CHEM 230 EXAM 2 QUESTIONS & ANSWERS
Enthalpy (ΔH)
What is the third law of thermodynamics? - Answer -The entropy of a perfect crystal is
zero.
What is the second law of thermodynamics? - Answer -As a whole, the universe always
tends towards increasing entropy.
What is the relationship between number of microstates and entropy? - Answer -More
microstates = higher entropy
What is the relationship between temperature and entropy? - Answer -Temperature
increases, entropy increases (direct)
What is the relationship between atomic weight and entropy? - Answer -Heavier =
increased entropy (direct)
What happens to entropy when a substance is dissolved in another? - Answer -Entropy
increases
What is the relationship between moles of gas and entropy? - Answer -When moles of
gas increase, entropy increases (direct)
What does change in G measure? - Answer -The extent of the spontaneity of a
process, and the useful energy available from it
What is the relationship between work and spontaneous processes? - Answer -
Spontaneous processes produce work
What is the relationship between work and non-spontaneous processes? - Answer -
Non-spontaneous processes require work
What does -w mean? - Answer -Work is being produced - spontaneous
What does a positive sign for w mean? - Answer -Work is required - non-spontaneous
What is the relationship between free energy, enthalpy, and entropy? - Answer -
Δ(system)=ΔH(system)-TΔS(system)
Can a change be spontaneous in both directions? - Answer -No
What is an extensive property? - Answer -Value depends on the amount of substance
,What does ΔG > 0 mean? - Answer -Non-spontaneous process - requires work
What does ΔG < 0 mean? - Answer -Spontaneous process - produces work
What does standard free energy of formation mean? ΔGo - Answer -The free energy
change that occurs when 1 mole of a compound is made from its elements
What is the standard free energy of formation for an element in its standard state? ΔGf -
Answer -0
What does reversing a reaction do to the standard free energy of formation? ΔG0 -
Answer -Changes its sign
Why are most exothermic reactions spontaneous? - Answer -Because the large
negative ΔH/large energy release makes the free energy change negative.
What happens when ΔH and ΔS have opposite signs? - Answer -The reaction occurs
spontaneously at all temperatures or at none.
What happens when enthalpy (ΔH) is negative and entropy (ΔS) is positive? - Answer -
Spontaneous at all temperatures, negative free energy
What is the likely sign of ΔH and ΔS for a combustion reaction? - Answer -Negative
ΔH, positive ΔS
What happens when ΔH is positive and ΔS is negative? - Answer -Nonspontaneous at
all temperatures
What happens when ΔH and ΔS are both positive? - Answer -Reaction becomes
spontaneous as temperature increases
What happens when ΔH and ΔS are both negative? - Answer -Reaction becomes
spontaneous as temperature decreases
Why can the maximum work of a system never obtained from a real process? - Answer
-Because such an irreversible process will always involve some free energy being
converted to heat
How do you find where a reaction becomes spontaneous? - Answer -Set ΔG equal to
zero and solve -- use T=deltaH/deltaS to find the temperature value.
Is a chemical reaction proceeding to equilibrium a spontaneous or non-spontaneous
change? - Answer -Spontaneous
How to determine reaction direction from ΔG? - Answer -ΔG<0 reaction proceeds to the
right
, ΔG>0 reaction proceeds to the left
Which direction does the reaction proceed if ΔG is positive? - Answer -To the left / to
reactants
Which direction does the reaction proceed if ΔG is negative? - Answer -To the right/ to
products
How to predict ΔS(system)? (change in entropy of a system) - Answer -Positive
ΔS(system) means available microstates increases/disorder increases
How to calculate the ΔSo (aka standard entropy change) of the formation of one mole of
gas from its elements? - Answer -1. Write the balanced chemical equation with the
proper coefficients that make the product ONE MOLE.
2. Find the value of the one mole of product in Appendix B. This is the deltaS(standard)
for the product.
3. Find the value of the reactants in Appendix B for deltaS(standard) and multiply them
by the coefficients you got in step 1.
4. Use total = products-reactants for your final value.
How to calculate ΔG (free energy) using enthalpy ΔH and entropy ΔS values? - Answer
-ΔG=ΔH-TΔS
What is the equation that relates ΔG to K? - Answer -ΔG=-RT ln K
What is the Boltzmann constant? - Answer -ln(# of microstates)
What does a big W mean? - Answer -More microstates
What does more microstates mean? - Answer -Big ΔS - more entropy
What does fewer microstates mean? - Answer -Smaller ΔS - less entropy
What happens to entropy when volume is increased? - Answer -Microstates increase,
entropy increases
What is the relationship between ΔS of a system, q, and T? - Answer -
ΔS(system)=qrev/T
What happens to entropy ΔS when temperature increases? - Answer -It increases
What is the relationship between mass and entropy ΔS? - Answer -As mass increases,
entropy increases
What is the relationship between molecular complexity and entropy ΔS? - Answer -As
molecular complexity increases, entropy increases
Enthalpy (ΔH)
What is the third law of thermodynamics? - Answer -The entropy of a perfect crystal is
zero.
What is the second law of thermodynamics? - Answer -As a whole, the universe always
tends towards increasing entropy.
What is the relationship between number of microstates and entropy? - Answer -More
microstates = higher entropy
What is the relationship between temperature and entropy? - Answer -Temperature
increases, entropy increases (direct)
What is the relationship between atomic weight and entropy? - Answer -Heavier =
increased entropy (direct)
What happens to entropy when a substance is dissolved in another? - Answer -Entropy
increases
What is the relationship between moles of gas and entropy? - Answer -When moles of
gas increase, entropy increases (direct)
What does change in G measure? - Answer -The extent of the spontaneity of a
process, and the useful energy available from it
What is the relationship between work and spontaneous processes? - Answer -
Spontaneous processes produce work
What is the relationship between work and non-spontaneous processes? - Answer -
Non-spontaneous processes require work
What does -w mean? - Answer -Work is being produced - spontaneous
What does a positive sign for w mean? - Answer -Work is required - non-spontaneous
What is the relationship between free energy, enthalpy, and entropy? - Answer -
Δ(system)=ΔH(system)-TΔS(system)
Can a change be spontaneous in both directions? - Answer -No
What is an extensive property? - Answer -Value depends on the amount of substance
,What does ΔG > 0 mean? - Answer -Non-spontaneous process - requires work
What does ΔG < 0 mean? - Answer -Spontaneous process - produces work
What does standard free energy of formation mean? ΔGo - Answer -The free energy
change that occurs when 1 mole of a compound is made from its elements
What is the standard free energy of formation for an element in its standard state? ΔGf -
Answer -0
What does reversing a reaction do to the standard free energy of formation? ΔG0 -
Answer -Changes its sign
Why are most exothermic reactions spontaneous? - Answer -Because the large
negative ΔH/large energy release makes the free energy change negative.
What happens when ΔH and ΔS have opposite signs? - Answer -The reaction occurs
spontaneously at all temperatures or at none.
What happens when enthalpy (ΔH) is negative and entropy (ΔS) is positive? - Answer -
Spontaneous at all temperatures, negative free energy
What is the likely sign of ΔH and ΔS for a combustion reaction? - Answer -Negative
ΔH, positive ΔS
What happens when ΔH is positive and ΔS is negative? - Answer -Nonspontaneous at
all temperatures
What happens when ΔH and ΔS are both positive? - Answer -Reaction becomes
spontaneous as temperature increases
What happens when ΔH and ΔS are both negative? - Answer -Reaction becomes
spontaneous as temperature decreases
Why can the maximum work of a system never obtained from a real process? - Answer
-Because such an irreversible process will always involve some free energy being
converted to heat
How do you find where a reaction becomes spontaneous? - Answer -Set ΔG equal to
zero and solve -- use T=deltaH/deltaS to find the temperature value.
Is a chemical reaction proceeding to equilibrium a spontaneous or non-spontaneous
change? - Answer -Spontaneous
How to determine reaction direction from ΔG? - Answer -ΔG<0 reaction proceeds to the
right
, ΔG>0 reaction proceeds to the left
Which direction does the reaction proceed if ΔG is positive? - Answer -To the left / to
reactants
Which direction does the reaction proceed if ΔG is negative? - Answer -To the right/ to
products
How to predict ΔS(system)? (change in entropy of a system) - Answer -Positive
ΔS(system) means available microstates increases/disorder increases
How to calculate the ΔSo (aka standard entropy change) of the formation of one mole of
gas from its elements? - Answer -1. Write the balanced chemical equation with the
proper coefficients that make the product ONE MOLE.
2. Find the value of the one mole of product in Appendix B. This is the deltaS(standard)
for the product.
3. Find the value of the reactants in Appendix B for deltaS(standard) and multiply them
by the coefficients you got in step 1.
4. Use total = products-reactants for your final value.
How to calculate ΔG (free energy) using enthalpy ΔH and entropy ΔS values? - Answer
-ΔG=ΔH-TΔS
What is the equation that relates ΔG to K? - Answer -ΔG=-RT ln K
What is the Boltzmann constant? - Answer -ln(# of microstates)
What does a big W mean? - Answer -More microstates
What does more microstates mean? - Answer -Big ΔS - more entropy
What does fewer microstates mean? - Answer -Smaller ΔS - less entropy
What happens to entropy when volume is increased? - Answer -Microstates increase,
entropy increases
What is the relationship between ΔS of a system, q, and T? - Answer -
ΔS(system)=qrev/T
What happens to entropy ΔS when temperature increases? - Answer -It increases
What is the relationship between mass and entropy ΔS? - Answer -As mass increases,
entropy increases
What is the relationship between molecular complexity and entropy ΔS? - Answer -As
molecular complexity increases, entropy increases
