Instantaneous & Average Rate of Change And 5 Factors

Tangent = line that touches a curve at only one point
Secant = line that crosses a curve at two points

determining slope:
http://cstl.syr.edu/fipse/graphb/Unit8/Unit8a.html

Rate of Reaction = change in concentration/change in time:
[conc]/t {where  = delta)

If change in concentration is plotted versus change in time:
Instantaneous rate of reaction = tangent to curve at specified time
Average rate of change of reaction = secant to curve at times specified

5 Factors Affecting Reaction Rate:
-nature of the reactants (somethings react differently than others)
-concentration (higher concentration = more particles colliding, greater likelihood that particles will collide in such a way as to result in a successful collision to  produce the desired product)
-temperature (higher temperature = faster moving particles =more collisions =greater likelihood that collisions will occur with sufficient energy to break reactant bonds to produce desired product)
-catalyst (reaction specific chemial that can speed up the rate of a reaction by altering the energy required to break reactant bonds and allowing new product bonds to form - more on the "how this happens" in another lesson)
-surface area (greater surface area = more particles exposed to react, more particles able to react, greater likelihood that successful collisions will occur resulting in desired product)

Collision Frequency Vs Collision Effectiveness
Frequency how often particles collide
      {concentration, surface area & temperature}
Collision Effectiveness - degree to which collisions result in desired product
      {nature of reactant, catalyst & temperature}
Q: What does Tarzan say when he sees a herd of elephants in the distance? A: "Look, a herd of elephants in the distance" Q: What does Tarzan say when he sees a herd of elephants with sunglasses? A: Nothing. He doesn't recognize them. How quickly did you react to the previous jokes? [i.e your reaction rate?]

Spontaneity?

Change in enthalpy (energy): H
Change in entropy (degree of order): S

Tendency to be spontaneous:  H = -ve (exo) and  S= +ve
Tendency not to be spontaneous: H = +ve (endo) and S= -ve

Predicting the sign of S
Two things to be able to do:
(1) When looking at a balanced chemical equation, a reaction will tend to be spontaneous if when comparing reactants and products
(a) there are more moles of gaseous products
(b) products are simpler, smaller, less complex
(2) Calculating S
  (products) - (reactants)
use data table pp 799-800 and additional elemental information

Next up
-calculating the slope of a line with a ruler and a pencil (slope = rise/run)
-drawing a tangent and a secant to a curve

In honour of the ether bunny funny from earlier (bunny-O-bunny), a few hare hystericals here now. Q: How many rabbits does it take to change alightbulb? Ans: Only one if it hops right to it.  Q: If you have a line of 100 rabbits in a row and 99 of them take 1 step backwards, what do you have? A: A receding hare line.  Q: How do you catch a unique rabbit? Ans: Unique up on it.  Observation: In order to catch a rabbit, you must hide behind a tree and make a noise like a carrot.

Bond Energies

Bond energies require knowledge of molecular structure
O2: O=O     N2: - has a triple bond (still need to figure a few symbols out)

Formation reactions: produce one mole of product from elements in their standard state
-remember HOFBrINCl & the making one mole of product
Na(s) + 1/2 H2(g) + C(s) + 3/2 O2(g) --> NaHCO3(s)

Last but not least when calculating a reaction's enthalpy change (H):
Bonds broken - bonds formed
              (reactants)                             (products)
Yes, it is the reverse of the other calculations.

Please note that H (enthalpy change) is not the same as Hx (molar enthalpy change)
One more thing before wrapping up. Consider the generic reaction:

2A  +  B  -->  A2B  H = -300kJ

molar enthalpy of A: HA = -150kJ/mol 
molar enthalpy of B: HB = -300kJ/mol
[That is all for today. Question: What is the name of the molecule bunny-O-bunny? Answer: An ether bunny  Question: What is a cation afraid of? Answer: A dogion.]

Hess’s Law and Hof [aka heats of formation]
**technical difficulties abound with little delta symbols and superscripts and subscripts - do you know how to include symbols, superscripts and/or subscripts?



With Hess’s law – use each of the given equations, add to arrive at the target equation
-if the equation is reversed, the sign of H is also reversed (+ to – and/or – to +)
-if the equation is multiplied by a value, multiply H by the same value

One more thing, consider the equation written below:

4CO2 (g) + 6 H2O(g) -->2 C2H6 (g) + 7 O2 (g)            H = 849.0KJ

the molar enthalpy of CO2: HCO2 = 212.25 kJ/mol        [849kJ / 4 mol]

Any equation can be interpreted per mole of any substance in the equation

Heats of Formation Hof

-be able to write the formation reaction for any substance [produce one mole of product from elements in their standard state]

-when calculating H with heats of formation (data tbl 799-800) remember that it is:

                 (products) – (reactants)

For a small change a few elephant jokes. Ready or not, here they come. Q: How do elephants communicate? [On the elephone.]  Q: Why are elephants large, grey and wrinkled? [Have you ever tried ironing one?]  What time is it when an elephant sits on your fence? [Time to get a new fence.]