Chemistry Argumentative Essay

This essay has a total of 3405 words and 21 pages.

Chemistry

Thermochemistry Answer Key
Assignments 1- 8
Page 1.

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Answers to Energy Assignment 1


1. Indicate whether the following scenarios illustrate matter possessing mainly potenial energy, or kinetic
energy.

a) There are 30 liters of fuel in the tank of a stationary automobile.
•Potential energy (chemical energy in fact)
b) A large rock is perched at the edge of a 40m cliff.
•Potential energy (of the gravitational type)
c) Energy is supplied to a beaker of water raising its temperature from 20oC to 40oC
•The warming water is increasing its kinetic energy content
d) The average speed of a molecule of a gas at room temperature is about 1000 m/s.
•Kinetic energy is possessed by moving molecules
e) Air in an automobile tire exerts pressure on the walls of the tire.
•Kinetic energy of the moving molecules in the tire volume. Some argue that if the tire could
"blow", it could also be considered potential energy. That's acceptable also.
f) A container of dynamite sits on a storage room floor
•Chemical potential energy
g) Millions of gallons of water is held behind a hydroelectric power dam.
•Gravitational potential energy
h) Thirty grams of ice at 0oC are melted to become liquid water at 0oC.
•The ice experiences an increase in its potential energy content as it melts to become liquid water.
There is no change in kinetic energy content because there is no increase in temperature.
2. Indicate whether the following scenarios illustrate the First law, or Second law of Thermodynamics (or both).
Explain.
a) A toboggan which possesses 100 units of energy at the top of a hill converts 85 units to
kinetic energy and 15 units to heat as it slides to the bottom of the hill.
•First Law - all forms of energy accounted for, none lost or gained
b) Hot iron is plunged into a bucket of cold water which warms up.
•Second Law - heat goes from hot iron to cold water
c) An open window in December cools the house down very quickly.
•Second Law - heat transfers from hot house to cold outside air
d) Burning wood converts chemical energy to heat, light and sound.
•Both Laws
First Law- energy is being converted to many different forms and their total should equal
the total energy the wood possesed in the first place.

Second Law- heat is transferred from the burning wood to the cool air
e) Chemical energy is converted to electrical energy by a battery.
•First Law (mainly) This is a simple conversion from one form to another but in reality,
a small amount of the energy is converted to heat


Energy Assignment 2 - Answers

1. The chart

Scenario System
(syst) Surroundings
(surr) Exothermic or
Endothermic Direction of
Energy Flow Energy
Conversion
* a) dissolving KBr water endothermic surroundings to system Ek Surr to Ep system
b) burning CH4 air in furnace exothermic system to surroundings Ep syst to Ek surr
c) dissociating H2SO4 water component of solution exothermic system to surroundings Ep syst to Ek surr
d) combustion of candle wax surrounding air exothermic system to surroundings Ep syst to Ek surr
e) melting ice water in glass endothermic surroundings to system Ek Surr to Ep syst
f) condensing water air in cloud exothermic system to surroundings Ep syst to Ek surr
g) your metabolic reactions air around you exothermic system to surroundings Ep syst to Ek surr
h) the chemical reaction immediate surroundings endothermic surroundings to system Ek Surr to Ep syst
yours


2. The equation for the fermentation of glucose to alcohol and carbon dioxide is

C6H12O6(s) -------> 2C2H5OH(l) 2CO2(g)

a) The enthalpy change for the reaction is -68.1 kJ. Is the reaction exothermic or endothermic?
Ans: Its exothermic as you can tell from the negative sign on the energy term.
b) Is energy absorbed or released as the reaction proceeds? Explain how you know this.
Ans: Released. All exothermic systems release energy to the surroundings.
3. The last step in the commercial production of sulfuric acid is
SO3(g) H2O(l) -----> H2SO4(aq) DH = -250 kJ
In designing a sulfuric acid plant, is it necessary to provide for heating or cooling the building
in which the reaction is taking place? Explain.
Ans: You would need to cool the building as the system is constantly releasing
energy into it.
4. 25.0 grams of molten silver metal is dropped into a container of cold water.
a) Write a chemical equation that describes the system.
Ag(l) -----> Ag(s) energy
b) What substance gains kinetic energy during this process?
The water into which the molten silver was dropped will gain kinetic energy.
c) What substance changes its potential energy content during this process?
The silver loses potential energy as it solidifies.

Energy Assignment 3 - Answers
Multiple choice

1. B
2. A
3. D
4. D
5. B
6. D


7. Convert each of the following equations to DH notation. Indicate whether each reaction is exothermic or endothermic.
a) Cd(s) 1/2 O2(g) ----------> CdO(s) DH = -261kJ

b) Cl2(g) 7/2 O2(g) ---------> Cl2O7(g) DH = 272 kJ

c) 2Au(s) 3/2 O2(g) --------> Au2O3(s) DH = -8.4 kJ

d) 2B(s) 3/2 H2(g) -------> B2H6(g) DH = 31 kJ

e) 1/2 Br2(l) 1/2 Cl2(g) -------> BrCl(g) DH = 15 kJ

8. Identify each of the following reactions as exothermic or endothermic. Which processes
store more energy in the products than in the reactants?
a) Hg(l) 1/2 I2(s) ------> HgI(g) DH = 138 kJ endothermic

b) 1/2N2(g) 3/2 F2(g) ------> NF3(g) DH = -114 kJ exothermic

c) Si(s) 2Br2(l)------> SiBr4(l) DH = -398 kJ exothermic

d) Na(s) --------> Na(g) DH = 109 kJ endothermic
Therefore (a) and (d) store more energy in the products than in the reactants.

9. At 100oC, solid ammonium nitrite decomposes to form nitrogen gas and gaseous water. At
this temperature, the decomposition releases 224 kJ per mole of ammonium nitrite
decomposed. The key word is "releases" indicating that the system is exothermic, so.....

(a) Write the balanced equation for this reaction including the energy term in the equation.
NH4NO2(s) -------> N2(g) 2H2O(g) 224 kJ
(b) Write the balanced equation for the reaction and use the DH notation for the energy term.
NH4NO2(s) -------> N2(g) 2H2O(g) DH = -224 kJ

10. At elevated temperatures, solid lead (II) nitrate decomposes to form solid lead monoxide,
oxygen gas and gaseous nitrogen dioxide. The decomposition absorbs 292 kJ of energy per
mole of lead (II) nitrate decomposed. The key word is absorbs, indicating the system is exothermic, so.......

(a) Write the balanced equation for the reaction including the energy term in the equation.

If balanced with whole numbers, it results in 2 moles of lead(II) nitrate so......
2Pb(NO3)2(s) 584 kJ -------> 2PbO(s) O2(g) 4NO2(g)

If balanced in order to express it in terms of one mole of lead (II) nitrate its....
Pb(NO3)2(s) 292 kJ -------> PbO(s) 1/2 O2(g) 2NO2(g)
(b) Write the balanced equation for the reaction using DH notation.
Either: 2Pb(NO3)2(s)-------> 2PbO(s) O2(g) 4NO2(g) DH = 584 kJ
Or: Pb(NO3)2(s) -------> PbO(s) 1/2 O2(g) 2NO2(g) DH = 292 kJ

Use the following diagram to answer the question which follows.


11. The reaction shown in the diagram is
A. exothermic and DH is negative C. endothermic and DH is negative
B. exothermic and DH is positive D. endothermic and DH is positive



Answers to Energy Assignment 4
Stoichiometry and the Energy Term







1. Consider the equation: I2(s) 14.9 J ------> I2(g)
Based on this data, determine the mass of I2(g) formed if 3.78 J of heat are absorbed by
a sample of solid iodine.

mass of I2(g) = 3.78J x 1 mol I2 (s) x 253.80 g I2(g)
14.9J mol I2(g)
mass of I2(g) = 64.4 g
2. Given the following equation:
Na2O(s) 2HI(g) --------> 2NaI(s) H2O(l) 503 kJ
a) If 9.00 grams of NaI(s) are formed by the process, how much heat will be involved?

DH = 9.00 g NaI(s) x 1 mol NaI(s) x -503 kJ
149.89 g NaI(s) 2 mol NaI(s)
DH = -15.1 kJ
b) Is the heat absorbed or released in this reaction? Explain how you arrived at this conclusion.
Since he energy term is on the product side of the given equation, it's an exothermic reaction, so heat must
be released.

3. When concentrated nitric acid (HNO3(aq)) is added to water, heat is liberated. This heat
of dilution is 30 kJ per mole of nitric acid diluted. How many grams of nitric acid must be
diluted to release 3620 kJ of heat?

If you had trouble with this one it's probably because you are writing the wrong equation.
Use this: HNO3(aq) -------> H (aq) NO3-(aq), 30 kJ and try again.
The answer is: 7.6 x 103 g



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The method of solution for the remaining questions follows the pattern set in questions 1
and 2a which illustrate the use of dimensional analysis for solving thermochemical
stoichiometry questions. Only the answers to questions 4-6 are provided below Good luck!


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4. (Ans: 3.46 kJ absorbed)

5. a) (Ans: 78.13 g)
b) (Ans: 208.2 g)
6. (Ans: 13.2 kJ)




Quiz 1 examines all the material to this point.



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