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- Instructor: Upali Siriwardane (Ph.D., Ohio State University)
- CTH 311, Tele: 257-4941, e-mail: upali@chem.latech.edu
- Office hours: 10:00 to 12:00 Tu & Th ; 8:00-9:00 and 11:00-12:00 M,W,& F
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- Tests will be given in regular class periods from 9:30-10:45
a.m. on the following days:
- September 22, 2004 (Test 1): Chapters 1 & 2
- October 8, 2004(Test 2): Chapters 3,
& 4
- October 22, 2004 (Test 3):
Chapter 5 & 6
- November 12, 2004 (Test 4): Chapter 7
& 8
- November 15 Brief survey of
chpater 9-10
- November 17, 2004 MAKE-UP: Comprehensive
test (Covers all chapters 1-8)
- Grading:
- [( Test 1 + Test 2 + Test3 +
Test4 + Test5)] x.70 + [ Homework + quiz average] x 0.30 = Final Average
- 5
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- Thermodynamics
- 1. Endothermic and exothermic based on heat flow between a system and
its surroundings.
- 2. Enthalpy (DH), entropy (DS) , and free energy (DG)
- 3. Experiments to get
thermochemical information and fuel values.
- Reaction Rates
- 4. Reaction rate and the role of kinetics in chemical and physical
change.
- 5. Activation energy and the activated complex and effects on reaction rate.
- 6. Predict the affect of, concentration, temperature, and catalysis on
the rate of a chemical reaction.
- 7. Write rate equations for elementary processes.
- Chemical Equilibrium
- 8. Equilibrium chemical reactions.
- 9. Equilibrium-constant expressions and equilibrium constants.
- 10.LeChatelier's principle for predicting equilibrium position.
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- Thermodynamics is study of energy, heat and work.
- As chemists we are interested in heat changes in chemical and physical
changes.
- Aim: to predict whether a change (both physical and chemical ) will
occur spontaneously when left to itself--do we have to do anything other
than mix the reactants together to make it occur?
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- In a chemical reaction heat is released or absorbed by our system to or
from the surroundings. In a chemical reaction convert energy in bonds
into heat energy (and vice versa).
- We can measure the energy changes in these processes.
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- System:
- Surroundings:
- Universe:
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- Law of Conservation of Energy:
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- Heat changes during chemical reactions
- Thermochemical equation. eg.
- H2 (g) + O2
(g) ---> 2H2O(l) DH =- 256
kJ;
- DH is called the enthalpy of
reaction.
- if DH is + reaction is called endothermic
- if DH is - reaction is called exothermic
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- In an endothermic reaction
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- Define enthalpy (H):
- Normally talk about a change in enthalpy (DH, DHo):
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- When solid NaOH is dissolved in water the soln gets hotter.
- S(s) + O2(g) g SO2(g) DHo = -71kcal
- N2(g) + 2O2(g
+16.2kcal g 2NO2(g)
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- A spontaneous reactions is one that occurs without us having to do
anything to it (once it has started). (No external energy input)
- Name some spontaneous processes.
- Note that one direction is spontaneous, the reverse is not.
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- We want to come up with a way of predicting whether something will be
spontaneous.
- It has been observed that many exothermic processes are spontaneous.
- Question: Does exothermicity guarantee that something is spontaneous?
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- Look at H2O(s) g H2O(l) DHorxn=1.44kcal
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- Entropy (S, So) is a measure of the disorder, or randomness
of a system.
- The greater
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- For a rxn: D Srxn= sum of entropy of all the products
minus the sum of the entropy of all the reactants.
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- A log burns in a fireplace
- Water vapor condenses on a cold surface
- A solid metal melts
- Water boils
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- Look at H2O(l) g H2O(s)
- DS<0 but
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- The entropy of the universe increases in a spontaneous process and is
equal to zero in a system at equilibrium.
- Not always easy to calculate the entropy change of the universe.
- Define a new quantity, G, Gibbs
free energy. G refers to the system we are studying.
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- DG = D H - T D S
- This equation combines the
exothermicity and positive entropy criteria.
- Criterion for spontaneity
- If DG < 0
- If DG > 0
- If DG = 0
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- Look at H2O(l) g H2O(g) water boiling
- DH= 10.6kcal and DS =0.0284kcal
- DG = DH -T DS
- at 50oC DG =
- at 100oC DG =
- at 120oC DG =
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- An exothermic rxn that has a positive entropy change is
- An endothermic rxn that has a negative entropy change is
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- An exothermic rxn that has a negative entropy change is spontaneous
- An endothermic rxn that has a positive entropy change is spontaneous
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- Measure heat change (temp inc or dec) in a quantity of water or solution
that is in contact with the reaction of interest and is isolated from
the surroundings.
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- Q= m x SH x DT
- Q = heat change; DT = temperature change = Tfinal-Tinitial
- Add heat, temp inc; remove heat temp dec
- Specific heat (SH) = the amount of heat (cal) required to raise the
temperature of one g of a substance by 1°C
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- Units of SH : cal/(g oC); [SH of H2O= 1.00
cal/(g oC); of Al = 0.21 cal/(g oC)]
- Calc the amt of heat liberated (in cal and kcal) from 366 g of aluminum
when it cools from 77.0oC to 12.0oC.
- 10kJ of heat is supplied to 1000g of H2O and to 1000g of Al.
Calc the inc in temp for both.
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- To raise the temp of a mass of water from 25oC to 50oC
requires 7.5 kcal. What is the mass of the water?
- A sample of Al weighs 67 g. If 854 cal of heat are required to raise the
temp of this sample from 25oC to 85oC, calculate
the specific heat of aluminum.
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- Fuel value is amt of energy per gram of food.
- One nutritional Calorie (C) = 1 kcal = 1000 cal; 1 cal = 4.184 J; 1 kcal=1 Cal =4.184
kJ
- 8.7: A 1.00 g sample of a candy bar (which contains a lot of sugar was
burned in a bomb calorimeter. A 3.0oC temp increase was
observed for 1.00 x 103 g of water. The entire candy bar
weighed 2.5 ounces. Calc the fuel
value (in nutritional Calories) of the sample and the total caloric
content of the candy bar.
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- 8.8: If the fuel value of 1.00g of a certain carbohydrate is 3.00
nutritional Calories, how many grams of water must be present in the
calorimeter to record a 5.00oC change in temp?
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- Thermodynamics tells us whether a reaction should occur spontaneously,
but does not tell us how fast the reaction will occur. Kinetics tells us
- For example, thermodynamics says that diamond will spontaneously change
into graphite.
- Kinetics tells us
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- Look at H2(g) + I2(g) g 2HI(g)
- Let’s envision how the rreaction might occur on a molecular basis.
- To react
- 1. H2 must collide with I2
- 2. There must be enough energy to break a H-H and a I-I bond to initiate
reaction
- 3. The molecules must collide in the correct geometry.
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- A molecule that is moving has kinetic energy; faster the motion, the
greater the KE. When molecules collide some of the KE is changed into
vibrational energy of the bonds . Sometimes there is enough energy
gained through collision to break a bond and initiate reaction. If the
energy is not enough to break the bond, the molecules bounce of one
another with no reaction occurring.
- So there is some minimum collision energy below which no reaction
occurs.
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- I. Structure of reacting species: A. oppositely charged species often
react faster than
neutral species. B. bond strength can influence rxn
rate (Ea)
C. size and shape of molecule can be important
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- II. Concentration of reactants: if
increase the conc of the reactants:
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- III. Increase the temperature:
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- IV. Physical state of reactants:
reactions in solution (liquids) are often very fast. In the solid
state molecules have limited motion,
in the gas phase have large distances between molecules and not
many collisions so these reactions may be slower. In the liquid phase
the molecules (ions) are able to move and are close to each other.
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- V. Add a catalyst. A catalyst speeds up the rate of reaction by
- . The catalyst generally works by giving a different pathway (of lower
energy) for the reaction to occur. A catalyst is not used up (consumed)
in a reaction. The catalyst appears to be unchanged at the end.
- Biological catalysts: enzymes
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- Object is to develop a mathematical relationship btn rate and concs of
various species: know as rate equation (law)
- for A +B + C g products we say
the
- or
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- For a rate law where rate = k [A]x[B]y[C]z
- say
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- H2(g) + I2(g) g 2HI(g) rate = k[H2][I2]
- H2(g) + I2(g) g 2HI(g) high P, Au catalyst rate = k
- 2N2O5 g 4NO2 + O2 rate =
k[N2O5]
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- 2NO(g) + O2(g) g 2NO2(g) rate = k[NO]2[O2]
- CHCl3(g) +Cl2(g) g CCl4(g) + HCl(g) rate
= k[CHCl3][Cl2]1/2
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- A reaction is found to be second order in A and third order in B. Write
the rate equation.
- The rate equation and the rate constant have to be determined
experimentally.
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- Write the general form of the rate equation for:
- CH4(g) + O2(g) g CO2(g) + 2H2O(g)
- 2NO2(g) g 2NO(g) + O2(g)
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Notes
