1. Thermochemistry
Mrs. Stoops
Chemistry

2. Thermochemistry or Thermodynamics
reactions involving heat.
Abbreviations to know –
rxn = reaction
E = Energy
 = delta = change
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3. Kinetic Energy vs Potential Energy
Potential – stored energy
Kinetic – movement energy
Units of Energy – Joule or J also KJ
calories 1 cal = J PE KE
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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
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5. Work & Heat
Do you work?
Work = force x distance moved
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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
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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.
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8. System vs Surroundings
Another way to solve for E
E = q + w = heat + work. Sign direction issues!
q +
Heat entering
q –
Heat leaving
System
W –
Do work
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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
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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.
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11. State Functions
State functions – depend on the current conditions only, not the path taken.
E, average temperature
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12. Homework Day 1
Page 203: 25, 29
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13. Enthalpy - H Heat gained or lost **yeah, and q is too! State function
H = H products – H reactants

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.
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15. Thermochemical equation
a reaction shown with the H value
Ex:
2H2 + O2 → 2H2O H = KJ
What this means:
Since H is negative, the enthalpy is a _____________ (which side of rxn?)
2H2 + O2 → 2H2O KJ
*** the sign tells the side of reaction ONLY!
product
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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 x 4 = KJ
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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
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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)
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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 mol O2
*** would the answer be negative? What does the sign tell us?
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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
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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
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22. Homework Day 2
page 204: 37a, 41, 43, 51, 53
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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 = x
2 B  C H = y
3 C  D H = z
Overall A  D H = x + y + z
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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 = KJ
2 CO (g) + ½ O2 (g)  CO2 (g) H = KJ
Work:
2 C (s) + O2 (g)  2 CO (g)
2C (s) + 2O2 (g)  2CO2 (g) H x 2 = x 2 = -787 KJ
2CO2 (g)  2CO(g) + O2 (g) H x 2, R = x 2 = 566 KJ
**Cancel**
2 C(s) + O2 (g)  2 CO(g) KJ
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25. Enthalpy of Formation Hf Depend on the state (s, l, g, aq)
Standard enthalpy = Hº
Standard enthalpy of formation = Hfº = are listed for 298 K
Units are KJ/mol
Listed in the text book = appendix C (page 1019)
***Element values are zero***
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26. Enthalpy of Formation Hrxn =  n Hfºproducts -  m Hfºreactants
What are these?
 - sigma, Greek letter, means sum of
n & m – coefficients from balanced equation
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27. Example Find the Hrxn for C2H5OH(l) + 3 O2 (g)  2 CO2(g) + 3 H2O (l)
Look up values:
C2H5OH(l) KJ
O2 (g) 0 KJ
CO2(g) KJ
H2O (l) KJ
Plug into the formula:
[(2* KJ) + (3* KJ)] – [(1* KJ) + (3*0)] =
H = KJ
PRODUCTS - REACTANTS
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28. Homework Day 3
P 205: 52, 61, 63, 69, 71b d,
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