Name: Kathryn Blankenship
Contact Info: [email protected]
Lesson Title: What is Heat
Unit #:
2
Activity Title: Behind the Scenes of Heat
Lesson #:
1
Date:16-Jul-13
Activity #:
2
Worksheet:
b
Worksheet Title: Behind the Scenes of Heat Worksheet
Behind the Scenes of Heat: Chapter 6.1-6.4 Problem Set
I. Energy (E)
A. the ability to do work and/or produce heat
B. the sum of all potential and kinetic energy in a system is known as the internal energy of
the system
C. Potential energy – energy by virtue of position.
1. In chemistry this is usually the energy stored in bonds (i.e., when gasoline burns
there are differences in the attractive forces between the nuclei and the electrons in the
reactants and the products)
2. When bonded atoms are separated, the PE is raised because energy must be added
to overcome the coulombic attraction between each nucleus and the shared electrons.
D. Kinetic energy – energy of motion (translational, rotational & vibrational motion of
particles)
1. proportional to Kelvin temperature; kinetic energy depends on the mass and the
velocity of the object: KE = ½ mv
Problem 1: What are examples of Energy and how do they relate to the definitions above?
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Problem 2: What is the Law of Conservation of Energy?
II. Heat (q)
A. Two systems with different temperatures that are in thermal contact will exchange
thermal energy, the quantity of which is call heat.
B. This transfer of energy in a process (flows from a warmer object to a cooler one,
transfers heat because of temperature difference but, remember, temperature is
not a measure of energy—it just reflects the motion of particles)
III. Work (w)
A. work = −PΔV where gases are involved; expressed in Joules or kJ
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IV. State Function
A. A property of a system that depends ONLY on its present state, not how it got there.
Problem 3: Describe how the following statement describes a state function:
A liter of water behind a dam has the same potential energy for work regardless of
whether it flowed downhill to the dam or was taken uphill to the dam in a bucket.
V. Chemical Energy
A. System vs Surroundings
1. System
2. Surroundings
Problem 4: What must be true about the energy that flows from the system to the surroundings?
B. Define the following types of processes and provide at least two examples of each.
a. Endothermic reaction
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b.
Exothermic reaction
Problem 5: Study the diagram below. Label each process as endothermic or exothermic.
**Energy is defined as: The ability to do work and/or produce heat.
VI. Internal Energy
A. ∆E = q + w
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B. Your answer must ALWAYS have a sign and a number. The sign indicated the direction of
the flow
1. If energy flows INTO a system by heat, it is ___________________________
The sign is _________________________
2. If the energy flows OUT of a system by heat, it is ___________________________
The sign is _________________________
3. If a system does work on the surroundings, w is __________________________
4. If the surroundings does work on the system, w is ________________________
Problem 6: Determine the signs of heat and work for the two systems below:
Left: q = _______________ w = ______________Right: q = _______________ w = ___________
Problem 7: Calculate ΔE for a system undergoing an endothermic process in which 15.6 kJ of heat
flows and where 1.4 kJ of work is done on the system.
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C. Work done by gases: NOTE the piston was PUSHED up
1. the signs (+/-) for the variables when work is done by a gas?
a. ΔV
b. w
c. Equation:
2. the signs (+/-) for the variables when work is done to a gas?
a. ΔV
b. w
c. Equation:
Problem 8: Calculate the work associated with the expansion of a gas from 46 L to 64 L at a
constant external pressure of 15 atm.
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Problem 9: A balloon is being inflated to its full extent by heating the air inside it. In the final
stages of this process, the volume of the balloon changes from 4.00 x 10 L to 4.50 x 10 L by the
addition of 1.3 x 10 J of energy as heat. Assuming that the balloon expands against a constant
pressure of 1.0 atm, calculate ΔE for the process.
(To convert between L x atm and J, use 1 L x atm = 101.3 J.)
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Problem 10: Enthalpy is a type of energy. In our course, we will use the term enthalpy to describe
heat, but this is only true at what condition? What is the symbol for enthalpy?
Problem 11: When 1 mole of methane is burned at constant pressure, 890 kJ of energy is released
as heat.
a. Write a balanced chemical reaction for this process with the corresponding ΔH.
b. Calculate the ΔH for a process in which a 5.8 gram sample of methane is burned at
constant pressure.
Problem 12: When 2.00 moles of water are frozen, 12.04 kJ of energy is released as heat.
a. Write a balanced chemical reaction for this process with the corresponding ΔH.
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b. If 20.0 g of water are frozen, calculate the ΔH.
Problem 13: What is the equation for calculating enthalpy from mass and change in temperature?
Problem 14: Constants known as heat capacity for substances are used to convert temperature
changes and mass data. Using the table below, describe how the value of specific heat capacity is
related to your everyday life experience:
When you put a copper pot on the stove top filled with water, the copper pot changes
temperature quickly, but the water takes longer to change temperature.
Problem 15: Constants of specific heat may have many units and must be watched carefully. Show
some of the possible units (be sure to include both the specific heat capacity and the molar heat
capacity).
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Problem 16: Calorimetry is the study of enthalpy as heat during chemical reactions. Below is a
simple calorimeter similar to what we will use in lab. Describe the importance of each component.
Problem 17: How is the heat lost in a system related to heat gained?
Problem 18: SOLID IN WATER
a. A small “coffee cup” calorimeter contains 110. g of water at 22.0°C. A 100. g sample
of lead is heated to 90.0°C and then placed in the water. The contents of the
calorimeter come to a temperature of 23.9°C. What is the specific heat of
lead? (0.132J/g .°C)
b. 5.00 kg of a hot metal at 200.0°C is added to 25.0 kg of water at 30.0°C. What is the
final temperature of the metal? The specific heat of the metal is 0.800 J/g°C and for
water it is 4.184 J/(g°C).
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Problem 19: SOLUTION
When 1.00 L of 1.00 M Ba(NO ) solution at 25.0°C is mixed with 1.00 L of 1.00 M Na SO solution at
25.0°C in a calorimeter, the white solid BaSO forms and the temperature of the mixture increases
to 28.1°C. Assuming that the calorimeter absorbs only a negligible amount of heat, that the specific
heat capacity of the solution is 4.18 J/°C•g, and that the density of the final solution is 1.0 g/mL,
calculate the enthalpy change per mole of BaSO formed.
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