Ch 5: Thermochemistry Flashcards
Define energy.
The capacity to do work or transfer heat.
Define thermal energy.
Energy associated with the random motion of atoms and molecules.
Define potential energy and give 2 important example types.
Energy possessed by an object by virtue of its position.
Chemical energy and electrostatic energy.
Define chemical energy.
Energy stored within the structural units of chemical substances.
Define electrostatic energy.
The potential energy that results from the interaction of charged particles. Opposites attract, like charges repel.
What is the equation for electrostatic energy?
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2 types of calorimeters:
bomb: used to measure combustion reactions (at constant VOLUME)
cup: used to measure heat of reactions (at constant PRESSURE)
q = (s)(m)(ΔT)
s is the specific heat of the substance
m is mass
ΔT is the change in temperature
What is the specific heat?
What are its units?
the amount of heat required to raise the temperature of 1 g of the substance by 1°C
measured in J / (g x °C)
What is enthalpy?
Enthalpy is the internal energy of a system plus the product of pressure and volume. It is measured in units of energy: J.
H = U + PV
U, P, and V are all state functions, that is, the changes in U + PV depend only on the initial and final states, therefore, the equation for enthalpy can also be written:
ΔH = ΔU = PΔV
For an endothermic process (where heat is absorbed by the system from the surroundings), ΔH is ________. For an exothermic process (where heat is released by the system to the surroundings) ΔH is ________.
positive
negative
(p. 197)
Consider the chemical reaction for the combustion of methane. Burning of natural gas releases heat to the surroundings, so it is an ________ process.
exothermic
p. 198
Keep in mind that the ΔH value in kJ/mol does not mean per mole of a particular product or reactant. It refers to all the species in a reaction in the molar amounts specified by the __________ in the balanced equation.
coefficients
When writing thermochemical equations, we must always specify the physical states of all reactants and products, because they help determine the actual ________ _________. In the equation for the combustion of methane, for example, changing the liquid water product to water vapor changes the value of ΔH.
enthalpy changes
If we multiply both sides of a thermochemical equation by a factor of n, then ΔH must also change by the same ______.
factor
p. 199
What are the units of specific heat?
J/ g °C
Consider a reaction in which we have found ΔH. It is important to remember that ΔU (the change in internal energy) is not equivalent to ΔHrxn. ΔU = q + w. We may have found the q, but that doesn’t mean we know what work was done.
HOWEVER, if the ______ in a reaction is constant, and we consider that W = -(PΔV), only then do we know ΔU, because in this scenario, W = 0.
volume
Said another way, if volume is constant, then ΔUrxn = ____
qrxn
When you see the word “bomb calorimeter” in a problem, what do you immediately know about it?
That ΔV = 0
In what units is heat capacity measured in?
kJ/ °C
or
J/°C
Define heat capacity (C).
The amount of heat required to raise the temperature of an object by 1°C.
p. 200
What formula do you need to solve most bomb calorimeter problems?
qrxn = -Ccal ΔT
(Ccal represents the heat capacity of the bomb calorimeter.)
Given w = 0, a reaction that absorbs heat has the following:
a. ) +ΔH and -ΔU
b. ) -ΔH and +ΔU
c. ) +ΔH and +ΔU
d. ) -ΔH and -ΔU
C
Which of the following statements is true about ΔU at constant pressure?
a. ) ΔU = ΔH
b. ) ΔU = qrxn
c. ) ΔU = -qsol
d. ) ΔU = w + qrxn
d.) ΔU = w + qrxn
The ΔHrxn in a chemical equation is an extensive property, meaning it depends on the ________ of materials used in the experiment.
quantity
If a reaction can be expressed as a series of steps, then the ΔHrxn for the overall reaction is the ___ of the heats of reaction for each step.
sum
According to Hess’s Law, if the chemical equation is reversed, then the sign for the enthalpy change is ALSO reversed, but the magnitude of the enthalpy change remains the same. See the example below.
A + 2B → C ΔH1
C → A + 2B ΔH2 = -ΔH1
Enthalpy change is also a state of function and this can be obtained from summing individual steps that lead to an overall reaction. See example:
A + 2B → C ΔH1
C → 2D ΔH2
=
A + 2B → 2D ΔH3 = ΔH1 + ΔH2
The same species found on the same side of the chemical equation are summed.
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