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Energy in Chemical Reactions — Test Answers

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1
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Which label shows the overall enthalpy change and is the reaction exothermic or endothermic?

A
A, exothermic.
B
B, exothermic.
C
C, endothermic.
D
D, endothermic.
2
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Molecules that have longer arrows are moving faster. Which statement describes what is happening in this system?

A
There is no change in average kinetic energy between these two systems.
B
Molecules vibrate more as temperature decreases.
C
Kinetic energy decreases as temperature decreases.
D
Temperature is not related to the average kinetic energy of a system.
3

What is the overall enthalpy change triangle cap h sub rxn for the system?

A
1 comma 300 kJ
B
negative 300 kJ
C
negative 1 comma 300 kJ
D
300 kJ
4

Which direction does thermal energy flow in this diagram?

A
The system is too cold for heat to move.
B
Heat flows from the hand to the ice cream.
C
Heat flows from the ice cream to the hand.
D
The system is in a state of equilibrium.
5

Nitrogen dioxide, NO sub 2 times g Δ⁢H_f=33.84kJ/mol , is decomposed according to the reaction: 2 NO sub 2 g rightarrow cap n sub 2 times g plus 2 cap o sub 2 g What is the enthalpy change when 2 point 5 0 mol of nitrogen dioxide decomposes? Use Δ⁢H_rxn=∑(Δ⁢H_f_1p⁢r⁢o⁢d⁢u⁢c⁢t⁢s)−∑(Δ⁢H_f_1r⁢e⁢a⁢c⁢tan⁡t⁢s)

A
84 point 6 kJ of energy released
B
13 point 5 kJ of energy absorbed
C
13 point 5 kJ of energy released
D
84 point 6 kJ of energy absorbed
6

Consider these intermediate chemical equations. cap c times s plus cap o sub 2 times g rightarrow CO sub 2 times g triangle cap h sub 1 is equal to negative 393 point 5 kJ 2 CO g plus cap o sub 2 times g rightarrow 2 CO sub 2 g triangle cap h sub 2 is equal to negative 566 point 0 kJ 2 cap h sub 2 cap o g rightarrow 2 cap h sub 2 g plus cap o sub 2 times g triangle cap h sub 3 is equal to 483 point 6 kJ The overall chemical equation is C(s)+H_2​O(g)→CO(g)+H_2​(g) To calculate the final enthalpy of the overall chemical equation, which step must occur?

A
Reverse the first equation, and change the sign of the enthalpy. Then, add.
B
Multiply the first equation by three, and triple the enthalpy. Then, add.
C
Divide the third equation by two, and double the enthalpy. Then, add.
D
Reverse the second equation, and change the sign of the enthalpy. Then, add.
7

Ammonia, NH sub 3 , Δ⁢H_f=−46.2kJ reacts with oxygen to produce water Δ⁢H_f=−241.8kJ and nitric oxide, NO , Δ⁢H_f=91.3kJ , in the reaction: 4 NH sub 3 g plus 5 cap o sub 2 g rightarrow 6 cap h sub 2 cap o g plus 4 NO g What is the enthalpy change for this reaction? Use Δ⁢H_rxn=∑(Δ⁢H_f_1p⁢r⁢o⁢d⁢u⁢c⁢t⁢s)−∑(Δ⁢H_f_1r⁢e⁢a⁢c⁢tan⁡t⁢s)

A
negative 900 point 8 kJ
B
negative 104 point 6 kJ
C
104 point 6 kJ
D
900 point 8 kJ
8

Consider these intermediate chemical equations. cap c times s plus 1 half cap o sub 2 g rightarrow CO times g CO times g plus 1 half cap o sub 2 g rightarrow CO sub 2 times g When you form the final chemical equation, what should you do with CO ?

A
Add the two CO molecules together, and write them as reactants in the final chemical reaction.
B
Cancel out CO because it appears as a reactant in one intermediate reaction and a product in the other intermediate reaction.
C
Write CO only once as a reactant in the final chemical reaction.
D
Write CO as a reactant and a product in the final chemical reaction.
9

Which reaction is endothermic?

A
2 Na plus Cl sub 2 rightarrow 2 NaCl plus energy
B
2 cap c sub 2 cap h sub 6 plus 7 cap o sub 2 rightarrow 4 CO sub 2 plus 6 cap h sub 2 cap o plus 2 comma 502 KJ
C
6 CO sub 2 plus 12 cap h sub 2 cap o plus energy rightarrow cap c sub 6 cap h sub 12 cap o sub 6 plus 6 cap o sub 2 plus 6 cap h sub 2 cap o
D
HCl plus NaOH rightarrow NaCl plus cap h sub 2 cap o plus 58 kJ
10

Which statement about enthalpy is true?

A
The enthalpy of formation for a pure element in its standard state is always negative.
B
Enthalpy is not a state function because its change depends on the identities of the reactants and products.
C
Enthalpy is a state function because its change depends only on initial and final conditions.
D
The enthalpy of formation for a pure element in its standard state is always positive.
11

Which is the best example of how electromagnetic energy is used in everyday life?

A
a car burning gasoline to power the engine
B
a patient receiving an X-ray in a hospital
C
a car of a roller coaster moving quickly down a slope
D
a pot of boiling water on a gas stove
12

Butane cap c sub 4 cap h sub 10 g , Δ⁢H_f=−125.7 , combusts in the presence of oxygen to form CO sub 2 times g , Δ⁢H_f=−393.5kJ/mol , and cap h sub 2 cap o g , Δ⁢H_f=−241.82 in the reaction: 2 cap c sub 4 cap h sub 10 g plus 13 cap o sub 2 g rightarrow 8 CO sub 2 g plus 10 cap h sub 2 cap o g What is the enthalpy of combustion, per mole, of butane? Use Δ⁢H_rxn=∑(Δ⁢H_f_1p⁢r⁢o⁢d⁢u⁢c⁢t⁢s)−∑(Δ⁢H_f_1r⁢e⁢a⁢c⁢tan⁡t⁢s)

A
2 comma 657 point 4 kJ over mol
B
5314 point 8 kJ over mol
C
negative 2 comma 657 point 4 kJ over mol
D
negative 5314 point 8 kJ over mol
13

Consider these intermediate chemical equations. cap c times s plus 1 half cap o sub 2 g rightarrow CO times g CO times g plus 1 half cap o sub 2 g rightarrow CO sub 2 times g How will oxygen appear in the final chemical equation?

A
cap o times g as a product
B
cap o sub 2 times g as a reactant
C
cap o sub 2 times g as a product
D
2 cap o g as a reactant
14

Which statement best describes the direction of heat flow by conduction between two samples of the same material?

A
Heat flows from faster molecules to slower molecules when they are near.
B
Heat flows from faster molecules to slower molecules when they collide.
C
Heat flows from slower molecules to faster molecules when they are near.
D
Heat flows from slower molecules to faster molecules when they collide.
15

Complete combustion of a 0 point 3 5 0 g sample of a compound in a bomb calorimeter releases 14 point 0 kJ of heat. The bomb calorimeter has a mass of 1 point 2 0 kilograms and a specific heat of 3 point 5 5 the fraction with numerator cap j and denominator open paren g times degree cap c close paren If the initial temperature of the calorimeter is 22 point 5 times degree C , what is its final temperature? Use q is equal to m cap c sub p triangle cap t

A
19 point 2 times degree C
B
72 point 3 times degree C
C
34 point 2 times degree C
D
25 point 8 times degree C
16

Reactant cap p contains 50 cap j of energy, and reactant cap q contains 35 point 3 cap j of energy. The reactants combine to form product PQ , which contains 104 cap j of energy. What is the energy transformation?

A
Energy is lost because the product has less energy than the reactants have.
B
Energy is absorbed because the product has less energy than the reactants have.
C
Energy is absorbed because the product has more energy than the reactants have.
D
Energy is lost because the product has more energy than the reactants have.
17

Which best describes heat?

A
energy that increases as a book is lifted to a higher shelf
B
energy that flows from a hot mug of tea to a cold hand
C
energy that is carried by electric and magnetic fields
D
energy that decreases as a car slows to a stop
18

A metal sample is heated and placed into the water in a calorimeter at room temperature. Which statement best describes how the calorimeter can be used to determine the specific heat capacity of the metal sample?

A
Energy transfers from the metal to the water and calorimeter until they are all at room temperature.
B
Energy transfers to the metal from the water and calorimeter until they all reach a single temperature.
C
Energy transfers to the metal from the water and calorimeter until they are all at room temperature.
D
Energy transfers from the metal to the water and calorimeter until they all reach a single temperature.
19

Which describes the enthalpy change associated with an endothermic reaction?

A
It is positive because the enthalpy of the reactants is greater than the enthalpy of the products.
B
It is negative because the enthalpy of the products is greater than the enthalpy of the reactants.
C
It is positive because the enthalpy of the products is greater than the enthalpy of the reactants.
D
It is negative because the enthalpy of the reactants is greater than the enthalpy of the products.
21

What explains the key difference between a bomb calorimeter and a coffee cup calorimeter?

A
A bomb calorimeter can measure heat gain or loss in gaseous reactions but is not useful for reactions that occur at high pressures and temperatures.
B
A bomb calorimeter is 10 times larger but works the same way.
C
A bomb calorimeter measures heat for liquid products only.
D
A bomb calorimeter has a separate chamber to hold substances and can even measure heat gain or loss for reactions that do not occur in water.

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