# Monthly Archives: February 2014

## Next Two Weeks Homework (Feb. 15 to March 01, 2014)

I. Stoichiometry

II. Some Representative Groups and Families

For all students:

• On Barron’s SAT book, Page 186 – 187, Chapter 6:  Stoichiometry (Chemical Calculation and the Mole Concept) Practice Excises, #1, #2, #3, #4, #5, #6 and #7.
• On Barron’s AP book, page 259- -260, Chapter 6 Stoichiometry, Practice Excises, #7 through #21.
• On Barron’s SAT book, Page 304– 305, Chapter 13: Some Representative Groups and Families, Practice Excises, #1 through #14. (You may review the contents on page 297 through 303 before you do the Practice Excises).

For the students who plan to take AP exam:

• On Barron’s AP book, page 261, Chapter 6 Stoichiometry,  Practice Excises, #22 through #25.

## 三月二号每一个学生在班内做演讲

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## enthalpy (H) v.s. internal energy (E)

If you are still having a problem with enthalpy vs. internal energy, please read:

ΔE = q + w      (ΔE is change in internal energy)    eqn (1)

Enthalpy (H) accounts for heat flow in processes occurring at constant pressure when no forms of work are performed other than P-V work.

H = E + PV   eqn (2)

H is a state function because E, P, and V are all state functions.

Δ H = Δ (E + P V)   eqn (3)

When a change occurs at constant pressure,

Δ H = Δ E + P Δ V      (notice that V Δ P   = 0, since Δ P = 0 at constant P), eqn (4)

Recalling ΔE = q + w, eqn (1),

The work involved in the expansion or compression of gases is w = P Δ V,

Substitute w for P Δ V and q + w for ΔE in eqn (4):  Δ H = Δ E + P Δ V = (qp + w) – w =  qp , eqn (5)

Δ H = qp,   eqn (6)

qis the heat flow in the process at constant pressure.

Therefore, Δ H (change in enthalpy) equals the heat gained or lost at constant pressure.

Because we can either measure or readily calculate qp, and because so many physical and chemical changes of interest to us occur at constant pressure, enthalpy H is a more useful function than internal energy E.

For most reactions the difference in Δ H and Δ E is small because P Δ V is small.

Last note: relationship between Δ H and heat qp has specific limitations that only P-V work is involved and the pressure is constant.

## Homework for the week of Feb. 03 to 08, 2014

For all students:

On Barron’s AP book, page 445 -447, Practice Excises, #5, ,#6, #9, #10, #18, #19, #20, and #21.

For the students whose high school chemistry class has not covered the Thermodynamics chapter, you must study the following before you try to do this assignment:

• Eqn (12.31) and the example on Barron’s AP book, page 437, and Appendix 3 on page 750,
• Eqn (12.32) on Barron’s AP book, page 438,
• Eqn (12.33) and the example on Barron’s AP book, page 439,
• Example 12.4 and Example 12.5 on Barron’s AP book, page 440 and 441.

For the students who plan to take AP exam:

On Barron’s AP book, page 448 -449, Free-Response (a), (b) and (c)*.