Accelerated Chemistry Chapters 16 and 17 Notes
(Student edition)
Chapter 16/17 problem set: page 552: 7-12, 15, 27;page 581: 1, 5, 11, 13-15
Reaction Energy and Reaction Kinetics
Thermodynamics: the study of in chemical reactions
Substances have stored in them. It is stored in , stored in , and any
other form of energy.
Substances also have energy ( of the molecules).
The sum of the energy is the heat content known as
Heat of reaction ( change): heat or during a chemical reaction
Exothermic versus Endothermic:
Reactants Products
Products Reactants
Reactants Products + Reactants + Products
The reaction is . The reaction is .
Heat of Formation (Hf) – heat released or absorbed when of a compound
is from its elements
2 H2+O22 H2O+ 572 kJ( not Hf )
H2+½ O2 H2O+ 286 kJ( yes! Hf)
Hf = - 286 kJ
Example 1: The heat of formation of NaCl is -411.15 KJ. Express this in three ways.
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Example 2: The heat of formation of NaCl is -411.15 KJ. Express this in three ways.
Stability of Compounds
A high negative Hf means that the compound is very .
Why? The compound a lot of energy during its formation so it
takes just as much to the compound apart.
A high positive Hf means that the compound is very .
Why? The compound took quite a bit of to make the elements join
and will probably require very little activation energy to make the
compound apart to go to lower energy.
Which is more stable?
CS2(g) Hf = +117.07 Kcal/mole
FeCl3(s) Hf = -399.49 Kcal/mole
SO3(g) Hf = -395.72 Kcal/mole
Solid is more stable because it has a heat of formation
that is more
Hess’ Law: The overall enthalpy change in a reaction depend on the number
of . The enthalpy change is equal to the of the enthalpy changes
for the individual steps in the process.
Example 1: Calculate the heat change using Hess’ Law:
2 CO(g)+O2(g)2CO2(g)
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Bond Energy Problems:
Example 1:
2 H2(g)+O2(g)2 H2O(g)
The Driving Force of Chemical Reactions
In regard to enthalpy, is more favorable (- H).
For example;C8H18+O2CO2+H2O+ heat
However, this is not the only driving force.
Entropy (S): the measure of in a system. The higher disorder (more __S), the
likely the reaction is to occur (messy room, leaves on trees).
Systems tend to go towards energy (-H) and randomness (+S).
General trends in entropy:
slg=+ S
gg + g + g=+ S
temperature=+ S
Spontaneous reactions take place outside influence (they can be fast or slow).
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Ex1:2 C8H18(l) +25 O2(g) 16 CO2(g) + 18H2O(g) + heat
pieces and heat
Entropy (S) is and enthalpy (H) is .
Thus, the reaction is .
Ex2:CO2(g) + N2(g)+ H2O(g)+ heat C3H5(ONO2)3(l)
pieces and heat
Entropy (S) is and enthalpy (H) is .
Thus, the reaction is .
Ex3: H2O (s)H2O (l)
does it happen?
Entropy (S) is and enthalpy (H) is .
Thus, we determine the spontaneity based on the information given. Gibbs
Free energy formula is used to determine the spontaneity.
Free energy of a system: G = Gibbs Free Energy (combined enthalpy/entropy
function)
G = (use absolute temperature)
if G is negative, the reaction is
Possible combinations of entropy and enthalpy:
Spontaneous / Non-Spontaneous / Maybe / MaybeEnthalpy
Entropy
What determines a maybe?
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Is this reaction spontaneous?
H2O+ CCO+H2
H = + 131.3 kJ/mole
S =+ 0.134 kJ/mole.K at 25 Co
G =
G = The reaction is .
How about at 900 Co?
G =
G = The reaction is .
The Reaction Process
Reaction Mechanism: The series of in a reaction.
We have learned the following:
H2+I2 2 HIH = + 26.5 kJ
However, we now know that it is really:
Step 1I22 I
Step 2I+H2H2I
Step 3H2I+ I2 HI
H2+I22 HI
Reaction Intermediate: a species that appears in but not in the
reaction. It is relatively . So, in the above
example, is the reaction intermediate.
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Collision Theory: In order for molecules to react, they must , but doesn’t
guarantee reaction.
For a reaction to take place,
1. the collisions need enough
2. the particles need proper
Not enough energy to make new products
Activated complex: a structure existing when old bonds are broken and new
bonds are being formed.
Energy of activation: energy needed to transform .
Reactants must have sufficient .
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Energy Diagrams:
50
h 40
e
a 30
t
20
10
time
Energy of the reactants = Energy of the products =
Energy of the activated complex = Activation energy =
Change in the heat = The reaction is
50
h 40
e
a 30
t
20
10
time
Energy of the reactants = Energy of the products =
Energy of the activated complex = Activation energy =
Change in the heat = The reaction is
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Reaction Rate – reaction rates are affected by:
• nature of (type of bond – ionics react than covalents)
•
• concentration of
• (gases only)
• a (A substance that is in a reaction, but is not in the reaction.
Catalysts work by changing the of a reaction and lowering the
energy).
• area
All of these factors can be explained in relation to the collision theory.
Writing Rate Laws
For:A + B C
rate = k [ A ]x [ B ]y(determined experimentally)
If a reaction is one step, then the coefficients equal the .
If a reaction is multi step, then the coefficients do equal exponents.
Ex.2 H2 +2 NON2 + 2 H2O
If the reaction is one step, the reaction rate formula is …
If the reaction had multiple steps, the reaction rate formula would be something
else. Perhaps something like …
Notice that the coefficients and exponents are the same. An would have
to be performed to show this.
Ex.X+2YXY2 (a single step reaction)
Write the rate law:
If you double X, the rate .
If you double Y, the rate .
If Y is reduced to 1/3, the rate is .
If X is cut in half and Y is doubled, the rate .
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