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REVIEW ACS V

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Question 1

Calculate the change in enthalpy for the chemical reaction N2(g) + 3H2(g) --> 2NH3(g)

Select one:
 Correct

 

Explanation

 

 Δ H = Bonds Broken - Bonds Formed

Broken = N2 + 3H2 = 946 +1308 = 2254

Formed = 2NH3 = 2334

  Δ H = 2254 - 2334 = -80
 

Question 2

In a Styrofoam coffee cup calorimeter with a lid, the experiment takes place

Select one:
 Correct

 

Explanation

 The coffee cup calorimeter is used to calculate heat flow so that mean pressure needs to remain constant.

 

Question 3

Correct

 

Explanation

 Enthalpy of Combustion: Product - Reactant


Product: (2 * -393.5) + ( 3 * -285.8) = -1644.4

Reactant: ( -84.7 )


Enthalpy of Combustion: -1644.4 + ( -84.7) = -1559.7

 

Question 4

The following question is from the ACS Study Guide. It would be better to ask which diagram best represents the change in enthalpy for the dissolution process.

Select one:
 Correct

 

Explanation

 The dissolution process - where solid, liquid, gas particles are broken into solutes. To break apart molecules from each other requires energy therefore this is an endothermic reaction as shown in graph A. 

 

Question 5

When a 86 g sample of an alloy at 100.0 oC is dropped into 90.0 g of water at 26.2 oC, the final temperature is 30.3 oC. What is the specific heat of the alloy? (The specific heat of water is 4.184 J/(goC).)

Correct

 

Explanation

 

q(alloy) = q(water)

(86.0)(cp alloy)(100 - 30.3) = (90)(4.184)(30.3 - 26.2)

c alloy = (90)(4.184)(30.3 - 26.2) / (86.0)(100 - 30.3) 0.2575  J/g C
 

Question 6

For which of the following reaction is the enthalpy of formation of the compound equal to the ΔH of reaction?

Select one:
 Correct

 

Explanation

 Only 1 mole of the compound is formed from its constituent elements. Therefore, the first option is right. 

 

Question 7

What is the enthalpy of formation of CO(g) in kJ? 

2CO(g) + O2(g) --> 2CO2(g).

 ΔH reaction = -566 kJ ; ΔH CO2(g) =-393.5 kJ/mol.

Correct

 

Explanation

 

 ΔH reaction =  Product - Reactant

Product = ΔH 2 CO2(g) = 2 (-393.5 kJ/mol )

Reactant = ΔH 2CO(g) = ?

ΔH reaction = -566 kJ

(-566 kJ) =   2 (-393.5 kJ/mol) - 2X

-110.5 = X

 

Question 8

When a liquid freezes and then melts, the steps are

Select one:
 Correct

 

Explanation

Freezing a liquid requires energy to be removed so it is an exothermic process. Melting requires adding energy so it is an endothermic process. 

 

Question 9

It takes more energy to boil away a pan of water at 100 oC than to heat it from room temperatue to 100 oC, because

Select one:
 Correct

 

Explanation

Heat of Vaporization is 40.7 kJ/mol

 

Question 10

Correct

 

Explanation

 2 H2O2 ---> 2H2 + 2 O2   ΔH = 376

2H2 + O2 ----> 2H2O        ΔH = -572


2H2O ---> O2 + 2H2O  ΔH =-196

 

Question 11

When 192 J of energy are added to a sample of gallium that is initially at 25.0 oC, the temperature rises to 35.0 oC. What is the volume of the sample in cubic centimeter?

Correct

 

Explanation

 q = 192 J


q = mC ΔT


m = q / (C ΔT)


m = 192 / ( 0.372 x 10) = 51.6 g


51.6 g x ( 1cm^3/  5.904 g) = 8.74 cm 3


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