Free energy Change Flashcards
What makes a reaction feasible?
- 🔼G must be negative or zero
What is free energy change
Is a measure used to predict whether a reaction is feasible
What does free energy change take into account of a reaction system?
It takes into account, the:
- changes in enthalpy
- Changes in entropy
What is the formula for 🔼G (free energy change)
🔼G = 🔼H- T x 🔼S
What are the units for free energy change equation
- 🔼H= enthalpy change in ( J mol-1)
- T = temperature (in K)
- 🔼S = entropy change (in J K-1mol-1)
Note
- 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
- 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
Check physical flashcards for answers
What does feasibility depend on?
Temperature
If a reaction is exothermic (negative) and has a positive entropy change, what is the temperature of the reaction and its feasibility?
- The reaction is always negative
- These reactions are feasible at any temperature
If a reaction is endothermic (positive) and has a negative entropy change, what is the temperature of the reaction and its feasibility
- The reaction is always positive
- These reactions are not feasible at any temperature
If 🔼H is positive (endothermic) and 🔼S is positive how does this affect the temperature and feasibility of the reaction?
- The reaction wont be feasible at some temperatures but will be at high enough temperatures
How is kjmol-1 converted jmol-1
x1000
How does entropy increase
- The number of moles increase
- A gas product is formed
How does entropy decrease
- When liquids are formed
If 🔼H is negative (exothermic) and 🔼S is negative then the negative, how does this affect the feasibility and temperature of the reaction
- The reaction will be feasible at lower temperatures
- But wont be feasible at higher temperatures
When is a reaction feasible?
when 🔼G has a negative value/ zero
When does T🔼S=🔼H
when 🔼G=0
Give the formula for temperature
T= 🔼H/🔼S
Give the units for the free energy change equation
- 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)
Give the formula for entropy
S products- S reactants
How are elements reduced?
Using H
- 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
Check physical flashcards for answers
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?
Check physical flashcards for answers
What are the units for 🔼G
Jmol-1
How can we determine if a reaction is feasible?
• 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
What determines if a substance is soluble or insoluble in water
• 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
