Unit 1: Energy Changes and Rates of Reaction Chapters 5:
Thermochemistry Chapter 6: Chemical Kinetics
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Different forms of energy
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Law of Conservation of Energy energy cannot be created or
destroyed, only transformed from one form to another (1 st Law of
Thermodynamics)
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Thermochemistry study of energy changes in physical, chemical
and nuclear changes
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Energy and Molecules Kinetic Energy: energy of motion/work
(i.e. collision theory). Substances may have Thermal Energy (energy
from motion of molecules- amount of substance matters). Temperature
is the average kinetic energy of the particle (amount of substance
negated). Potential Energy: (due to position and composition)
stored in molecules because of the arrangement of nuclei and
electrons in its atoms, electrical forces b/w charged
particles
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Endo vs. Exothermic Which process requires energy? releases
energy? breaking bonds or forming bonds What is the net E change
if: a) E (required to break bonds) > E (released when bonds are
formed)? net change = E is absorbed (endothermic) reactants +
energy products b) E (required to break bonds) < E (released
when bonds are formed)? net change = E is released (exothermic)
reactants products + energy
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IMPORTANT TERMINOLOGY Chemical System: substances (reactants
and products) we are studying, represented by the chemical
equations Open System: Energy and matter can move in or out
Isolated System: an ideal system where no energy or matter can move
in or out Closed System: only energy can move in or out, but not
matter Surroundings: all the matter around the system that absorbs
or releases heat Universe Surroundings System Exchanges matter and
energy
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3 TYPES OF SYSTEMS Identify the systems below:
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Why is E released during an exothermic reaction? What is the
source of E released during a reaction?
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Discuss the combustion of gasoline in terms of PE, KE, energy
absorbed, energy released, net energy change, exo or endothermic,
work. PE of bonds in reactants > PE of bonds in products KE of
reactants < KE of products E absorbed to break bonds < E
released when bonds are formed net energy change is release
(exothermic) High KE of product gases does work on car parts to
make the car move. gasoline + oxygen carbon dioxide + water +
energy
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Homework: p. 282 #1,2,4,5 p. 285 A-C p. 291 #1,2,4,7-9
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Follow-up to 5.1 1.What is the difference between thermal
energy and temperature of a substance? 2. Compare the potential
energies of the reactants to that of the products for the two types
of chemical processes below: (a) endothermic process (b) exothermic
process 3. Label the following reactions as endothermic,
exothermic, or not enough information. (a) O 2 O + O (b) O 2 + O O
3 (c) H 2 + Br 2 2HBr (d) NaCl Na + + Cl 4. Classify each of the
following as an open system or a closed system. (a) a pot of
boiling water (b) a sealed bottle of water (c) a helium balloon
(d)a hot-air balloon
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How do we measure energy changes during a physical or chemical
change? A Simple Calorimeter Key Assumptions: 1. Any thermal energy
transfer b/w the calorimeter and the universe is negligible. 2. Any
thermal energy absorbed by the calorimeter itself is negligible. 3.
All dilute, aqueous solutions have d = 1.00 g/mL and c = 4.18
J/gC.
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Calculations Involving Thermal Energy Transfer Understanding
the formula: q is thermal energy transferred, in J or kJ m is mass,
in g T = T 2 -T 1 the temperature change, in C c is the specific
heat capacity, in J/(g C)(Table 1 p. 292) q=mcT Substancec Ice
Water Steam Aluminum Substancec (J/(g C)) Ice2.01 Water4.18
Steam2.01 Aluminum0.900 q > 0 exo q < 0 endo
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p. 295: Tutorial 1 Sample Problem 1: A student places 50.0 mL
of liquid water at 21.00C into a calorimeter. She places a sample
of gold at 100.00C into the calorimeter. The final temp of the
water is 21.33C. Calculate the quantity of thermal energy, q,
absorbed by the water in the calorimeter. G iven R equired A
nalysis S olution P araphrase
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Sample Problem 2 Using the value of q from #1, calculate the
specific heat capacity of the sample of gold if its mass is 6.77 g.
Assume that the final temp of the gold sample was the same as the
final temp of the water in the calorimeter.
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Sample Problem 3 A 50.0 mL sample of 1.0 mol/L HCl (aq) was
mixed with 50.0 mL of 1.0 mol/L NaOH (aq) at 25C in a calorimeter.
After the solutions were mixed by stirring, the temperature was
31.9C. Determine the quantity of thermal energy transferred by the
reaction to the water and state whether the reaction was
endothermic or exothermic.
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p. 297 #1-3
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Mini-Investigation: p. 297
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Demos KMnO 4 and glycerol in fume hood Ba(OH) 2(s) + NH 4 SCN
(s) less stable more stable system surroundings enthalpy change H
sys heat (q surr )
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Re-draw for an endothermic rxn!
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Enthalpy Change Internal energy - sum of all KE and PE of all
the components of the system We cant measure internal energy, so
measure enthalpy change the energy absorbed from or released to the
surroundings during a reaction Recall: 2 types of energy Potential
Energy includes: Protons and neutrons within nuclei Electrons in
bonds Kinetic energy includes: Moving e- within atoms Vibrational,
rotational, and translational motion
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ENTHALPY CHANGE (H) aka heat of reaction, enthalpy of reaction,
change in heat content H > 0, q < 0 Endothermic H 0
Exothermic H means @SATP: 100kPa, pure liquids/solids, 1 M, 25C or
298K H system = - q surroundings Calorimetry Calculations: q surr =
mcT H = -q enthalpy per gram = H/m molar enthalpy = H/n where n =
m/M
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MOLAR ENTHALPY (H X ) H X (molar enthalpy) energy
released/absorbed by system per mole of reacting substance units of
kJ/mol good for reference or comparison H (enthalpy change) energy
released/absorbed by system as R P units of kJ depends on amount of
substance reacting H x =H n H =nH x n = H H x
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Mini-Lab p. 285 Calculate the molar enthalpy of solution for
the salt used in the expt.
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Molar Enthalpy Examples Ex 1: The molar enthalpy of
vaporization of Freon-12 is 34.99 kJ/mol. If 100.0 g of Freon-12 (M
= 120.91 g/mol) is vaporized, calculate the expected enthalpy
change. Ex 2: 50.0 mL of 0.300 mol/L CuSO 4(aq) is mixed with and
equal volume of 0.600 mol/L NaOH (aq). The initial temperature of
both solutions is 21.4C. After mixing, the highest temperature
reached is 24.6C. Determine the enthalpy change of the
reaction.
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Ex 3: A chemist wants to determine the molar enthalpy of
neutralization for the following reaction. HCl (aq) + NaOH (aq)
NaCl (aq) + H 2 O (l) The chemist uses a coffee cup calorimeter to
neutralize 61.1 mL of 0.543 mol/L HCl (aq) with 42.6 mL of NaOH
(aq). The initial temperature of both solutions is 17.8 C. After
neutralization, the highest recorded temperature is 21.6C.
Calculate the molar enthalpy of neutralization, in kJ/mol HCl. Ex
4: What mass of KCl must have dissolved if the temperature of 200.0
g of water increased by 5.5C? (H soln,KCl = 1.7 x 10 4 J/mol)
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Investigation 5.2.1 Molar Enthalpy of a Chemical Change p. 333
Title Purpose Observations Analyze and Evaluate (a-g) Apply and
Extend (h,i)
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Homework Practice Problems p. 301 #1-4 Section Review p. 306
#1,2,4
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REPRESENTING ENTHALPY CHANGES 4 Methods- 1. Include energy
value in the thermochemical equation H 2(g) + O 2 H 2 O (l) + 285.8
kJ 2. Write the chemical equation and enthalpy change H 2(g) + O 2
H 2 O (l) H= - 285.8 kJ 3. State the molar enthalpy for a specific
reaction & substance H Form = - 285.8 kJ/mol H 2 O (l) 4. Draw
a chemical potential energy diagram Exothermic= product side
Endothermic= reactant side Exothermic= product side Endothermic=
reactant side Equation must be balanced Watch +/- signs Equation
must be balanced Watch +/- signs Molar Enthalpy!!! (kJ/mol)
Standard molar enthalpies (H x ) are @ SATP Molar Enthalpy!!!
(kJ/mol) Standard molar enthalpies (H x ) are @ SATP
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4. Draw a chemical potential energy diagram Changes to E p as
bonds are broken and formed Exothermic Reactions= Energy released=
E p =products lower than reactants Endothermic Reactants= Energy
gained= E p =products higher than reactants REPRESENTING ENTHALPY
CHANGES 4 Methods- (kJ) Remember: Title Label axis Units Reactants
on LEFT Products on Right Potential Energy Diagram for the
Formation of Water
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REPRESENTING ENTHALPY CHANGES 4 Methods- What about the reverse
reaction, the decomposition of water? 1. Include energy value in
the thermochemical equation 2. Write the chemical equation and
enthalpy change. 3. State the molar enthalpy for a specific
reaction. 4. Draw a chemical potential energy diagram.
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REPRESENTING ENTHALPY CHANGES Ex. What is the thermochemical
equation for the following chemical equation and molar enthalpy of
combustion for butane: 2 C 4 H 10 (g) + 13 O 2 (g) 8 CO 2 (g) + 10
H 2 O (l) H comb = -2871 kJ/mol Remember that thermochemical
equations contain H values!