Ch 5: Thermochemistry

Question 1

Define energy.

Answer 1

The capacity to do work or transfer heat.

Question 2

Define thermal energy.

Answer 2

Energy associated with the random motion of atoms and molecules.

Question 3

Define potential energy and give 2 important example types.

Answer 3

Energy possessed by an object by virtue of its position.
Chemical energy and electrostatic energy.

Question 4

Define chemical energy.

Answer 4

Energy stored within the structural units of chemical substances.

Question 5

Define electrostatic energy.

Answer 5

The potential energy that results from the interaction of charged particles. Opposites attract, like charges repel.

Question 6

What is the equation for electrostatic energy?

Answer 6

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Question 7

2 types of calorimeters:

Answer 7

bomb: used to measure combustion reactions (at constant VOLUME)
cup: used to measure heat of reactions (at constant PRESSURE)

Question 8

q = (s)(m)(ΔT)

Answer 8

s is the specific heat of the substance

m is mass

ΔT is the change in temperature

Question 9

What is the specific heat?

What are its units?

Answer 9

the amount of heat required to raise the temperature of 1 g of the substance by 1°C

measured in J / (g x °C)

Question 10

What is enthalpy?

Answer 10

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

Question 11

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:

Answer 11

ΔH = ΔU = PΔV

Question 12

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 ________.

Answer 12

positive

negative

(p. 197)

Question 13

Consider the chemical reaction for the combustion of methane. Burning of natural gas releases heat to the surroundings, so it is an ________ process.

Answer 13

exothermic

p. 198

Question 14

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.

Answer 14

coefficients

Question 15

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.

Answer 15

enthalpy changes

Question 16

If we multiply both sides of a thermochemical equation by a factor of n, then ΔH must also change by the same ______.

Answer 16

factor

p. 199

Question 17

What are the units of specific heat?

Answer 17

J/ g °C

Question 18

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 ΔH_rxn. 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.

Answer 18

volume

Question 19

Said another way, if volume is constant, then ΔU_rxn = ____

Answer 19

q_rxn

Question 20

When you see the word “bomb calorimeter” in a problem, what do you immediately know about it?

Answer 20

That ΔV = 0

Question 21

In what units is heat capacity measured in?

Answer 21

kJ/ °C

or

J/°C

Question 22

Define heat capacity (C).

Answer 22

The amount of heat required to raise the temperature of an object by 1°C.

p. 200

Question 23

What formula do you need to solve most bomb calorimeter problems?

Answer 23

q_rxn = -C_cal ΔT

(C_cal represents the heat capacity of the bomb calorimeter.)

Question 24

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

Answer 24

C

Question 25

Which of the following statements is true about ΔU at constant pressure?

a. ) ΔU = ΔH
b. ) ΔU = q_rxn
c. ) ΔU = -q_sol
d. ) ΔU = w + q_rxn

Answer 25

d.) ΔU = w + q_rxn

Question 26

The ΔH_rxn in a chemical equation is an extensive property, meaning it depends on the ________ of materials used in the experiment.

Answer 26

quantity

Question 27

If a reaction can be expressed as a series of steps, then the ΔH_rxn for the overall reaction is the ___ of the heats of reaction for each step.

Answer 27

sum

Question 28

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.

Answer 28

A + 2B → C ΔH₁

C → A + 2B ΔH₂ = -ΔH₁

Question 29

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:

Answer 29

A + 2B → C ΔH₁

C → 2D ΔH₂

=

A + 2B → 2D ΔH₃ = ΔH₁ + ΔH₂

Question 30

The same species found on the same side of the chemical equation are summed.

Answer 30

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