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Thermochemistry
Mrs. StoopsChemistry
Thermo Day 1 2
Thermochemistry or Thermodynamics
• reactions involving heat.• Abbreviations to know –
– rxn = reaction– E = Energy = delta = change
Thermo Day 1 3
Kinetic Energy vs Potential Energy
• Potential – stored energy• Kinetic – movement energy
PE
KE
Units of Energy – Joule or J also KJcalories 1 cal = 4.184 J
Thermo Day 1 4
System vs Surroundings
We will focus on the system – the system changes depending on what we are looking at. Could be a beaker, or the room, or the Earth, or the solar system
Thermo Day 1 5
Work & Heat
• Do you work?• Work = force x distance moved
Thermo Day 1 6
Heat
• Heat is energy transferred. Heat is transferred from HOT to COLD.
• Energy is the capacity to do work or transfer heat.• NEW symbol = delta – means change T means change in temperature = Tfinal - Tinitial
Thermo Day 1 7
1st Law of Thermo
1st law of Thermodynamics = Energy is CONSERVED.
E = Ef - Ei
E is positive means Ef Ei. Also Energy has moved from surroundings into the system
E is negative means Ef Ei. Energy has moved from system out to surroundings.
Thermo Day 1 8
System vs Surroundings
Another way to solve for EE = q + w = heat + work. Sign direction issues!
System
q +Heat entering
q –Heat leaving
W –Do work
Thermo Day 1 9
Example problem:
Calculate the change in energy on the system when 140 J of heat are absorbed and 85 J of work are done.
E = (+140J) + (- 85J) = 55 J
Thermo Day 1 10
Exothermic & Endothermic
Cue words are important!!!Absorbs heat = goes INTO the system
Endothermic = q +Releases heat = EXITS the system
Exothermic = q –When something feels cold – the heat is being taken
away and used, leaving the object “feeling” cold.
Thermo Day 1 11
State Functions
• State functions – depend on the current conditions only, not the path taken.
E, average temperature
Thermo Day 1 12
Homework Day 1
Page 203: 25, 29
Enthalpy - H• Heat gained or lost **yeah, and q is too!• State function H = H products – H reactants
Thermo Day 2 14
Enthalpy - H
• Sign indicates the direction of heat transfer only!!!
• +H = gained heat (into system), Endothermic, REACTANT
• -H = lost heat (out of system), Exothermic, PRODUCT
Hrxn – heat of reaction - we will calculate this later, for now it is given.
Thermo Day 2 15
Thermochemical equation
• a reaction shown with the H valueEx:
• 2H2 + O2 → 2H2O H = -483.6 KJ
What this means:• Since H is negative, the enthalpy is a
_____________ (which side of rxn?)
• 2H2 + O2 → 2H2O + 483.6 KJ
*** the sign tells the side of reaction ONLY!
product
Thermo Day 2 16
Rules of using Enthalpy:
1. Enthalpy is an extensive property. The coefficients matter!
Ex: CH4 + 2 O2 → CO2 + 2 H2O H = -890 KJ
• This is for 1 mole CH4 = based on the stoichiometry
For 4 moles CH4 the H becomes -890 x 4 = -3560 KJ
Thermo Day 2 17
Rules of Enthalpy
2. The enthalpy sign changes when the reaction is reversed
CH4 + 2 O2 → CO2 + 2 H2O H = -890 KJ
But if we switch products for reactants
CO2 + 2 H2O → CH4 + 2 O2 H = +890 KJ
Thermo Day 2 18
Rules of Enthalpy
3. Enthalpy depends on the states of the reactants and the products.
The states have been listed in the reactions – you need to pay attention to.
(s) solid, (l) liquid, (g) gas, & (aq) aqueous – dissolved in water
H2 (g) + O2 (g) → H2O (g)
Thermo Day 2 19
Using these ideas for stoichiometry . . .
If you have a Thermochemical equation, you can be asked to solve a problem to find the heat transferred.
Ex: CH4 + 2 O2 → CO2 + 2 H2O H = -890 KJ
If you have 50 KJ of heat, how many grams of oxygen can be reacted?
50KJ x 2 mol O2 x 32 g O2 = 3.6 g O2
890 KJ 1 mol O2
*** would the answer be negative? What does the sign tell us?
Thermo Day 2 20
Calorimetry
• Measures heat flow by measuring the temperature changes
• Heat capacity = amount of heat needed to raise the temperature by 1 (degree) Kelvin
• Molar heat capacity = amount of heat needed to raise the temperature by 1 degree for 1 mole
• Specific heat = heat capacity for 1 gram
Thermo Day 2 21
To find specific heat or use specific heat
Heat transferred = mass (in grams) x specific heat x change in temperature
Equation form:• q = m c t• Values for c are on page 181 of text, given, or
you are asked to solve for them
Thermo Day 2 22
Homework Day 2
page 204: 37a, 41, 43, 51, 53
Thermo Day 3 23
Hess’s Law
If the reaction is carried out in a series of steps, the H for the overall reaction will be equal to the sum of the H for all the steps.
Generic example:Overall reaction A D H = ?
Steps:1 A B H = x2 B C H = y3 C D H = z
Overall A D H = x + y + z
Thermo Day 3 24
Example:Find the H for the reaction: 2 C (s) + O2 (g) 2 CO (g)
Steps:
1 C (s) + O2 (g) CO2 (g) H = -393.5 KJ
2 CO (g) + ½ O2 (g) CO2 (g) H = - 283.0 KJ
Work:
2 C (s) + O2 (g) 2 CO (g)
2C (s) + 2O2 (g) 2CO2 (g) H x 2 = -393.5 x 2 = -787 KJ
2CO2 (g) 2CO(g) + O2 (g) H x 2, R = +283.0 x 2 = 566 KJ
**Cancel**2 C(s) + O2 (g) 2 CO(g) -221 KJ
Thermo Day 3 25
Enthalpy of Formation
Hf
Depend on the state (s, l, g, aq)Standard enthalpy = HºStandard enthalpy of formation = Hfº = are listed
for 298 KUnits are KJ/molListed in the text book = appendix C (page 1019)***Element values are zero***
Thermo Day 3 26
Enthalpy of Formation
Hrxn = n Hfºproducts - m Hfºreactants
What are these? - sigma, Greek letter, means sum ofn & m – coefficients from balanced equation
Thermo Day 3 27
ExampleFind the Hrxn for C2H5OH(l) + 3 O2 (g) 2 CO2(g) + 3 H2O
(l)
Look up values:
C2H5OH(l) -277.7 KJ
O2 (g) 0 KJ
CO2(g) -393.5 KJ
H2O (l) -285.83 KJ
Plug into the formula:
[(2*-393.5 KJ) + (3*-285.83 KJ)] – [(1*-277.7 KJ) + (3*0)] =
H = -1366.79 KJ PRODUCTS - REACTANTS
Thermo Day 3 28
Homework Day 3
P 205: 52, 61, 63, 69, 71b d,