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Chapter 5

CHE 101 Chapter Notes - Chapter 5: Calorie, Granola, Carbohydrate


Department
Chemistry
Course Code
CHE 101
Professor
Atwood Jim
Chapter
5

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Chapter 5 - Thermochemistry
Jim Atwood
A. Definitions (p 164 - 181)
Chemical reactions are accompanied by energy changes
a. Conservation of Energy - Energy is neither created nor destroyed
b. Processes that liberate heat are exothermic
c. Processes that absorb heat are endothermic
c.i. Ex: cold pack
d. Enthalpy − H − is the heat content of a substance
d.i. State function
e. The enthalpy change for a reaction (at constant pressure) is our focus
e.i. ΔHrxn = Hproducts − Hreactants
e.ii. ΔHrxn is negative for exothermic and positive for endothermic processes
2H2(g) + O2(g) → 2H2O(g)
ΔH = −483.6 kJ
Guidelines:
1. ΔHrxn depends on amount. 2 moles of H2(g) reacting with 1 mole O2(g) released
483.6 kJ. 4 moles H2(g) reacting with 2 moles O2(g) gives 967.2 kJ.
4H2 + 2O2 → 4H2O ΔH = −967.2
*** change depend on amount
2. For the reverse reaction, the sign of ΔHrxn changes.
2H2O → 2H2(g) + O2(g)
ΔHrxn = 483.6 kJ
3. The value of ΔHrx depends on the state of the reactants and products
2H2(g) + O2(g) → 2H2O(l) ΔH = −571.0 kJ
Kinetic Energy: energy of motion
KE = 1/2mv2
Potential Energy: energy of object
Electrical potential energy is given by
E (Q1Q2) / d
Work: energy to move object against a force
ΔE = q + w
q is heat
w is work
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B. Calorimetry
Calorimetry measures heat flow
Where will heat go?
1. Definitions
a. Heat capacity: amount of heat required to raise an object’s temperature
1K.
b. Specific heat: the heat capacity of 1g of a substance
c. Specific heat = (quantity of heat transferred) / [(grams of substance) x
(temperature change)]
= q / mΔT
2. Constant pressure calorimetry
a. Measuring the heat change from a reaction at constant pressure (as in
laboratory).
b. Can be used to measure enthalpy of a reaction. qrxn = ∆H
Example (5.7 p. 184):
100 ml of 0.500M HCl is reacted with 100 ml of 0.500M NaOH in a constant
pressure calorimeter that has a heat capacity of 335 J/K. The initial temperature of
each solution is 22.50°C. After the reaction the temperature is 24.90°C. Calculate
∆H for the reaction assuming that the density is 1.00 g/ml and the specific heat (c)
is 4.184 J/gK.
qrxn = ΔH → calculate ΔH of rxn heat capacity = 334 J/K
HCl + NaOH → H2O + Na+ + Cl
−qrxn = qsolution + qcalorimeter
qsolution = (c)(m)(ΔT)
= (4.184 J/(gK))(200 mL)(1.00 g/mL)(2.40 K)
= 2.01 x 103 J
qcalorimetry = (335 J/K)(2.40 K)
= 804 J
−qrxn = 2.01 x 103 J + 804 J = 2.81 x 103 J
ΔH = qrxn = −2.81 x 103 J
C. Enthalpies of Formation
1. Standard enthalpy change (ΔHrxn) for conversion
CaO(s) + CO2(g) → CaCO3
ΔHrxn = ΔHproducts − ΔHreactants
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