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General Chemistry IIGeneral Chemistry IICHEM 152 Unit 4CHEM 152 Unit 4
Week 13
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A 3.41 x 10-6 g sample of a compound is known to
contain 4.67 x 1016 molecules. This compound
is:
1. CO2
2. CH4
3. NH3
4. H2O
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Solution
A 3.41 x 10-6 g sample of a compound is known to contain 4.67 x 1016 molecules. This compound is?
Gameplan: Find the molecular weight (g/mole), then assess what compounds could have that MW
4.67 x 1016 molecules 1 mole = 7.65 10-8
6.02 1023 molecules moles
3.41 x 10-6 g = 44.0 g/mole
7.65 10-8 moles What molecule has this MW?
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A 3.41 x 10-6 g sample of a compound is known to contain 4.67 x 1016 molecules.
This compound is? 44.0 g/mole
1. CO2 44 g/mole
2. CH4 16 g/mole
3. NH3 17 g/mole
4. H2O 18 g/mole
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In a neutral COMPOUND some elements often have a fixed OXIDATION NUMBER
H almost always is +1O almost always is -2F always is -1
3) In a NEUTRAL COMPOUND the SUM of all the atomic oxidation numbers is ZERO
CH4
CH3COOH
? +1CH4 1C + 4H = 0
C + 4(+1) = -4
-4 +1 CH4
? +1 ? -2 -2 +1 CH3C O O H 2C + 4H + 2O = 0
2C + 4(+1) +2(-2) = 02C + 4 – 4 = 0
0 +1 0 -2 -2 +1 CH3 C O O H
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4) In a COMPLEX ION the SUM of all the atomic oxidation numbers is equal to the TOTAL CHARGE on the ion.
ClO3−
MnO4−
PO33-
? -2 ClO3
1Cl + 3O = -1Cl + 3(-2) = -1
+5 -2 ClO3
? -2 MnO4
1Mn + 4O = -1Mn + 4(-2) = -1
+7 -2 MnO4
? -2 PO3
3 1P + 3O = -3P + 3(-2) = -3
+3 -2 PO3
3
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What is the oxidation number of P in H3PO4?
P = +5What is the oxidation number
of Cr in Cr2O72-?
Cr = +6What is the oxidation number
of C in (NH4)2CO3?C = +4
What is the oxidation number of C in C3H8(propane)?
C = -2 2/3 or -8/3
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When the carbon-carbon bond lengths in ethane (C2H6),
ethene (C2H4), and benzene (C6H6) are arranged in order of
increasing bond length (shortest first), which is the
correct order?
1. C2H6 < C2H4 < C6H6
2. C2H4 < C2H6 < C6H6
3. C6H6 < C2H4 < C2H6
4. C2H4 < C6H6 < C2H6
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Propane again…
Using the structure method, what is the oxidation number of each carbon in propane (C3H8)?
C
C
C
H H
H H
HHH H
12
3
1 4 7 = -32 4 6 = -23 4 7 = -3
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What is the oxidation number of the oxygen-bonded C in
CH3COOH (acetic acid)?
CC
HH
O
HO—H
=
:
:
:
:
1
22 is the O-bonded CO.N. 4-1 = +3
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What is the oxidation number of the C in NH2CONH2? In
ClCH2CH2Cl?
H—N—C—N—H
H H
O=
: :
: :
H—C — C—H
H H
Cl Cl: : : : : :
O.N. 4-0 = +4both carbons are equivalentO.N. 4-5 = -1
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S8 + 12 O2 8 SO3
Is it redox?
If so,What is oxidized?What is the reducing agent?
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0 0 +6 -2S8 + 12 O2 8 SO3
Is it redox?
If so,What is oxidized?
0 +6Sulfur is oxidized0 -2Oxygen is reduced
What is the reducing agent? S8
What is the reducing agent? O2
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2 AgNO3 + Na2S Ag2S + 2 NaNO3
Is it redox?
5 As4O6 + 8 MnO4- + 18 H2O
20 AsO43- + 8 Mn2+ + 36 H+
What is oxidized and reduced?What is the oxidizing agent (OA) and reducing agent (RA)?
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NOT REDOX
+1 +5-2 +1 -2 +1 -2 +1 +5-22 AgNO3 + Na2S Ag2S + 2 NaNO3
Is it redox?
Na will always be +1 in ionic compoundsAg is a transition element, but because we know nitrate and sulfide, it is +1 in both
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Is it redox?
+3 -2 +7 -2 +1 -25 As4O6 + 8 MnO4
- + 18 H2O +5 -2 +2
+1 20 AsO4
3- + 8 Mn2+ + 36 H+
As is oxidizedMn is reducedAs4O6 is the reducing agentMnO4
- is the oxidizing agent
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Predict the outcome of the following reaction. Classify each of the reactions as processes as product-favored or reactant-favored.
Fe(s) + Sn2+(aq)
O2(g) + Au(s)
Na(s) + H2O(l)
Al3+ (aq) + Hg(l)
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Predict the outcome of the following reaction. Classify each of the reactions as processes as product-favored or reactant-favored.
Fe(s) + Sn2+(aq)
O2(g) + Au(s)
Na(s) + H2O(l)
Al3+ (aq) + Hg(l)
product favored
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Predict the outcome of the following reaction. Classify each of the reactions as processes as product-favored or reactant-favored.
Fe(s) + Sn2+(aq)
O2(g) + Au(s)
Na(s) + H2O(l)
Al3+ (aq) + Hg(l)
reactant favored
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Predict the outcome of the following reaction. Classify each of the reactions as processes as product-favored or reactant-favored.
Fe(s) + Sn2+(aq)
O2(g) + Au(s)
Na(s) + H2O(l)
Al3+ (aq) + Hg(l)
product favored
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Predict the outcome of the following reaction. Classify each of the reactions as processes as product-favored or reactant-favored.
Fe(s) + Sn2+(aq)
O2(g) + Au(s)
Na(s) + H2O(l)
Al3+ (aq) + Hg(l)
reactant favored
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Calculate the Ecell for these reactions.
Fe(s) + Sn2+(aq)
O2(g) + Au(s)
Na(s) + H2O(l)
Al3+ (aq) + Hg(l)
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Fe(s) Fe2+ + 2 e- Eox = +0.44 V (the negative of the Ered)Sn2+ + 2e- Sn(s) Ered = -0.14 V
Overall reaction: Fe(s) + Sn2+ Fe2+ + Sn(s) Ecell = +0.30 Vpositive Ecell , therefore spontaneous (product-favored)
You could choose either oxygen reaction, I just chose this one to do the calculation.
4[Au(s) Au3+ + 3 e-] Eox = -1.50 V (the negative of the
Ered)3[O2(g) + 2 H2O(l)+ 4e- 4 OH-] Ered = +0.40 V
multiply the reactions to get charge balance, but NOT the Eox and Ered Overall reaction: 4 Au(s) + O2(g) + 6 H2O(l) 4 Au3+ + 2 OH-
Ecell = -1.10 Vnegative Ecell , therefore non-spontaneous (reactant-favored)
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2[Na(s) Na+ + 1 e-] Eox = +2.714 V (the negative of the Ered)
2 H2O(l)+ 2e- H2 + 2 OH- Ered = -0.8277 Vmultiply the reactions to get charge balance, but NOT the Eox and Ered
Overall reaction: 2 Na(s) + 2 H2O(l) 2 Na+ + H2 + 2 OH- Ecell = +1.886 V
positive Ecell , therefore spontaneous (product-favored)
3[Hg(l) Hg2+ + 2 e-] Eox = -0.855 V (the negative of the
Ered)2[Al3+ + 3e- Al(s)] Ered = -1.66 V
multiply the reactions to get charge balance, but NOT the Eox and Ered
Overall reaction: 3 Hg(l) + 2 Al3+ 3 Hg2+ + 2 Al(s) Ecell = -2.52 V
negative Ecell , therefore non-spontaneous (reactant-favored)
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Summary Activity
A lead acid battery uses the following two reactions to provide a voltage:
Pb + SO42- PbSO4 + 2e- +0.36V
PbSO4 + 2H2O PbO2 + 4H+ + SO42- + 2e- -
1.70V
What is the overall reaction?
What is the overall standard cell potential?
Which reaction is the oxidation and which reaction is the reduction?
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A lead acid battery uses the following two reactions to provide a voltage:
Pb + SO42- PbSO4 + 2e- E1 = +0.36V
PbSO4 + 2H2O PbO2 + 4H+ + SO42- + 2e- E2 = -1.70V
What is the overall reaction?
(reversing the 2nd reaction produces a spontaneous E; electrons are equal in both reactions, so no multiplication)
Pb + PbO2 + 4H+ + 2SO42- 2PbSO4 + 2H2O
What is the overall standard cell potential?
(since the second reaction was reversed, the E2 = +1.70)
Ecell = 0.36 + 1.70
Ecell = 2.06 V
Which reaction is the oxidation and which reaction is the reduction?
The 1st reaction proceeds as-is, with the electrons being lost. Therefore the 1st reaction is an oxidation.
The 2nd reaction is written as an oxidation, but proceeds in the reverse direction, therefore it is a reduction.
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CH4(g) + 2O2(g) CO2(g) + 2H2O(g)
If 0.10 mol of methane is reacted with 0.10 mol oxygen gas, how many moles of CO2
can form?
1. 0.050 mol2. 0.10 mol3. 0.20 mol4. 0.15 mol
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CH4(g) + 2O2(g) CO2(g) + 2H2O(g)If 0.10 mol of methane is reacted with 0.10
mol oxygen gas, how many moles of CO2 can form?
Limiting reagent problem: calculate the moles of CO2 can form from both 0.10 mol CH4 and 0.10 mole O2. The lower number is the correct answer.
0.10 mol CH4 1 mol CO2 = 0.10 moles CO2
1 mol CH4
0.10 mol O2 1 mol CO2 = 0.050 moles CO2
2 mol O2