmosphere and at a specified temperature (usually taken as 298K)

Question 3

how can the standard enthalpy of a reaction be calculated and what’s the formula?

Answer 3

-the standard enthalpies of formation of the reactants and products

ΔH° = ΣΔH°f (products) - ΣΔH°f (reactants)

Question 4

what is the entropy (S) of a system?

Answer 4

a measure of the degree of disorder of the system

Question 5

the greater the degree of disorder….

Answer 5

the greater the entropy

Question 6

what disorder do solids and gases have?

Answer 6

solids: low
gases: high

Question 7

state the relationship between entropy and temperature

Answer 7

entropy increases as temp increases

Question 8

what happens to entropy at the melting point of a substance?

Answer 8

rapid increase in entropy

Question 9

what happens to entropy at boiling point?

Answer 9

even more rapid and larger change in entropy

Question 10

state the second law of thermodynamics

Answer 10

the total entropy of a reaction system and its surroundings always increases for a spontaneous process

Question 11

heat energy is released by the reaction system into the surroundings. what happens to the entropy of the surroundings?

Answer 11

increases

Question 12

heat energy is absorbed by the reaction system from the surroundings. what happens to the entropy of the surroundings?

Answer 12

decreases

Question 13

state the third law of thermodynamics

Answer 13

he entropy of a perfect crystal at 0 K is zero

Question 14

what is the standard entropy of a substance?

Answer 14

the entropy value for the substance in its standard state

Question 15

how can the change in standard entropy for a reaction system be calcuated? whats the formula?

Answer 15

-standard entropies of the reactants and products

ΔS° = ΣS°(products) - ΣS°(reactants)

Question 16

what is the change in free energy for a reaction related to?

what’s the formula?

Answer 16

enthalpy and entropy changes

ΔG° = ΔH° - TΔS°

Question 17

when would a feasible reaction occur?

Answer 17

change in free energy (ΔG°) between reactants and products is negative

Question 18

what is a feasible reaction?

Answer 18

one that tends towards the products rather than the reactants (this doesn’t give an indication of the rate of the reaction)

Question 19

how is the standard free energy change for a reaction calculated?

what’s the formula?

Answer 19

-using free energies of formation of the reactants and prodcuts

ΔG° = ΣΔG° (products) - ΣΔG° (reactants)

Question 20

how can the feasibility of a chemical reaction under certain conditions be predicted?

Answer 20

from the calculated value of the change in standard free energy (ΔG°)

Question 21

how can the temperatures at which a reaction may be considered feasible be estimated?

Answer 21

by considering the range of values of T for which ΔG° < 0

Question 22

when is a reaction feasible under non-standard conditions?

Answer 22

if ΔG is negative

Question 23

what does ΔG equal at equilibrium?

Answer 23

0

Question 24

when will a reversible reaction proceed spontaneously until?

Answer 24

the composition is reached where ΔG=0