Honors chemistry

Spring Examination Study Questions

 

1.  (Chapter 6/Heat Changes)  A calorimeter containing water is used to measure the heat produced by a chemical reaction.  If the water absorbs 58.5 kJ when the temperature is raised from 21.2°C to 77.2°C, how much water was in the calorimeter?  (The specific heat of water is 4.18 J/g °C.)

 

2.  (Chapter 6/Heat Changes)  For the reaction between solid iron and water to form iron(III) oxide and hydrogen gas,

      a)  write a balanced chemical equation.

      b)  use the appropriate table to calculate DH for this reaction.

      c)  determine whether this reaction is endothermic or exothermic.

 

3.  (Chapter 5/Gases)  A sample of gas occupies a volume of 5.60 liters at STP.

      a)  What is the pressure of this sample when it is allowed to expand to 18.0 liters at 78°C?

      b)  How many moles of gas are in the sample?

      c)  If the sample contains 7.50 grams of gas, what is the molar mass of this gas?

      d)  The above gas is an alkane.  Give its formula, name the alkane, and draw its structure.

 

4.  (Chapter 5/Gases)  For the reaction between solid iron and water to form iron(III) oxide and hydrogen gas,

      a)  write a balanced chemical equation.

      b)  How many liters of hydrogen gas are produced from 29.8 grams of iron at 1.60 atmospheres and 117°C?

      c)  What is the density of hydrogen gas in (b)?

 

5.  (Chapter 11/Solutions)  You may use the equation below to solve the following problems:

                        DTf   =   -1.86°C  x  moles solute particles/kg water

      a)  What is the freezing point of a solution containing 117 g NaCl in 500 g of water?

      b)  How many moles of a nonelectrolyte in 50 g of water are required for a solution to have a freezing point of -2.79°C?

 

6.  (Chapter 11/Solutions)

      a)  How many g of NaNO3 are needed to make 157 ml of a 3.00 M NaNO3 solution?

      b)  Describe how you would make the solution in (a).

      c)  What is the concentration of NaNO3 in a solution prepared by diluting 240 ml of 0.500 M NaNO3 to 2.00 liters?

      d)  The density of a 26.0% solution of NaNO3 is 1.19 g/ml.  What is the molarity of the 26.0% solution?

 

7.  (Chapter 12/Reaction Rate)  Explain how and why each of the following affect reaction rate:

      a)  concentration of reactants                      c)  temperature

      b)  surface area of reactants                        d)  a catalyst

 

8.  (Chapter 13/Equilibrium)

      a)  Write a balanced chemical equation for the equilibrium reaction in which bromine gas and chlorine gas combine to form bromine chloride gas.

      b)  Write an expression for K, the equilibrium constant, for this reaction.

      c)  A one-liter flask initially contains 0.70 M bromine and 0.55 M chlorine.  When the system reaches equilibrium, the bromine concentration is 0.35 M.  Calculate the value for K for this system.

 

9.  (Chapter 13/Equilibrium)

      a)  For the system at equilibrium,

                        2 NO2(g)        N2O4(g)       DH  =  -58 kJ,

            what affect will each of the following have?

            i)  decreasing the volume                                   iii)  adding a catalyst

            ii)  increasing the temperature                            iv)  adding more N2O4

b)  Write an expression for K for this equilibrium.  Calculate [N2O4] if the [NO2] = 0.010 M when the value for K is 10.0.

 

10.  (Chapter 14/pH)  Find the pH of each of the following solutions:

      a)  0.0050 M HClO4

      b)  1.0 g NaOH dissolved in 250 ml water

      c)  a 0.10 M solution of a weak acid with a Ka of 10-7

      d)  0.00435 M NaHCO3 (Kb for HCO3- is 2.3 x 10-8).

 

11.  (Chapter 15/Buffers)

      a)  What is the pH of a buffer made up of 100 mL of 0.10 M HF and 174 mL of 0.10 M NaF?

      b)  What ratio of ClO-/HClO is needed to produce a buffer with a pH of 7.4?

 

12.  (Chapter 15/Titration)

      a)  What is the molar mass of an acid, if 0.864 grams of the acid are neutralized by 36.0 ml of  0.400 M NaOH?

      b)  What is the molarity of an NH3 solution if 12.0 liters of the NH3 solution are neutralized by 360 ml of 4.00 M HCl?

 

13.  (Chapter 16/Ksp)

      a)  The Ksp for zinc hydroxide is 4.5 x 10-17.  Find the concentration of zinc hydroxide dissolved in a saturated solution.

      b)  What concentration of potassium hydroxide must be added to 0.0040 M zinc chloride to form a precipitate?

 

14.  (Chapter 18/Oxidation-Reduction)  For the following oxidation-reduction equation:

                        NO3-(aq)  +  Cu(s)   ®   NO(g)  +  Cu2+(aq)

      a)  Balance the equation adding H+ and H2O as needed.

      b)  Use standard reduction potentials to determine E° for the reaction.

      c)  Which substance is acting as an oxidizing agent?  as a reducing agent?


Honors Chemistry

Answers to Spring Examination Study Questions

 

1.  Q  =  C x m x DT;   Q = 58.5 kJ = 58,500 J; m = ?; DT = 77.2-21.2 = 56.0°C; C = 4.18 J/g °C

      m  =   =  250 g  =   2.50 x 102 g   (3 sig fig)

2.  a)  2 Fe(s)  +  3 H2O(l)   ®   Fe2O3(s)   +   3 H2(g)

      b)  From Table 4.2:            2 Fe(s)  +  1˝ O2(g)   ®   Fe2O3(s)     DH  =  -822.2 kJ

                                                H2(g)  +  ˝ O2(g)   ®   H2O(l) DH  =  -285.8 kJ

            2 Fe(s)  +  1˝ O2(g)   ®   Fe2O3(s)                             DH  =  -822.2 kJ

                        3 H2O(l)         ®   3 H2(g)  +  1˝ O2(g)           DH  =  +3(285.8) kJ = +857.4

            2 Fe(s)  +  3 H2O(l)   ®   Fe2O3(s)   +   3 H2(g)          DH = -196.5 + 204.9 =  +35.2 kJ

      c)  endothermic

 

3.  a)  V1 = 5.60 L; P1 = 1 atm, T1 = 273 K; P2 = ?; V2 = 18.0 L; T2 = 78 + 273 = 351 K.

      =  0.400 atm

      b)  5.60 L = 0.250 mol;     or use at either T and P.

      c)  molar mass =  =   30.0 g/mol

      d)  C2H6, ethane, CH3–CH3

 

4.  a)  2 Fe(s)  +  3 H2O(l)   ®   Fe2O3(s)   +   3 H2(g)

      b)  29.8 g Fe =  0.800 moles  H2

V =   =  16.0 L

      c)  mass(H2)  =  0.800 mol x=  1.61 g;  density  =   =  0.101 g/L

 

5.  a)  moles particles  =  117 g NaCl  =  4.00 mol particles

            DTf  =  1.86   =  14.9°C;   Tf  =  0 – 14.9°C = -14.9°C

      b)  DTf  =  2.79°C,  moles = ?,  50.0 g  =  0.0500 kg H2O

            DTf = 1.86 ; moles  =  = 0.0750 mol

6.  a)  157 mL  =  40.0 g NaNO3

     

      b)  Review how to prepare a solution from either solid solute or by diluting a concentrated

solution:  see Procedure from the “Solution Preparation” Experiment.

      c)  V1 x M1 = V2 x M2; V1 = 240 mL; M1 = 0.500 M; V2 = 2.00 L = 2000 mL; M2 = ?

            M2  =    =  0.0600 M

d)    =  3.64 M

7.  a) concentration of reactants:  Reaction rate increases as concentration of reactants increases because number of collisions increases, making reaction more likely to occur.

      b)  surface area of reactants:  Rate increases as surface area of reactants increases because the greater the area of reactant exposed, the more likely are collisions that will result in product formation.

      c)  temperature:  As temperature increases, rate increases because at higher temperature, a greater proportion of reactant molecules have a kinetic energy greater than the activation energy so a greater proportion of collisions result in product formation.

      d)  catalyst:  Catalysts increase reaction rate by lowering the activation energy.

 

8.  a)    Br2(g)  +  Cl2(g)     2 BrCl(g)

      b) 

      c)                Br2(g)  +    Cl2(g)           2 BrCl(g)

            init.    0.70 M       0.55 M                     0

            D      -0.35 M     -0.35 M              +0.70 M       = 7.0

          equil.    0.35 M       0.20 M                0.70 M

 

 

9.  a)  i)  shift to right (shift to side with fewer moles);  ii)  shift to the left (shift in endothermic direction to use up heat);  iii)  no shift;  iv)  shift to left (use up some of the N2O4 added).

      b)  K  =  ;  [N2O4]  =  K x [NO2]2  =  10.0 (0.010)2  =  0.0010 M

10.  a)  HClO4  =  strong acid, so [HClO4]  =  [H+] =  0.0050 M; pH = - log(0.0050) =  2.3

      b) NaOH = strong base, so [NaOH] = [OH-] = = 0.10 M NaOH

            [OH-]  =  0.10 M  =  1 x 10-1 M;    [H+] = 1 x 10-13 M;     pH  =  13.0

      c)  Ka = ;  [H+] = [A-] = x;  [HA] » 0.10 M;  1.0 x 10-7 =

            x2 = (1.0 x 10-7)(0.10) = 1.0 x 10-8 M;  x = [H+] = 1.0 x 10-4 M;  pH = 4.0

      d)  HCO3- + H2O    H2CO3 + OH-;  Kb = ; x = [OH-] = [H2CO3]

            2.3 x 10-8  =  ;  x2 = (2.3 x 10-8)(0.00435) = 1.0 x 10-10

            x = [OH-] = (1.0 x 10-10)1/2  =  1.0 x 10-5 M;  pOH = 5.0;  pH = 9.0

 

11.  a)  pH  =  pKa  +  log;   pKa(HF)  =  - log (7.2 x 10-4)  =  3.14;   =  1.74

            pH  =  3.14  +  log(1.74)  =  3.14  +  0.24  =  3.40;  pH  =  3.40

 

      b)  Ka(HClO)  =  3.5 x 10-8; [H+] = 10-7.4 M  =  4.0 x 10-8 M

             = 0.88

 

12.  a)  moles acid = moles base = 0.0360 L   =  0.0144 moles

            molar mass =  = 60.0 g/mol

      b)  since moles acid = moles base: VA x MA = VB x MB

            0.360 L  x  4.00 M  =  12.0 L x MB;      MB  =    =  0.120 M

 

13.  a)  Zn(OH)2(s)    Zn2+(aq)  +  2 OH-(aq);   Ksp  =  [Zn2+][OH-]2;  Ix = [Zn2+]; [OH-] = 2x

            Ksp  =  x(2x)2  =  4x3;   4.5 x 10-17  =  4x3;    x3 = 1.1 x 10-17;    x = 2.2 x 10-6 M

b)  Ksp  =  [Zn2+][OH-]2;  x = [KOH];  [OH-] = x; [Zn2+] = 0.0040 M

            4.5 x 10-17  =  (0.0040)(x)2

            x2 = (4.5 x 10-17)/(0.0040);  x = (1.1 x 10-14)1/2 = 1.1 x 10-7 M

14. a)  oxidation: Cu(s) ® Cu2+(aq) + 2 e-;  reduction: 3 e- + 4 H+ + NO3-(aq)® NO(g) + 2 H2O

            overall: 2 NO3-(aq)  +  3 Cu(s)  +  8 H+(aq)  ®   2 NO(g)  +  3 Cu2+(aq) +  4 H2O

      b)  E° = E°ox(Cu) + E°red(NO3-)  =  - 0.34 v  +  0.96 v  =  0.62 volts

      c)  NO3- is the oxidizing agent (it’s reduced); Cu is the reducing agent (it’s oxidized).