1. Flashcards for Unit 13

2. All physical & chemical changes are accompanied by energy changes.
Thermochemistry is the study of heat flow in physical or chemical change.
Thermochemistry

3. 2 ways to transfer energy
Heat flow
Work
2 ways to transfer energy

4. What is the relationship between stability and potential energy?
The higher the potential energy, the lower the stability, and vice versa.
What is the relationship between stability and potential energy?

5. Conservation of Energy
Energy of the universe is a constant.
Energy can be transformed from one form to another.
Energy can move between the system and the surroundings.
Energy CANNOT be created or destroyed.
Conservation of Energy

6. Conservation of Energy
Total energy before a change =
Total energy after the change
Conservation of Energy

7. Energy of motion
Kinetic Energy

8. Potential Energy Stored energy or energy of position.
In a molecule, potential energy depends on which atoms and how they are arranged.
(i.e., composition & structure)
Potential Energy

9. system
surroundings
Universe
In chemistry, energy flow is always labeled from the system’s perspective.
Chemistry Model

10. Energy is released: It flows from system to environment
Energy is released: It flows from system to environment. Temperature of environment increases.
system
surroundings
Universe
Exothermic Reaction

11. Energy is absorbed: It flows from environment to system
Energy is absorbed: It flows from environment to system. Temperature of environment drops.
system
surroundings
Universe
Endothermic Reaction

12. Coffee Cup Calorimetry
Way to measure heat flow, Q, at constant pressure.
Heat flow at constant pressure is the enthalpy change for that process.
Coffee Cup Calorimetry

13. Coffee Cup Calorimetry
The water is the environment.
Whatever we put into the water is the system.
Coffee Cup Calorimetry

14. H Enthalpy or Heat content in a system at constant pressure.
We can only measure changes in enthalpy. H

15. Hproducts - Hreactants
Heat of reaction, H

16. Potential Well Diagram: Useful when we can only measure changes
2Ag + ½ O2  Ag2O
Potential well diagram shows NET change in enthalpy.
Going downhill is exo. Going uphill is endo.
The deeper the well, the more stable the compound.
Enthalpy 
2Ag + ½ O2 H
Ag2O
Potential Well Diagram: Useful when we can only measure changes

17. Hproducts – Hreactants or H
The difference between the potential energy of the products & the potential energy of the reactants?

18. H is negative. System has net loss in energy.
H for exothermic rxn

19. Exothermic reaction. Products have less potential energy than reactants.
H is negative?

20. Endothermic reaction. Products have more potential energy than reactants.
H is positive?

21. H is positive. System has net gain in energy.
H for endothermic rxn

22. Exothermic Reaction. Heat term is on product side.
A + B  C + D + heat

23. Endothermic Reaction. Heat term is on reactant side.
A + B + heat  C + D

24. Which phase change is endothermic?
Gas to liquid
Gas to solid
Solid to gas
Liquid to solid
Solid
Liquid
Gas
Up: Endother mic
Down: Exother mic
Potential Energy  as the particle spacing 
Which phase change is endothermic?

25. Which phase change is exothermic?
Solid to gas
Liquid to gas
Solid to liquid
Liquid to solid
Solid
Liquid
Gas
Up: Endother mic
Down: Exother mic
Potential Energy  as the particle spacing 
Which phase change is exothermic?

26. Endothermic Energy flowed from the water into the system.
A reaction occurs in water & the temperature of the water decreases. Endo or Exo?

27. Exothermic
The water is the surroundings! Heat flow into surroundings is exothermic.
A reaction occurs in water & the temperature of the water increases. Endo or Exo?

28. If you flip an equation, you flip the sign of H.
What is H for C + D  A + B?
H = +45 kJ
If you flip an equation, you flip the sign of H.
A + B  C + D. H = -45 kJ.

29. A + B  C + D. H = -45 kJ. What is H for: 2A + 2B  2C + 2D?
H depends on how much stuff. If you double the amounts, you double H.
A + B  C + D. H = -45 kJ.

30. Math Facts for H If you flip an equation, you flip the sign of H.
H depends on how much stuff. If you double or triple the amounts, you have to double or triple H. If you cut the amount in half, then cut H in half.
Math Facts for H

31. Which reference table has information about enthalpy changes?
Table I
Heats of combustion, lines 1-7
Heats of formation, lines 8-18
Heats of solution, lines 19-24
Heat of neutralization, line 25
Which reference table has information about enthalpy changes?

32. The more negative the heat of formation, the more stable the compound is.
Al2O3 has the most negative heat of formation, so it is the most stable compound in table I.
How do the heats of formation in table I relate to the stability of the compound?

33. Degree of disorder or chaos in a system.
Entropy

34. The entropy of the universe is increasing.
Law of Disorder

35. The more degrees of freedom a system has, the higher the entropy.
Degrees of freedom = number of ways you can move + number of places you can be.
Entropy

36. Entropy S

37. 0 entropy? A perfect crystal at 0 Kelvin.
Unlike enthalpy, we can measure entropy, not just changes in entropy.
0 entropy?

38. Change in entropy Sproducts – Sreactants

39. Which phase has the greatest entropy?
Gas phase – the molecules have more ways of moving & more places to be!
Which phase has the greatest entropy?

40. Which phase has the greatest entropy?
Sgas >> Sliquid > Ssolid
Which phase has the greatest entropy?

41. Did the entropy increase or decrease as the solid dissolves?
NaCl(s)  Na+(aq) + Cl-(aq)
Increase!
Did the entropy increase or decrease as the solid dissolves?

42. Did the entropy increase or decrease as the gas dissolves?
CO2(g)  CO2(aq)
Decrease!
Did the entropy increase or decrease as the gas dissolves?

43. Did the entropy increase or decrease?
2H2O(l)  2H2(g) + O2(g)
Increase!
Did the entropy increase or decrease?

44. How does entropy change with temperature?
As temperature increases, entropy increases.
The random thermal motion of the molecules increases.
How does entropy change with temperature?

45. Physical or Chemical change that occurs with no outside intervention
Physical or Chemical change that occurs with no outside intervention. (Supplying activation energy does not count!)
Spontaneous Process

46. What factors determine the spontaneity of a reaction?
The enthalpy change.
The entropy change.
The temperature at which the change occurs.
What factors determine the spontaneity of a reaction?

47. Nature is lazy & disorganized.
Nature favors:
Negative value for H: Net loss
Positive value for S: Net gain
Nature is lazy & disorganized.

48. Possible combos of H & S
What can you tell just from the signs of H and S? +
endo
chaos 
H against nature, S with nature. Spontaneity depends on the temperature. -
chaos 
Both changes are going against the direction preferred by nature. NEVER spontaneous.
exo
Both changes are going in the direction preferred by nature. ALWAYS spontaneous.
H with nature, S against nature. Spontaneity depends on temperature.
Possible combos of H & S

49. When will a change always be spontaneous?
When both factors go with nature:
Negative value for H: Net loss in energy.
Positive value for S: Net gain in chaos
When will a change always be spontaneous?

50. When will a change never be spontaneous?
When both factors go against nature:
Positive value for H: Net gain in energy.
Negative value for S: Net loss in chaos
When will a change never be spontaneous?

51. Nature is lazy & disorganized!
exothermic – nature likes.
chaos  because 3 additional moles of gas were generated – nature likes.
Always Spontaneous!
Predict the spontaneity for: C4H9OH(l) + 6O2(g)  4CO2(g) + 5H2O(g) kJ

52. Predict the spontaneity for: 6C(s) + 3H2(g) + 49 kJ  C6H6(l)
Nature is lazy & disorganized!
Endothermic – against nature.
Becomes more organized – lose 3 moles of gas – against nature.
Never Spontaneous!
Predict the spontaneity for: 6C(s) + 3H2(g) kJ  C6H6(l)

53. What about the other two possible combos of H & S?
When one term is with nature & one term is against, spontaneity depends on the temperature. Have to do a full calculation.
So for (+ H & + S) or for (- H & - S): can’t tell by looking at equation.
What about the other two possible combos of H & S?

54. Potential Energy diagrams for reactions
Hill diagrams!
Don’t just show the net change.
They show how the enthalpy changes over the entire reaction.
Potential Energy diagrams for reactions

55. Endo: starts low, ends high
Endo or Exo?

56. Exo: starts high, ends low
Endo or Exo?

57. Absorbs Energy
Breaking Bonds …

58. Releases Energy
Making Bonds …