# 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

————————————————————————–

## 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)*.

* Hint for Free-Response (c):

# Measuring Entropy

One useful way of measuring entropy is by the following equation:

ΔS = q/T    (1)

Where S represents entropy, DS represents the change in entropy, q represents heat transfer, and T is the temperature. Using this equation it is possible to measure entropy changes using a calorimeter. The units of entropy are J/K.

The temperature in this equation must be measured on the absolute, or Kelvin temperature scale. On this scale, zero is the theoretically lowest possible temperature that any substance can reach. At absolute 0 (0 K), all atomic motion ceases and the disorder in a substance is zero.
The absolute entropy of any substance can be calculated using equation (1) in the following way. Imagine cooling the substance to absolute zero and forming a perfect crystal (no holes, all the atoms in their exact place in the crystal lattice). Since there is no disorder in this state, the entropy can be defined as zero. Now start introducing small amounts of heat and measuring the temperature change. Even though equation (1) only works when the temperature is constant, it is approximately correct when the temperature change is small. Then you can use equation (1) to calculate the entropy changes. Continue this process until you reach the temperature for which you want to know the entropy of a substance (25 ºC is a common temperature for reporting the entropy of a substance).The Thermodynamics Table lists the entropies of some substances at 25 ºC. Note that there are values listed for elements, unlike DHfº values for elements. The reason is that the entropies listed are absolute, rather than relative to some arbitrary standard like enthalpy. This is because we know that the substance has zero entropy as a perfect crystal at 0 K; there is no comparable zero for enthalpy. The fact that a perfect crystal of a substance at 0 K has zero entropy is sometimes called the Third Law of Thermodynamics.