CSI Online: Eyewitness Basics Websites

Site #1: Art of Crime Scene Detection

Site #2: Spot the Differences

Site #3: Powers of Observation

Site #4: FBI Games -- Choose the Matching Game

Site #5: QUIA: Crime Scene Basics

--From the Worksheet Found at, www.sciencespot.net

Entropy Homework Answers

∆S and ∆G Homework

For the first three reactions below, indicate whether you believe the entropy for

the reaction is positive or negative:

1) ____ NaN3(s) à  ____ Na3N(s) + ____ N2(g)               POSITIVE

2) ____ Al(s) + ____ Br2(l) à ____ AlBr3(s)                    NEGATIVE

3) ____ Hg(l) + ____ O2(g) à ____ HgO(s)                       NEGATIVE

Solve the following problems regarding entropy and the spontaneity of chemical

reactions:

4) A chemical reaction has a ∆Hrxn of – 157 kJ and a ∆Srxn of – 221 J/K.  Is

this reaction spontaneous at 525 K?

ΔG = -157 --  (525)(-0.221)

ΔG = -40.975

YES, this reaction will be SPONTANEOUS BECAUSE,

+ΔG = non-spontaneous

-ΔG = spontaneous

ΔG = 0 = equilibrium

5) At what temperature is the reaction from problem #4 at equilibrium?

0 = -157 – (T)(-0.221)

157 = -(T)(-0.221)

-157 = T(-0.221)

T = 710 K

6) If a chemical reaction is at equilibrium at 298 K and ∆Srxn = -89 J/K, what

is ∆Hrxn for this process?

ΔH = -26.522

7) What is ∆G for the reaction in problem 6 at 398 K?  Is this reaction

spontaneous?

ΔG = 8.9

NO, this reaction is NON-SPONTANEOUS

Potential Energy Diagrams

Calculating Heat of Reaction Using Heats of Formation Answers

Tips:
- Use the steps provided in problem #1 for each problem.
- In problems 3 and 4 you are not solving for the heat of reaction, but rather the heat of formation of one of the molecules.


1. -90.566 kJ
2. -1057.9 kJ
3. -415.5 kJ
4. -157.3 kJ
5. -2069.6 kJ

Hess's Law Answers

1. -1220 kJ

2. Here's what will happen:
Multiply (1) by 2, Leave (2) as it is, reverse (3).

Heat of Reaction= 15.3 kJ

3. This is wicked hard. If  you do this I'll give you extra credit. For better practice, try this,


(1)  Find the ΔH for the reaction below, given the following reactions and subsequent ΔH values:
PCl₅(g)  →  PCl₃(g)  +  Cl₂(g)

P₄(s)  +  6Cl₂(g)  →  4PCl₃(g)            ΔH = -2439 kJ

4PCl₅(g)  →  P₄(s)  +  10Cl ₂(g)         ΔH = 3438 kJ
answer = 249.8 kJ

Stoichiometry Help/Answers

Stoichiometry Answers:

1. 7904.5 kJ


2. 209.4 kJ


3. 3.33 kJ

Specific Heat Help/Answers

Specific Heat Practice:
1.
q=???
m=250.0g
c=0.385
deltaT=45

2.
q=435J
m=3.4g
c=?
deltaT=85-21

3.
q=4500.J
m=2.50g
c=0.385
deltaT= (F--30.0)






Answers:
1. 4,331.25J
2.1.999 or 2.0
3. 4705 degrees