u3s4 L1 & L2
What fuels you?
Thermodynamics STSE issues
Textbook Readings
page 692 -702
Textbook Practice Items
page 702: item1
page 705: item 22
page 705: item 23
Upon completion of this lesson, you should be able to:
• identify and describe sources of energy including present sources and
possible new sources
• compare physical, chemical, and nuclear changes in terms of the species
and the magnitude of energy involved
• define, calculate and compare fuel values
• propose and analyze solutions to energy problems using the concept of
fuel value
• analyze examples where technologies were developed based on
thermochemistry using the idea of choice of fuel in a device
• identify perspectives that influence a decision involving fuels used in
devices and choices made in the Calorie content and servings of the food
we eat
• analyze scientific and technological activities that take place when making
choices for maintaining a healthy lifestyle
• analyze the risks and benefits to society and the environment in relation
to fossil fuel use and healthy lifestyle choices
• analyze the knowledge acquired in the study of thermochemistry to
identify areas of further study, specifically chemical engineering and
nutrition
Sources of energy:
1.
Fossil fuels
•
•
•
2.
carbonized organic matter
Non-renewable
Greenhouse gas emissions
Solar
•
3.
solar collectors / panels
Hydrogen
•
electrolysis of water. Burned or used in fuel
cells to create electricity
3. Ethanol
•
fermentation of grains
4. Nuclear
•
•
•
•
Nuclear reactors boil water to create steam which then turns
turbines.
A chunk of uranium the size of your fist releases a quantity of
heat equal to that of the combustion of three tonnes of coal.
clean energy - comes without the production of oxides of
carbon, nitrogen and sulfur.
the potential of nuclear fuel is too valuable and too clean to
be ignored.
5. Mechanical
•
wind, tidal, hydro,
Photsynthesis
Fermentation
Comparisons of molar heats
Nuclear > chemical > phase changes
• Physical changes involve enthalpies in the order of 10's of kiloJoules.
(∆Hvap=40.7 kJ/mol)
• Chemical changes involve enthalpies in the order of 100's or 1000's of
kiloJoules. (∆Hcombustion, coal = -3900.0 kJ/mol)
• Nuclear changes (fusion / fission of nuclei) involve enthalpies in the order
of billions (109) of kiloJoules. (∆Hfusion, U = -2.1 x 10+10 kJ/mol)
• 21 billion kilojoules per mole!!!!
Fuel Value
• A fuel is any substance from which energy can be
derived
• Fuel value is defined as the heat evolved per
gram of fuel burned.
– Fuel values allow comparisons based on mass - a unit of
measurement that most people are familiar with and
one that is easily measured.
m
FV 
q
Calculate the fuel value of this juice pack.
Molar Enthalpy of
Combustion
(kJ/mol)
Fuel Value
(kJ/g)
hydrogen, H2
-285.8
141.5
ethanol, C2H5OH
-1366.7
29.66
methane, CH4
-890.5
55.48
propane, C3H8
-2020.0
50.33
butane, C4H10
-2658.7
51.65
octane, C8H18
-5450.2
47.70
2,2,4-trimethylpentane, C8H18
-5455.9
47.75
Fuel / Chemical Formula
Based on this data, why do they use liquid H2 as a fuel on rockets?
Per gram of mass, hydrogen releases about three times as much heat than the
hydrocarbons. A smaller fuel load means a larger payload.
Fueling your body:
• Cellular respiration is a process that occurs in the cells of your
body.
– It is a complex series of catalysed chemical reactions that convert the
potential energy of glucose to potential energy in molecules known as
ATP - adenosine triphosphate.
– ATP provides the energy that drives other reactions like protein synthesis
in your body. The overall equation for cellular respiration is:
What do you notice about this equation?
Food Calorimetry
• Energy content of food can be determined by bomb calorimetry.
• The procedure is similar to that for any bomb calorimetry trial, except that
foods are typically dehydrated so that the energy needed to heat and
evaporate the water content does not affect the results.
– Example:
• A 2.50 g sample of Brand X granola bar is placed in a bomb calorimeter
with a heat capacity of 13.17 kJ/°C. Upon complete combustion of the
sample, the temperature of the calorimeter increases by 3.86°C. Calculate
the number of Calories in a 23.0 g serving of a Brand X granola bar.
4-9-4 method
• A more useful method of determining the energy content of foods
is to determine the masses of carbohydrate, fat and protein in a
sample and multiply these values by their corresponding fuel
values 4.2 Cal/g, 9.1 Cal/g and 4.2 Cal/g. This method is known
as the 4-9-4 method.
(1 Food Calorie = 4.184 kJ)
• It is a convenient way to to keep track of your Calorie intake,
especially if you have to follow a strict diet.
The Nutrition Label on a 270g bag of Krusty® potato chips states
that a serving is 28g (approximately 20 chips). The Calorie
content in one serving is 644 kJ ( 154 Cal ).
(a) How many servings are in a single bag of Krusty® potato
chips?
(b) How much energy is consumed if the full bag is eaten?
(c) How many calories are consumed if the full bag is eaten?
(d) For the following activities, calculate the number of hours it
would take doing the activity to "burn off" the full bag of
Krusty® chips:
(i) running at 10km/h (3140 kJ/h)
(ii) rowing (3500 kJ/h)
(iii) Walking at 6 km/h (1675 kJ/h)