AnswersChemistryEnergy in Chemical Reactions

Energy in Chemical Reactions — Test Answers

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1
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Which direction does thermal energy flow in this diagram?

A
Heat flows from the hand to the ice cream.
B
Heat flows from the ice cream to the hand.
C
The system is in a state of equilibrium.
D
The system is too cold for heat to move.
2
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What is the overall enthalpy change triangle cap h sub rxn for the system?

A
300 kJ
B
negative 1 comma 300 kJ
C
1 comma 300 kJ
D
negative 300 kJ
3

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.
4

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 sub 2 times g as a product
B
2 cap o g as a reactant
C
cap o times g as a product
D
cap o sub 2 times g as a reactant
5

A welding torch produces a flame by burning acetylene fuel in the presence of oxygen. This flame is used to melt a metal. Which energy transformation represents this process?

A
kinetic energy into potential energy
B
kinetic energy into electromagnetic energy
C
chemical energy into thermal energy
D
potential energy into chemical energy
6

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 decreases as a car slows to a stop
D
energy that is carried by electric and magnetic fields
7

cap h sub 2 cap o has a triangle cap h sub vap is equal to 40 point 7 kJ over mol What is the quantity of heat that is released when 27 point 9 g of cap h sub 2 cap o condenses? Use q is equal to n triangle cap h

A
60 point 0 0 kJ
B
63 point 0 9 kJ
C
68 point 6 0 kJ
D
61 point 0 5 kJ
8

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 absorbed
B
13 point 5 kJ of energy released
C
84 point 6 kJ of energy released
D
13 point 5 kJ of energy absorbed
9

Which statement about enthalpy is true?

A
The enthalpy of formation for a pure element in its standard state is always positive.
B
The enthalpy of formation for a pure element in its standard state is always negative.
C
Enthalpy is a state function because its change depends only on initial and final conditions.
D
Enthalpy is not a state function because its change depends on the identities of the reactants and products.
10

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
Divide the third equation by two, and double the enthalpy. Then, add.
C
Multiply the first equation by three, and triple the enthalpy. Then, add.
D
Reverse the second equation, and change the sign of the enthalpy. Then, add.
11

How is the kinetic energy of the particles of a substance affected during a phase change?

A
Kinetic energy does not change, but the potential energy does.
B
Kinetic energy increases during exothermic changes and decreases during endothermic changes.
C
Kinetic energy decreases during exothermic changes and increases during endothermic changes.
D
Kinetic energy changes in the opposite way that the potential energy changes.
12

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 positive because the enthalpy of the products is greater than the enthalpy of the reactants.
C
It is negative 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.
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 When you form the final chemical equation, what should you do with CO ?

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

Which statement defines calorimetry?

A
a device that is used to measure the heat that is gained or lost during a chemical change
B
the conservation of energy
C
a unit of energy that is used when calculating energy exchange
D
the measurement of energy that is given off or absorbed in a physical or chemical process
16

A sample of iron Fe with a mass of 200.0g releases 9 comma 840 cal when it freezes at its freezing point. What is the molar heat of fusion for iron? Use q is equal to n triangle cap h

A
2 comma 771 point 8 cal over mol
B
2 comma 747 point 7 cal over mol
C
3 comma 280 point 0 cal over mol
D
2 comma 811 point 7 cal over mol
17

Which reaction is endothermic?

A
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
B
HCl plus NaOH rightarrow NaCl plus cap h sub 2 cap o plus 58 kJ
C
2 Na plus Cl sub 2 rightarrow 2 NaCl plus energy
D
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
18

Which is the most important factor in determining the state of a substance?

A
the size of the atoms in a substance
B
the balance between intermolecular forces and kinetic energy
C
the number of molecules in a substance
D
the position of the electrons on the outer valence shells
19

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 to the metal from the water and calorimeter until they are all at room temperature.
B
Energy transfers from the metal to the water and calorimeter until they all reach a single temperature.
C
Energy transfers to the metal from the water and calorimeter until they all reach a single temperature.
D
Energy transfers from the metal to the water and calorimeter until they are all at room temperature.
20

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

A
A bomb calorimeter is 10 times larger but works the same way.
B
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.
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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