Free energy Change

Question 1

What makes a reaction feasible?

Answer 1

• 🔼G must be negative or zero

Question 2

What is free energy change

Answer 2

Is a measure used to predict whether a reaction is feasible

Question 3

What does free energy change take into account of a reaction system?

Answer 3

It takes into account, the:
- changes in enthalpy
- Changes in entropy

Question 4

What is the formula for 🔼G (free energy change)

Answer 4

🔼G = 🔼H- T x 🔼S

Question 5

What are the units for free energy change equation

Answer 5

• 🔼H= enthalpy change in ( J mol-1)
• T = temperature (in K)
• 🔼S = entropy change (in J K-1mol-1)

Question 6

Note

Answer 6

• Even if 🔼G shows that a reaction is theoretically feasible, it might have a very high activation energy or be slow that you wouldn’t notice it happening at all

Question 7

• Calculate the free energy change for the following reaction at 298K
• MgCO3(g) ->MgO(s) + CO2(g)
• 🔼H= +117000 Jmol-1
• 🔼S= +175J K-1 mol-1

Answer 7

Check physical flashcards for answers

Question 8

What does feasibility depend on?

Answer 8

Temperature

Question 9

If a reaction is exothermic (negative) and has a positive entropy change, what is the temperature of the reaction and its feasibility?

Answer 9

• The reaction is always negative
• These reactions are feasible at any temperature

Question 10

If a reaction is endothermic (positive) and has a negative entropy change, what is the temperature of the reaction and its feasibility

Answer 10

• The reaction is always positive
• These reactions are not feasible at any temperature

Question 11

If 🔼H is positive (endothermic) and 🔼S is positive how does this affect the temperature and feasibility of the reaction?

Answer 11

• The reaction wont be feasible at some temperatures but will be at high enough temperatures

Question 12

How is kjmol-1 converted jmol-1

Answer 12

x1000

Question 13

How does entropy increase

Answer 13

• The number of moles increase
• A gas product is formed

Question 14

How does entropy decrease

Answer 14

• When liquids are formed

Question 15

If 🔼H is negative (exothermic) and 🔼S is negative then the negative, how does this affect the feasibility and temperature of the reaction

Answer 15

• The reaction will be feasible at lower temperatures
• But wont be feasible at higher temperatures

Question 16

When is a reaction feasible?

Answer 16

when 🔼G has a negative value/ zero

Question 17

When does T🔼S=🔼H

Answer 17

when 🔼G=0

Question 18

Give the formula for temperature

Answer 18

T= 🔼H/🔼S

Question 19

Give the units for the free energy change equation

Answer 19

• T= temperature at which a reaction becomes feasible (K)
• 🔼H= enthalpy change (in J mol-1)
• 🔼S= entropy change (in J K-1 mol-1)

Question 20

Give the formula for entropy

Answer 20

S products- S reactants

Question 21

How are elements reduced?

Answer 21

Using H

Question 22

• Magnesium carbonate decomposes to form magnesium oxide and carbon dioxide:
• MgCO3(s) -> MgO(s) + CO2(g)
• 🔼H= +117kJmol-1, 🔼S= +175JK-1mol-1
  a) Calculate the free energy change of this reaction at the following temperatures
  i) 500K ii)760K

b) At which of the temperatures in part a) is the reaction feasible? Explain your answer

Answer 22

Check physical flashcards for answers

Question 23

2a) Use the equation below and the table on the right to calculate the free energy change for the complete combustion of methane at 298K.
- CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)
- 🔼H= -730KJmol-1
- CH4(g)= 🔼S=186 JK-1mol-1
- O2(g)= 🔼S= 205 JK-1 mol-1
- CO2(g)= 🔼S= 214JK-1mol-1
- H20(l)= 🔼S= 69.9JK-1mol-1

b) What is the temperature at which the reaction is feasible?

Answer 23

Check physical flashcards for answers

Question 24

What are the units for 🔼G

Answer 24

Jmol-1

Question 25

How can we determine if a reaction is feasible?

Answer 25

• Using the equation
• 🔼G = 🔼H-T🔼S
• If 🔼S is positive and T is high enough, T🔼S will be greater than 🔼H and 🔼G will be negative, making the reaction feasible

Question 26

What determines if a substance is soluble or insoluble in water

Answer 26

• The actual sizes of the hydration enthalpies and the lattice dissociation enthalpy individually aren’t important
• It’s their total value that determines whether a compound is soluble or not.
• For a soluble substance, this total needs to be less than or very close to zero, to give an exothermic enthalpy of solution