Chem 120A Exam 2 Name:___________________________ IMPORTANT: This exam is designed for you to learn how to apply the concepts you have been practicing. Read the entire question and answer as fully as you can. Be sure to attempt all parts of a question. Always show your work to maximize your points awarded, unsupported answers may not receive full credit and partial answers will receive some partial credit. Circle your answers. Write your name on each page in case your exam falls apart during grading. Question 1: Write the nuclear reaction equation for when a radioactive element undergoes  emission to form Pu: →
Pu Question 2: A radioactive element has a half‐life of 19 min. How many minutes will it take for 93.75% of the atoms to decay? Start with 100% and you are decaying all but 6.25% 100% (1/2)(1/2)(1/2)(1/2) = 6.25% 4 half‐lives. 19 min * 4 half‐lives = 76 minutes Question 3: Match the particles on the left with the minimum shielding required to block them on the right. A. Beta______II_______________ B. Gamma____III______________ C. Alpha_______I_____________ I. Paper II. Heavy clothing III. Lead Stanich – SU12 Page 1 Chem 120A Exam 2 Name:___________________________ Question 4: Rank the four attractive forces in order of weakest (1) to strongest (4). ___3_________Hydrogen bonds _____2_______Dipole‐dipole attractions _____4_______Ionic bonds _____1_______Dispersion Forces Question 5: Draw Lewis Dot structures and fill in the table for the following. This problem continues on the next page. A. CF4 What is the shape of this molecule according to VSPER theory? Tetrahedral How many polar Is this molecule bonds are in this polar? molecule? 4 No Name this molecule Carbon tetrafluoride B. N2 How many polar bonds are in this molecule? 0 Is this molecule polar? Name this molecule No Nitrogen Stanich – SU12 Page 2 Chem 120A Exam 2 Name:___________________________ Question 5: Continued… C. OBr2 What is the shape of this molecule according to VSPER theory? Bent How many polar Is this molecule bonds are in this polar? molecule? 2 Yes Name this molecule Oxygen dibromide D. NO2‐ (Including all resonance structures) What is the shape of this molecule How many polar Is this molecule according to VSPER theory? bonds are in this polar? molecule? Bent 2 Yes Name this molecule Nitrite ion 2 Stanich – SU12 Page 3 Chem 120A Exam 2 Name:___________________________ Question 6: For the elements Cs, F, and Cl, indicate the order of increasing electronegativity: LEAST Cs < Cl < F MOST Question 7: For the molecules Br2, I2, Cl2, and F2, indicate the order of increasing melting point: Lowest F2 < Cl2 < Br2 < I2 Highest Question 8: Given the following Lewis structure, answer the three questions below: H H H
H2C C C C CH3
1
2
3
4
5
I. How many lone pairs of electrons are present in this molecule? A) 0 B) 1 C) 2 D) 3 E) 4 II. How many electrons are shared between carbons 1 and 2? A) 0 B) 2 C) 4 D) 6 E) 8 III. How many bonding pairs of electrons are in this molecule? A) 7 B) 9 C) 14 D) 18 E) 28 Stanich – SU12 Page 4 Chem 120A Exam 2 Name:___________________________ Question 9: Write the formulas for:
A. cobalt(II) chloride CoCl2 B. Magnesium nitride Mg3N2 C. Iron metal rusts in oxygen to create iron(II)oxide and iron(III)oxide. Write the ionic formulas for both. FeO and Fe2O3 Question 10: Use the following unbalanced chemical equation to answer the following questions: Mg(s) + H3PO4(aq)  Mg3(PO4)2(s) + H2(g) A. Balance the chemical equation.
3 Mg(s) + 2 H3PO4(aq)  Mg3(PO4)2(s) + 3 H2(g) B. What atom is reduced in this reaction?
H is reduced from +1 to 0. C. In this reaction, the phosphate ion is (circle one answer below):
1. Oxidized.
2. Reduced.
3. A spectator ion. 4. The limiting reagent.
Stanich – SU12 Page 5 Chem 120A Exam 2 Name:___________________________ Question 11: Sulfuric acid may be produced by the following process (all equations are balanced): 4FeS2 + 11O2  2Fe2O3 + 8SO2
2SO2 + O2  2SO3
SO3 + H2O  H2SO4
Assuming the reaction is 85% efficient, how many moles of H2SO4 will be produced from 5.00 mol of FeS2? 5.00
85%
1.25
.
100%
1.06
A BETTER ANSWER, I should have given you full credit for this. Please turn in a regrade by Friday for this answer:
5.00
8
4
2
2
85%
1
1
10.0
100% 10.0
Actual yield = 8.50 mole Question 12: Hydrogen gas and oxygen gas react to form water vapor. If you mixed 15.0L of hydrogen gas and 20.0L of oxygen gas at sea level and at 150oC, answer the following questions: A. What is the limiting reactant, and how many liters of the excess reactant are left?
Find moles of H2 and O2 you start with
1
15.0
150 273
2
2
1
15.0
0.0821
1
0.0821
423
20.0
0.431924396
423
0.575899
Now you need to know what the balanced equation is: 2 H2(g) + O2(g)  2 H2O (g) If we used up all of the H2 we would need how many moles of O2? 0.431924396
0.215962198
If we used up all of the O2, we would need how many moles of H2? 0.575899
1.151798
needed. H2 is going to be used up first and is therefore the limiting reactant. O2 left over is = 0.575899 mole – 0.215962198 mole = 0.3599 mole ≈ 0.360 mole O2 left Stanich – SU12 Page 6 Chem 120A Exam 2 Name:___________________________ 1 atm V = 0.360 mole 0.0821 L atm/mole K 423K plug in excess moles to get liters V = 12.5 L B. How many moles of water will be formed? 0.431924396
0.431924396
0.432
C. What is the final volume of the mixture of excess reactant and water vapor? Water vapor + excess reactant = 0.43192 mole + 0.21596 mole = 0.648 mole
1
0.648
0.0821
423
22.5
It is also valid to find the volume of the water vapor then add the volume of O2 excess and volume of water
together for 22.5 L
You could have also used Avogadro’s law throughout this question for far less work and the same answer.
Question 13: An experiment was conducted by heating water by burning ethanol (C2H5OH) contained in the spirit burner shown on the left. Ethanol burns oxygen to create carbon dioxide and water. Use the data in the table below to answer the following questions: Mass of water: Initial temperature of the water: Final temperature of the water: 200.0g 20.0 oC 75.0 oC Initial mass of ethanol: Final mass of ethanol: Atmospheric pressure: 10.0 g 8.25 g in 1.05 atm Thermometer Water Spirit burner A. Write the balanced reaction equation for combustion of ethanol.
C2H5OH +3 O2  2 CO2 +3 H2O
Stanich – SU12 Page 7 Chem 120A Exam 2 Name:___________________________ B. According to the data, calculate how much energy was absorbed by the water during combustion.
∆
4.184
200.0
75.0
20.0
46,024
4.6
10 Question 13: Continued... C. Calculate the heat of reaction (ΔH) for 1 mole of ethanol. Report your answer in J/mole.
The answer in part B is for the amount of ethanol we burned. We started with 10.0g and ended with 8.25g, which means 1.75 g were burned. We need to know moles, so find MW of ethanol: 46.07 g/mole 1.75 .
0.037985
of ethanol were burned in the experiment The heat from part B should be negative now that we are thinking about heat released by the reaction. ,
.
1211636.17
1.21
10
D. What amount of heat is released for one molecule of ethanol? 1211636.17
.
201268.467
2.01
10
/
E. Is this reaction exothermic or endothermic? Exothermic F. The actual heat of reaction for the combustion of ethanol is higher than the heat of reaction you measured in the laboratory. Explain in one or two sentences why this is not surprising.
The beaker of water could be losing heat to the surroundings, making the change in temperature lower than it should have been. Another reason could be that we didn’t have an efficient combustion of our fuel. Stanich – SU12 Page 8 Chem 120A Exam 2 Name:___________________________ You got 1 point if you said something like human error. While this may be true, I wanted more about the physical reality of laboratory practices. Congratulations! You are done. Be sure to check that you have units, correct sig figs and circled answers. Be sure to write your name on each page in case the exam falls apart during grading. You may hand in your exam and leave quietly once you are done UNLESS there is only 5 minutes remaining. Stay seated to be courteous to your finishing classmates. Stanich – SU12 Page 9