Chemistry 534

Pretest 2.1

 

 

1.         A student was working on her pretest. Meanwhile 1000 kJ from the slab of cheesecake she ate earlier was used for basic bodily functions, and 600 kJ were used to store vital course-related information. If the slab of cheesecake contained 2000 kJ in all, how much energy was lost in the form of heat? Why?

 

2000 kJ = 600 kJ + 1000 kJ + x (based on the conservation of energy)

x = 400 kJ.

 

2.         Draw two H2O2 molecules. Label all intermolecular and intramolecular bonds.

 

 

 

 

 

 

           

 

 

 

 

An  intermolecular attraction occurs between two molecules, as shown by the two headed arrow. Each H2O2 also has 3 intramolecular attractions because there are three bonds per molecule.

 

 

 

3.         a.         Draw a reaction profile for an exothermic reaction.

 

                        E(kJ)

           

 

 

                                    Reaction progress

 

b.                   b.                  Do the same for an endothermic reaction.

 

 

E(kJ)

 

 

                                    Reaction progress

4.         When nitric acid, sulfuric acid and glycerin are mixed, they react to form a new compound and a great deal of heat is released. Where is this heat coming from?

 

            From the excess enthalpy of nitric acid, sulfuric acid and glycerin. More specifically, the energy comes mostly from the intramolecular bonds of the reactants mentioned.

 

5.         Which of the following defines enthalpy?

 

 

A)

 

The energy absorbed or released during a chemical reaction

 

B)

 

The change in potential energy that results from a chemical or physical change

 

C)

 

The energy required to start a chemical reaction

 

D)

 

The internal energy stored in a substance during its formation

 

Answer (D)     (A) and (B) are definitions of the change in enthalpy (DH). (C) is the definition of activation energy.

                        Note that enthalpy can also be defined as the sum of potential and kinetic energies of a substance.

 

6.         Which of the following changes are exothermic?

 

1.         Logs burning on a camp-fire

 

2.         The action of an ice-salt mixture used to freeze ice cream

 

3.         Laundry that dries on a clothesline

 

4.         A chicken roasting

 

5.         A bolt of lightning

 

Answer:            1, 2 and 5.

 

 

7.         This was , as you can see, #10 on the June 2001 exam:

 

 

            Reverse eq(1):

            CO2+ 22.61 kJ à C(diamond) + O2                        DH = 22.61 ( if heat is a “reactant”, DH = +)

            Keep eq(2) as it is:

            C(graphite) + O2à CO2+ 22.50 kJ              DH = -22.50 ( heat is released)

 

            Sum:     C(graphite) à C(diamond)                              DH = 0.11 kJ                

 

8.         Some automobiles and buses are equipped to burn propane gas, C3H8, as a fuel.

 

The complete combustion of propane is shown by the following chemical equation:

 

 

Given the following heats of formation.

 

                                    H2(g)  +   O2(g)  ®  H2O(g)                  DH = -242.0

 

                                    C(s)  +  O2(g)  ®  CO2(g)              DH = -393.5

 

What is the heat of formation of propane?

 

                                    3 C(s)  +  4 H2(g)  ®  C3H8(g)                   DH = ?

 

Solution:

 

 

 

3 C(s)  +  4 H2(g)  ®  C3H8(g)

DH = ?

Reverse the «combustion of propane» equation and add to it the heats of formation of CO2(g) and H2O(g)

(Reverse)

4 H2O(g)  +  3 CO2(g)  ®  C3H8(g)  +  5 O2(g)

DH = + 2044.5

(mult ´ 4)

4 H2(g)  +  2 O2(g)  ®  4 H2O(g)

DH = - 968

(mult ´ 3)

3 C(s)  + 3 O2(g)  ®  3 CO2(g)

DH = - 1180.5

 

 

3 C(s)  + 4 H2(g)  ®  C3H8(g)

DH = - 104

 

Answer :         The heat of formation of propane is - 104 .

 

 

9.         A bathtub contained  200.0 kg of water.  The water's temperature increased from 20.4 to 38.4 oC when it absorbed the heat from the combustion of of  420.0 g of C3H6 . Find the molar DH for the combustion of C3H6.                                                                                                                                   

(Ans: DH =-1.51 X 103 kJ/mole)

 

 

 10.        0.20 moles of H2SO4(aq) were completely neutralized by aqueous NaOH. The resulting 600.0 ml of mixture experienced a 15 degree increase in temperature. What is the molar heat of neutralization of aqueous NaOH? Density = 1.00 g/ml)

 

Q = mcDT

=600.0ml(1.00g/ml)(4.19J/(g oC))(15)oC

=

                             38011.68000J

=38.01168 kJ

DH =-Q =-38.01168 kJ

Since the ratio is 2:1  2 NaOH +  H2SO4 à Na2SO4 + 2 H2O, 0.40 moles of NaOH reacted with 0.20 moles of H2SO4(aq).

DH/n =-38.01168 kJ/0.40moles =-95kJ/mole NaOH

Flashback

 

1.         What is the minimum number of litres of O2 needed to completely burn 320. g of CH3OH if the reaction occurs at 150. kPa and 400. oC?

           

            2 CH3OH(l)       + 3 O2(g)            à        2 CO2(g)            +          4 H2O(l)

 

            320 g of CH3OH/(32 g/ mole of CH3OH) = 10 moles of CH3OH

 

            The ratio of O2(g) to CH3OH from the equation is 3 to 2, so we need 1.5*10 = 15 moles of O2(g).

 

            PV = nRT;

            V = nRT/P = 15(8.31)(400 + 273)/150 =  559 L

 

2.         Which 3 of the following factors influence the volume occupied by a gas?(JUNE 2001)

            __pressure        ___temperature            ___density        __solubility        __# of moles

           

P,T and n because V = nRT/P.