Chapter 5 - Thermochemistry Flashcards
-Interconvert energy units -Express the relationships among the quantities q, w, delta E, delta H. Under their sign conventions, including how the signs of q and delta H relate to whether a process is exo or endothermic. -State the 1st law of thermodynamics -Understand the concept of a state function & be able to give examples -Use thermochemical equations to relate the amount of heat energy transferred in reactions at constant pressure (delta H) to the amount of substance involved in the re
5.1
Define “energy”.
The capacity to do work or to transfer heat
5.1
Define “work”.
The energy used to cause an object with mass to move against a force
5.1
Define “heat”.
The energy used to cause the temperature of an object to increase
5.1
What’s the equation for the magnitude of kinetic energy?
Ek = 1/2mv^2
Ek = kinetic energy m = mass v = speed
5.1
Define “kinetic energy”.
Energy in motion
5.1
Define “potential energy”.
The energy that an object possesses as a result of its composition or its position with respect to another object.
5.1
What’s the equation for potential energy?
E = mgh
mass)(gravitational constant)(height
5.1
What’s the equation for electrostatic potential energy?
E = kQ1Q2 / d
k = 8.99 x 10^9 J-m/C^2
Q1 & Q2 = magnitude of the charge of the electron (1.60 x 10^-19 C)
d = Distance
The lower the E, the more stable (opposing charges = lower the E)
5.1
Define “system”.
The portion we single out for study
5.1
Define “surroundings”.
Everything we don’t single out for study
5.1
Define an open system.
Matter & energy can be exchanged with the surroundings
Ex. Boiling pot of H2O w/o the lid
5.1
Define a closed system.
Can exchange energy but not matter w/ its surroundings.
Ex. Mixture of H2 & O2 in a cylinder. Exchanges energy in the form of work and heat, but doesn’t exchange matter.
-Systems we can most readily study
5.1
Define an isolated system.
One in which neither energy nor matter can be exchanged with the surroundings.
Ex. An insulated thermos containing hot coffee
5.1 Give It Some Thought p. 168
Is a human being an isolated, closed, or open system? Explain.
Open. They exchange matter and energy with their surroundings.
5.2
Define the “1st Law of Thermodynamics”
Energy is conserved.
5.2
Define “internal energy”.
The sum of all the kinetic and potential energies of all its components.
5.2
A positive value of deltaE results when…
the system has gained energy from its surroundings.
Efinal > Einitial
5.2
A negative value of deltaE results when…
the system has lost energy to its surroundings.
Efinal < Einitial
5.2
deltaE =…
Efinal - Einitial
5.2
deltaE = …
q + w
5.2
A positive q means ___, while a negative q means ___
the system gained heat from its surroundings, the system lost heat to its surroundings
5.2
A positive w means ______, while a negative w means ____.
the surroundings did work on the system, the system did work on the surroundings
5.2
A postive deltaE means _____, while a negative deltaE means _____.
net gain of energy by system, net loss of energy by system
5.2
Define “endothermic”.
Heat goes into system
5.2
Define “exothermic”.
Heat goes out of system.
5.2
Define “state function”.
The value of a state function depends only on the present state of the system, not the path the system took to reach that state.
5.2
Is E a state function?
Yes.
E depends only on the initial and final states of the system, regardless of how the transfers of energy occur in terms of heat and work.
5.3
Define “pressure-volume work”.
The work involved in the expansion or compression of gases.
5.3
When the pressure is constant, the sign and magnitude of the pressure-volume work is given by…
w = -PdeltaV
5.3
What is the equation for enthalpy?
H = E + PV
5.3
Is enthalpy a state function?
Yes. Internal energy, pressure, and volume are state functions.
5.3
The equation for deltaH is…
deltaH = delta(E + PV) = delta E + PdeltaV
delta H = delta E + P deltaV = (qp + w) - w = qp
p = changes at constant pressure
5.3
The change in enthalpy equals …
the heat lost or gained at constant pressure.
5.4
delta H = ….
H products - H reactants
5.4 Samp. Ex.
How much heat is released when 4.50g of CH4 (g) is burned in a constant pressure system?
CH4 (g) + 2 O2 (g) —> CO2 (g) + 2 H2O (l)
delta H = -890 kJ
4.50g (1 mol / 16.0g)( -890 kJ / 1 mol) = .250 kJ
5.5
Define “calorimetry”.
The measurement of heat flow
5.5
Define “calorimeter”.
A device used to measure heat flow.
5.5
Define “heat capacity”. What does it determine?
The amount of heat required to raise its temperature by 1 K (or 1 C).
The temperature change experienced by an object when it absorbs a certain amount of heat.
5.5
Define “molar heat capacity”.
The heat capacity of one mole of a substance.
5.5
Define “specific heat”.
The heat capacity of 1g of a substance.
5.5
Specific heat =
q / m x delta T
5.5
q =
m * c * delta T
Which substance will undergo the greatest temperature change when the same mass for each substance absorbs the same quantity of heat? H2O (l) (4.18 J/gK) or Hg (l) (.14 J/gK)
Hg (l).
Specific heat is the heat capacity, or the amount of heat required to raise a substance’s temperature by 1 K, of 1g of a substance.
5.5
Calorimetry
qsoln =
(specific heat of solution) x (grams of solution) x (delta T) = -qrxn
5.5 Give It Some Thought (p. 183)
(a) How are the energy changes of a system and its surroundings related?
(b) How is the heat gained or lost by a system related to the heat gained or lost by its surroundings?
(a) Energy lost by a system is gained by its surroundings
(b) Same magnitude, opposite sign
5.5
Bomb Calorimetry
qrxn = ___
-C cal x delta T
5.1
Define “force”.
Any kind of push or pull exerted on an object
5.1 - Give It Some Thought (p, 167)
What are the terms for the energy an object possesses (a) because of its motion, (b) because of its position?
What terms are used to describe changes of energy qassociated with (c) temperature changes, (d) moving an object against a force?
(a) Kinetic (b) Potential
(c) Heat (d) Work
5.1
Define “joule”.
The SI unit for energy.
1 J = 1 kg-m^2/s^2
5.1
Define “calorie (cal)”.
1 cal = 4.184 J
1000 cal = 1 Cal
5.1
Define the equation for work.
w = F x d
w = magnitude of work F = force d = distance the object is moved
