Question

**this is a thermodynamics class**

4. We want to develop an intuition for what different amounts of energy look like in various forms. Calculate the energy in MegaJoules (MJ) present in the following systems. As a comment, many people will be surprised by some of the values, for example, a comparison of part e to parts a through c.

a) 1 liter (approximately 1 quart) of octane, a primary
component of gasoline. In this case, the chemical energy is
released by burning. Octane has a heat of combustion of 5.07 x
10^{3} kJ/mole, a density of 0.703 g/cm^{3}, and a
molecular weight of 114 g/mole.

b) A car with mass of 1500 kg (about 3300 pounds) traveling at a
velocity of 100 km/hour (about 60 miles per hour). In this case,
the energy is in the form of kinetic energy, calculated as
(1/2)*mv ^{2}*, where

c) A person with mass of 75 kg at the top of a cliff of height
100 meters (over 300 feet). In this case the energy is potential
energy, given by the formula *mgh*, where *m* is
mass, *g* is the gravitational acceleration constant of 9.8
m/sec^{2}, and *h* is the height.

d) A 12 Volt lead acid car battery that has mass of 10 kg (just over 20 pounds). From data on battery electrical energy storage density, the energy storage capacity of a battery of this type is 0.17 MJ/kg.

e) A breakfast burrito weighing 0.25 kg (about 1/2 pound), and having an energy density of 5 food calories per gram (that is, 5 kcal/g for thermodynamic calories).

We want to develop an intuition for what different amounts of energy look like in various forms. Calculate the energy in MegaJoules (MJ) present in the following systems. As a comment, many people will be surprised by some of the values, for example, a comparison of part e to parts a through c.

a) 1 liter (approximately 1 quart) of octane, a primary
component of gasoline. In this case, the chemical energy is
released by burning. Octane has a heat of combustion of 5.07 x
10^{3} kJ/mole, a density of 0.703 g/cm^{3}, and a
molecular weight of 114 g/mole.

b) A car with mass of 1500 kg (about 3300 pounds) traveling at a
velocity of 100 km/hour (about 60 miles per hour). In this case,
the energy is in the form of kinetic energy, calculated as
(1/2)*mv ^{2}*, where

c) A person with mass of 75 kg at the top of a cliff of height
100 meters (over 300 feet). In this case the energy is potential
energy, given by the formula *mgh*, where *m* is
mass, *g* is the gravitational acceleration constant of 9.8
m/sec^{2}, and *h* is the height.

d) A 12 Volt lead acid car battery that has mass of 10 kg (just over 20 pounds). From data on battery electrical energy storage density, the energy storage capacity of a battery of this type is 0.17 MJ/kg.

e) A breakfast burrito weighing 0.25 kg (about 1/2 pound), and having an energy density of 5 food calories per gram (that is, 5 kcal/g for thermodynamic calories).

**5. (4 points)** Use the data given in Question 4a
to find the energy density of octane in units of kcal/g. Compare
this value to our estimate of the average energy density of
biomaterials (a mix of proteins, carbohydrates, fats) of about 5
kcal/g. Based on this comparison, explain the advantage of powering
your car by burning gasoline versus burning dry wood.

Answer #1

a) density= mass/volume (mL)= 0.703g/mL

mass= 0.703g/mL x 1000mL = 703g

mol of octane= mass/MW= 703g/114.23g/mol= 6.154mol

Energy= 5.07x10^{3} kJ/mol x 6.154 mol= 31202.0485
kJ

1kJ= 0.001 MJ ----> **31.2 MJ**

b) 1m/s = 3.6 km/h

100km/h= 27.77777 m/s

E= 1/2 mv^{2} =1/2 1500kg (27.7777)^{2}=
578703.7037 J -----> **0.5787 MJ**

c) E= mgh= 75kg x 9.8 m/s^{2} x 100 m= 73500 J ---->
**0.0735 MJ**

d) Energy= 0.17 MJ/kg x 10kg= **1.7 MJ**

e) Energy= 5kcal/g x 250g= 1250 kcal

1 kcal= 4.184 kJ ----> 1250 kcal= 5230 kJ ---->
**5.23 MJ**

Question 5:

energy density= 5.07x10^{3} kJ/mol x (1/114.23 g/mol) x
1kcal/4.184 kJ= 10.608 kcal/g

If you burn 1 g of wood you will obtain 5kcal of energy, nut if you burn 1g of octane you will obtain 10.608 kcal, that means that burning octane is more efficient. Burning the same amount in mass gives different amount of energy, we will want to use the one that gives more kcal per gram.

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8 through 10 done please!!
3.13.6
Question 110 pts
A 319 kg motorcycle is parked in a parking garage. If the car has
35,494 J of potential energy, how many meters above ground is the
car? Report your answer to 1 decimal place. Please do not include
units or the answer will be marked incorrect.
Flag this Question
Question 210 pts
A box sitting on the top of a hill has 252 J of potential energy.
If the hill is...

just do questions 5 through 10
3.13.6
Question 110 pts
A 319 kg motorcycle is parked in a parking garage. If the car has
35,494 J of potential energy, how many meters above ground is the
car? Report your answer to 1 decimal place. Please do not include
units or the answer will be marked incorrect.
Flag this Question
Question 210 pts
A box sitting on the top of a hill has 252 J of potential energy.
If the hill...

answer all questions
3.13.6
Question 110 pts
A 319 kg motorcycle is parked in a parking garage. If the car has
35,494 J of potential energy, how many meters above ground is the
car? Report your answer to 1 decimal place. Please do not include
units or the answer will be marked incorrect.
Flag this Question
Question 210 pts
A box sitting on the top of a hill has 252 J of potential energy.
If the hill is 279 meters...

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