Chapter 4 Questions in Class PDF

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Net Ionic Equation: An Example NaOH + MgCl2 à฀ ? Decide whether a reaction occurs when the above reagents are mixed. If it does, write the molecular, ionic, and net ionic equations. First, by exchanging the reactant ions, determine the product formulas: The formula of the product containing Na is: (a)  NaCl (b)  NaCl2

The formula of the product containing Mg is: (a)  MgOH (b)  Mg(OH)2

Next, determine whether the products are soluble: Is NaCl soluble?

NaCl is soluble

Is Mg(OH)2 soluble?

Mg(OH)2 is insoluble

Since Mg(OH)2 is insoluble, a precipitate will be observed and a reaction will occur.

Net Ionic Equation: An Example NaOH + MgCl2 à฀ ? Decide whether a reaction occurs when the above reagents are mixed. A reaction does occur! If it does, write the molecular, ionic, and net ionic equations. First, write the balanced molecular equation, including state symbols. molecular equation: NaOH(aq) + MgCl2(aq) à฀ NaCl(aq) + Mg(OH)2(s)

Is it balanced? 2NaOH(aq) + MgCl2(aq) à฀ 2NaCl(aq) + Mg(OH)2(s)

Net Ionic Equation: An Example NaOH + MgCl2 à฀ ? Decide whether a reaction occurs when the above reagents are mixed. A reaction does occur! If it does, write the molecular, ionic, and net ionic equations. First, write the balanced molecular equation, including state symbols. molecular equation: 2NaOH(aq) + MgCl2(aq) à฀ 2NaCl(aq) + Mg(OH)2(s) Next, write the complete ionic equation, including state symbols. ionic equation: 2Na+(aq) + 2OH-(aq) + Mg2+(aq) + 2Cl-(aq) à฀ 2Na+(aq) + 2Cl-(aq) + Mg(OH)2(s) Finally, write the net ionic equation, including state symbols. net ionic equation: 2OH-(aq) + Mg2+(aq) à฀ Mg(OH)2(s)

Net Ionic Equation: An Example KBr + MgSO4 → ? Decide whether a reaction occurs when the above reagents are mixed. If it does, write the molecular, ionic, and net ionic equations. First by exchanging the reactant ions, determine the product formulas: The formula of the product containing K is: (a)  KSO4 (b)  K2SO4

The formula of the product containing Mg is: (a)  MgBr (b)  MgBr2

K+ and SO42- could make K2SO4 Mg2+ and Br- could make MgBr2 Next, determine whether the products are soluble: K2SO4 is soluble MgBr2 is soluble KBr + MgSO4 → no reaction There is no reaction, because the mixture ions (K+, Mg2+, Br- and SO42-) are soluble. A reaction would require the formation of a precipitate.

Reaction of a Zinc Bar with Copper Ions Zn(s) + CuSO4(aq)

ZnSO4(aq) + Cu(s)

A zinc bar is placed in an aqueous solution of CuSO4.

What is the oxidation state of Cu in CuSO4? (a)  Cu2+ (b)  Cu+ (c)  Cu3+ What is the oxidation state of Zn in ZnSO4? (a)  Zn2+ (b)  Zn+ (c)  Zn3+

Reaction of a Zinc Bar with Copper Ions Zn(s) + CuSO4(aq)

ZnSO4(aq) + Cu(s) A zinc bar is placed in an aqueous solution of CuSO4. • Zn atoms enter the solution as Zn2+ ions. • Cu2+ ions are converted to Cu atoms.

Zn Zn2+ + 2eZn is oxidized Zn is the reducing agent Cu2+ + 2eCu Cu2+ is reduced Cu2+ is the oxidizing agent Zn + Cu2+

Zn2+ + Cu

The zinc bar becomes copper plated.

Oxidation Numbers: Examples What are the oxidation numbers of all the elements in IF7 (iodine hepafluoride)? What is the oxidation state of fluorine? F = -1

IF7 What is the oxidation state of iodine?

Fluorine likes to form which oxidation state? (a)  F2(b)  F-

but, the sum of oxidation numbers on F

7 x (-1) = -7

? + (-7) = 0 I = +7 Also known as iodine(VII) fluoride

Iodine heptafluoride

Oxidation Numbers: Examples What are the oxidation numbers of all the elements in NaIO3 (sodium iodate)? What is the oxidation state of sodium?

What is the oxidation state of oxygen?

Na = +1

O = -2

NaIO3

but, the sum of oxidation numbers on O

3 x (-2) = -6 What is the oxidation state of iodine? 1 + ? + (-6) = 0 I = +5

Na+

Oxidation Numbers: Examples What are the oxidation numbers of all the elements in K2Cr2O7 (potassium dichromate)? What is the oxidation state of potassium?

What is the oxidation state of oxygen?

K = +1 2 x (+1) = +2

O = -2 7 x (-2) = -14

K2Cr2O7 What is the oxidation state of chromium? +2 + [2 x (Cr)] + (-14) = 0 [2 x (Cr)] = 12 Cr = +6

Classify the Following Reactions:

Ca2+ + CO32NH3 + H+

CaCO3 NH4+

Zn + 2HCl

ZnCl2 + H2

Ca + F2

CaF2

Precipitation Acid-Base Redox (H2 Displacement) Redox (Combination)

Choices: Precipitation Acid-Base Oxidation-Reduction (Redox) 1.

Combination reaction

2.

Decomposition reaction

3.

Displacement reaction

4.

Combustion reaction

5.

Disproportionation reaction

A Molarity Problem in Solution Example: A sample of 0.0341 mol iron(III) chloride, FeCl3, was dissolved in water to give a 25.0 mL solution. What is the molarity of the solution?

M = Molarity =

moles of solute liter of solution (L)

=

0.0341 mol 0.0250 L

Another way to think about this is: I have 0.0341 mol FeCl3 in 25 mL How many moles would this be in 1000 mL? 0.0341 mol x 1000 mL = 1.36 M 25 mL

= 1.36 M

A Dilution Problem Example: How would you prepare 60.0 mL of 0.2 M HNO3 from a stock solution of 4.00 M HNO3?

MiVi = MfVf Mi = 4.00 M Vi = ? L Vi =

MfVf Mi

=

Mf = 0.2 M Vf = 0.06 L

0.2 x 0.06 4.00

= 0.003 L = 3 mL

3 mL of acid + 57 mL of water = 60 mL of solution MiVi = 4.00 x 0.003 = 0.012 MfVf = 0.2 x 0.06 = 0.012

Quantitative Analysis: Gravimetric Analysis

Example: Suppose a 1.00 L sample of polluted water was analyzed for lead(II) ion, Pb2+, by adding an excess of sodium sulfate to it. The mass of lead(II) sulfate that precipitated was 229.8 mg. What is the mass (mg) of lead in a liter of the water? Na2SO4(aq) + Pb2+(aq) à฀ 2Na+(aq) + PbSO4(s) 229.8 mg PbSO4 How much Pb?

Quantitative Analysis: Gravimetric Analysis Example: Suppose a 1.00 L sample of polluted water was analyzed for lead(II) ion, Pb2+, by adding an excess of sodium sulfate to it. The mass of lead(II) sulfate that precipitated was 229.8 mg. What is the mass (mg) of lead in a liter of the water? • 

First, we must obtain the % mass of lead in lead(II) sulfate, by dividing the molar mass of lead by the molar mass of PbSO4, then multiplying by 100. 207.2 g/mol Pb % Pb =

x 100 = 68.32 % 303.3 g/mol PbSO4

•

Then, calculate the amount of lead in the PbSO4 precipitated. Amount of Pb in 1L sample: = 229.8 mg PbSO4 x 0.6832 = 157.0 mg Pb

Molar masses: Pb = 207.2 g/mol S = 32.07 g/mol O = 16.00 g/mol PbSO4 = 207.2 + 32.07 + (4 x 16.00) = 303.27 g/mol

A Neutralization Example Milk of Magnesia (MOM): • MOM is the common name of magnesium hydroxide. • MOM tablets are sold as an antacid, because they neutralize stomach acid. • The reaction is: magnesium hydroxide + hydrochloric acid à฀ water + magnesium chloride (solid) (soluble)

Write the chemical equation for this reaction using complete formulas with phase labels. Classify the reaction by its type. What is the formula for magnesium hydroxide? What is the formula for hydrochloric acid? What is the formula for magnesium chloride?

A Neutralization Example Milk of Magnesia (MOM): • MOM is the common name of magnesium hydroxide. • MOM tablets are sold as an antacid, because they neutralize stomach acid. • The reaction is: magnesium hydroxide + hydrochloric acid à฀ water + magnesium chloride (solid) (soluble)

Write the chemical equation for this reaction using complete formulas with phase labels. Classify the reaction by its type. Neutralization Reaction:

Mg(OH)2(s) + 2HCl(aq)

MgCl2(aq) + 2H2O(l)

Mg2+ + 2OH- + 2H+ + 2Cl-

Mg2+ + 2Cl- + 2H2O

2H+ + 2OHi.e. H+ + OH-

2H2O H2O

Volumetric Analysis: Titrations Example: What volume of a 1.420 M NaOH solution is required to titrate 25.00 mL of a 4.50 M H2SO4 solution? Step 1: WRITE THE CHEMICAL EQUATION!

H2SO4(aq) + 2NaOH(aq) à฀ 2H2O(l) + Na2SO4(aq) 4.50 M 25.00 mL

1.420 M ?L

M

volume acid

25.00 mL x

acid

M

rx

moles acid

4.50 mol H2SO4 1000 mL soln

x

coef.

moles base

2 mol NaOH 1 mol H2SO4

x

base

volume base

1000 mL soln 1.420 mol NaOH

= 158 mL

Volumetric Analysis: Titrations Example: What volume of 0.250 M HNO3 (nitric acid) reacts with 42.4 mL of 0.150 M Na2CO3 (sodium carbonate) in the following reaction? 2HNO3(aq) + Na2CO3(aq) à฀ 2NaNO3(aq) + H2O(l) + CO2(g) 0.250 M ?L

0.150 M 0.0424 L

0.0424 L Na2CO3 x

x

0.150 mol Na2CO3 1 L Na2CO3 1 L HNO3 0.250 mol HNO3

x

2 mol HNO3 1 mol Na2CO3

= 0.0509 L HNO3

Net Ionic Equations Magnesium metal reacts with hydrobromic acid to produce hydrogen gas and a solution of magnesium bromide. • •

Write the molecular equation for this reaction. Then write the corresponding complete ionic and net ionic equations.

First, write the balanced molecular equation, including state symbols.

Magnesium + hydrobromic acid → hydrogen gas + magnesium bromide The balanced reaction is: (a) 

Mg(s) + HBr(aq) → H2(g) + MgBr2(aq)

(b) 

Mg(s) + 2HBr(aq) → H2(g) + MgBr2(aq)

Net Ionic Equations Magnesium metal reacts with hydrobromic acid to produce hydrogen gas and a solution of magnesium bromide. • •

Write the molecular equation for this reaction. Then write the corresponding complete ionic and net ionic equations.

First, write the balanced molecular equation, including state symbols. molecular equation:

Mg(s) + 2HBr(aq) à฀ H2(g) + MgBr2(aq) Next, write the complete ionic equation, including state symbols. ionic equation:

Mg(s) + 2H+(aq) + 2Br-(aq) à฀ H2(g) + Mg2+ (aq) + 2Br- (aq) Finally, write the net ionic equation, including state symbols. Ions that occur in both the reactants and products, but do not participate in the chemical reaction are referred to as spectators. net ionic equation:

Mg(s) + 2H+(aq) à฀ H2(g) + Mg2+ (aq)...