CHE112     EXAM 1 __________________ID#__KEY______

1). When [A] = 0.1 M, the reaction has a rate of 0.010 M s-1. When [A]=0.2 M , the rate is 0.040 M s-1

What is the order of the reaction?

[A]1/[A]2 =1:2, the concentration increased 2-fold The rate have increased 4-fold: 4=2 x 2 = 22

Answer: second order

2) A reaction has a first order in A and zero order in B.  Write units of the rate constant k.

v=k[A]1[B]0 = k[A] – a first order reaction. Therefore, the units of k are 1/s

3) For zero order reaction, it took 10 min to decompose 20 g of NH3 in a certain catalytic reactor. How long it will take to decompose 25 g of ammonia in the same reactor?

t= 10 min x 25/20 = 12.5 min  (the rate is constant for zero order reaction)

4) An introduction of a catalyst had accelerated the forward reaction of the reversible process by factor of 2. What can you say about the rate of the reverse   reaction?

The catalyst changes the forward and the reverse reaction exactly the same way. So, the reverse reaction will go 2 times faster

Write the equilibrium constant expressions for the following processes (7-10):

5)  CO2(solid, “dry ice”) ® CO2 (in air)    K= PCO2

6) H2 + Br2 ® 2 HBr   (all are gases!)          K = PHBr2 / PH2 PBr2

7) HCOOH ® H+ + HCOO-  (in aqueous solution) K = [H+][HCOO-]/[HCOOH]

8) For reaction CO2 (g) + CF4 (g)= 2 COF2 (g)   Kc = 0.5 at 1300 K

If at some moment we have all concentrations being equal to 1 M, is this system at equilibrium? Will it go forward or reverse? Explain!

K =[COF2]2/[CO2][CF4]  Reaction quotient Q=1 if all concentrations are 1 M

Q>K , reaction is not at equlibrium and will go reverse

 

9). How will the equilibrium of the reaction

            N2 + O2  = 2 NO     More O2 will force reaction go forward (to the right side of the equation)

be affected by increase of partial pressure of oxygen?

 

10). Consider the equilibrium 2NO + Cl2 ® 2NOCl. The reaction is exothermic. How will the amount of NOCl at equilibrium be affected by raising the temperature?

Raise T = more heat – will shift equilibrium to the left side = less NOCl

 

11). The solubility of a gas is 0.10 mM  at partial pressure P = 0.20 atm. Calculate its solubility at P=1.00 atm.

        X = 0.010 mM x 1.00/0.20 = 0.050 mM

12). A solution is 0.020 M in BaCl2. Calculate the molarities of the barium ion and of the chloride ion in the solution.

BaCl2 ® Ba2+ + 2 Cl-    the same number of moles for Ba and twice larger for Cl

0.020 M   Ba2+        and 0.040 M Cl-   

13). Estimate the boiling point of 2 m sucrose solution in water.      Kb=0.51 °C/m.

                        t = 100o+ 0.51 x 2 = 101.2 o

14) Calculate the osmotic pressure of 0.1 M KCl in water at 300 K.    R=0.082 M-1 atm K-1

KCl ® K+ +  Cl-   Total concentration is 0.1 + 0.1 = 0.2 M

  Osmotic pressure = M R T = 0.2 x 0.082 x 300 = 4.92 atm

15). A first rate reaction has a rate of 0.02 M s-1 when [A] is 0.05 M.  Estimate the half-life of this reaction.

v= k [A]  k= 0.02/0.05 = 0.4 s-1   half-life = ln2/k = 0.69/0.4 = 1.73 s