Chapter 4 - Chemical Reactions

A 0.784 g sample of Mg is added to a 250 mL flask & dissolved in 150 mL water. Magnesium Hydroxide obtained from the reaction required 215.0 mL of 0.300 M Hydrochloric acid to completely react. How many moles of Mg were used?

• find moles HCl

  • M = n/V = moles/Volume

  • 0.300 M HCl = moles HCl/215.0 mL HCl

  • mol HCl = 0.0645

• chemical equation for reaction

  • Mg + 2HCl → MgCl2 + H2

  • mol HCl = 0.645 mol HCl * 1 mol Mg/2 mol HCl = 0.0323 mol Mg

A sample of an oxide of an unknown metal R contains 46.0 g R & 16.0 g O. If the formula of the metal oxide is R2O, what is the Molar Mass of R?

Molar Mass = mass/moles

Molar Mass R = 2*Atomic mass R + Atomic mass O

Molar Mass R = 2*Atomic mass R + 16.0 g/mol

total mass R2O = 46.0 + 16 = 62.0 g/mol

atomic mass R = total - O

Molar Mass R = 2(62.0 g/mol - 16.0 g/mol) + 16.0 g/mol = 23.0 g/mol

solubility

how much of a molecule (especially ionic molecules) will dissolve in water varies greatly

molarity

how much “stuff” is dissolved in water; M = moles/L

19.4 g Potassium Chromate dissolved in 1 L, find the molarity & [K+]

• 19.4 g K2CrO4 × 1 mol/194.20 g K2CrO4 = 0.100 moles

• M = 0.100 mol/1 L = 0.1 M

• 0.100 mol K2CrO4 × 2 mol K+ / 1 mol K2CrO4 = 0.200 mol K+

• M = 0.200 mol K+/1 L = 0.2M

How much 0.100 M CaCl2 solution and water must be mixed together to make 2.00 L of 0.0250 M Cl- solution?

goal: 2.00 L * 0.0250 mol/L = 0.05 mol Cl-

start: 0.100 mol/L CaCl2 × 2 Cl-/1CaCl2 = 0.200 mol/L Cl-

divide moles in target by moles in the start: 0.05 mol Cl-/(0.200 mol/L Cl-) = 0.25 L of original CCl2 solution + 1.75L H2O to make full 2.00L

0.265 L of 0.1253 M FeCl3. How many mol FeCl3?

0.1253 M = x/0.265 L = 0.0332 mol

How many L do you need to get 1.000 mol FeCl3? M = 0.1253

0.1253 M = 1.000 mol / V

V = 7.981 L

If you need 0.001 mol Ca and you have a 1.023 M Calcium Phosphate solution, what is the minimum volume of Calcium Phosphate solution you must use?

1.023 M Ca3(PO4)2 × 3 Ca^2+/Ca3(PO4)2 = 3.069 M Ca^2+

3.069 mol/L = 0.001 mol Ca^2+/V

V = 3×10^-4 L

You have a 0.13 M sample of Cl-. How much of this stock solution do you need to make 2.5 L of a 0.05 M solution of Cl-?

M1V1 = M2V2

0.13 M * V1 = 0.05 M * 2.5 L

0.13 M * V1 = 0.125 mol Cl-

V1 = 0.962 L

When does M1V1 = M2V2 work without needing other steps?

When the starting and ending solutions are the same

You have a 0.13 M sample of MgCl2. How much of this stock solution do you need to make 2.5 L of a 0.05 M solution of Cl-?

• 0.13 M MgCl2 × 2 Cl-/1 MgCl2 = 0.26 M Cl-

• M1V1 = M2V2

• 0.26 M Cl- * V1 = 0.05 M Cl- * 2.5 L

  • V1 = 0.48 L

3 types of chemical reactions

precipitation, acid-base, and oxidation reduction

precipitation reaction

• solid formed from a clear solution

• double-displacement reactions: compounds split into ions and switch pairs

acid-base reaction

acid and base are mixed to make water

oxidation reduction reactions (redox reactions)

• electrons move

• battery/electronic/energy

most nitrate (NO3-) salts are…

soluble

most salts containing the alkali metal ions (Li+, Na+, K+, Cs+, Rb+) and the ammonium ion (NH4+) are…

soluble

most chloride, bromide, & iodide salts and halogens are…

exceptions: …

soluble

exceptions: Ag+, Pb^2+, & Hg2^2+

most sulfates are…

exceptions: …

soluble

exceptions: BaSO4, PbSO4, Hg2SO4, & CaSO4

most hydroxides…

exceptions: …

form a solid

exceptions: NaOH, KOH, first row

sulfide, carbonate, chromate, and phosphate salts…

form a solid

metals…

like to be solid

How much 0.100 M CaCl2 solution and water must be mixed together to make 2.00 L of 0.0250 M Cl- solution?

0.100 M CaCl2 × 2 mol Cl-/1 mol CaCl2

0.200 M Cl- * V1 = 0.0250 M * 2.00 L

V1 = 0.25 L

0.265 L of 0.1253 M FeCl3. How many mol FeCl3?

0.1253 M = x/0.265 L = 0.0332 mol

How many L do you need to get 1.000 mol FeCl3? M = .1253

0.1253 M = 1.000 mol / V

V = 7.981 L

If you need 0.001 mol Ca and you have a 1.023 M Calcium Phosphate solution, what is the minimum volume of Calcium Phosphate solution you must use?

1.023 M Ca3(PO4)2 × 3 Ca^2+/Ca3(PO4)2 = 3.069 M Ca^2+

3.069 mol/L = 0.001 mol Ca^2+/V

V = 3×10^-4 L

What is the precipitate? HgCl2 + KI

HgI2(s) + 2KCl(aq)

HgI2 is the precipitate

When does M1V1 = M2V2 work without needing other steps?

When the starting and ending solutions are the same

precipitate reaction example: water softener

substances dissolved in water (Ca²+) react with phosphate tank to form a solid to be flushed out

acid base reactions (neutralization reactions)

• H+ + OH- → H2O

• products: water and an ionic complex (salt)

What is the precipitate? BaCl2 + Na2SO4.

BaSO4

What is the precipitate? CuCl2 + Na2CO3

CuCO3

precipitation reaction

• solid formed from a clear solution

• double-displacement reactions: compounds split into ions and switch pairs

Brønsted-lowry acid base

• acid gives H+

• base takes H+ (often OH-)

strong acids

• HBr, HCl, HI, HNO3, HClO4, H2SO4

• HBr → H+ + Br-

• split into ions completely when dissolved

strong bases

• Group I or Group II with OH-, NaOH or Ba(OH)2

• split into ions completely when dissolved

• NaOH(s) → Na+ (aq) + OH- (aq)

weak base

everything but strong acids and group 1 & group 2 with OH-

• Ammonia (NH3)

complete and net ionic equation of HCl (aq) + NaOH (s)

• strong acid - strong base

• complete: H+ (aq) + Cl- (aq) + Na+ (aq) + OH- (aq) → H2O (l) + Cl- (aq) + Na+ (aq)

• net: H+ (aq) + OH- (aq) → H2O (l)

equation of HF + NaOH

• weak acid - strong base

• weak acid: HF (aq) <=> H+ (aq) + F-

  strong base: H+ (aq) + OH (aq) → H2O (l)

• H+ (aq) + OH- (aq) → F- (aq) + H2O (l)

equation of NH3 + H+

• weak base - strong acid

• weak base: NH3 (aq) + H2O (l) <=> NH4+ (aq) + OH- (aq)

  strong acid: H+ (aq) + OH- (aq) → H2O (l)

• NH3 (aq) + H+ (aq) → NH4+

difference between acid and bases

• acid: something that starts with H

• base: second reactant with OH-

• products: water and an ionic complex

oxidation number rules

1. multi-atom molecules: overall oxidation number is the charge of the molecule

   • Fe³+ = oxidation number +3

   • ClO- = -1

2. any element in elemental form = 0

3. any element in a monatomic ion = charge on ion (Fe example above)

4. Group 1 = +1, Group 2 = + 2

5. Oxygen usually = -

oxidation reduction reaction

• redox reaction

• electrons move from one species to another

if an ion is oxidized it...

loses electrons

if an ion is reduced it…

gains electrons

Fe²+ (aq) → Fe³+ (aq) + e^-

Was Fe oxidized or reduced

lost an electron, oxidized

Fe³+ (aq) + e^- → Fe²+ (aq)

Was Fe oxidized or reduced

gained an electron, reduced

Zn (s) + 2H+ (aq) → Zn²+ (aq) + H2 (g)

What is oxidized and what is reduced? What is the oxidizing agent vs reducing agent?

Zinc: loses electrons, oxidized, reducing agent

H: gains electrons, reduced, oxidizing agent

oxidation number rules

1. multi-atom molecules: overall oxidation number is the charge of the molecule

   • Fe³+ = oxidation number +3

   • ClO- = -1

2. any element in elemental form = 0

3. any element in a monatomic ion = charge on ion (Fe example above)

4. Group 1 = +1, Group 2 = + 2

5. Oxygen usually = -2, Hydrogen usually = +1

6. Oxidation numbers sum to 0 or whatever the charge of the poly atomic ion is

Find the oxidation number for Cl: ClO4-

Oxygen = -2, Cl = ?

Cl + 4(-2) = -1

Cl = +5

What is the oxidation number for Cl? ClO3-

Cl + 3(-2) = -1

Cl = +5

What is the oxidation number for N? NH3

3(1) + N = 0

N = -3

What is the oxidation number for N? NO3^-

N + 3(-2) = -1

N = +5

Oxidation numbers in H2O

H = +1, O = -2

NO3^- (aq) → NO (g)

Balance this redox reaction

1. balance non-oxygen (redox) atoms

   • NO3^- (aq) → NO (g)

2. balance oxygen (add water)

   • NO3^- (aq) → NO (g) + 2H2O

3. balance protons (add H+ to reactants)

   • NO3^- (aq) + 4H+ → NO (g) + 2H2O

4. add up charge numbers on left and right sides

   • left: -1 + 4(+1) = +3

   • right: 0 + 0 = 0

5. balance charges: add electrons to appropriate side

   • NO3^- (aq) + 4H+ + 3e-→ NO (g) + 2H2O

HCl (aq) + HNO3 (aq) → NO2 (g) + 2H2O

Balance these redox half reactions

• HCl → Cl2

  1. 2HCl → Cl2

  2. 2HCl → Cl2

  3. 2HCl → Cl2 + 2H+

  4. 2HCl: 0, Cl2 + 2H+: 0+2 = +2

  5. 2HCl → Cl2 + 2H+ + 2e^-

• HNO3 → NO2

  1. HNO3 → NO2

  2. HNO3 → NO2 + H2O

  3. H+ + HNO3 → NO2 + H2O

  4. H+ + HNO3: +1, NO2 + H2O: 0

  5. e- + H+ + HNO3 → NO2 + H2O

• combine half reactions to get complete equation

  6. cancel out electrons: (e- + H+ + HNO3 → NO2 + H2O)2 to match 2e- in first eq.

  7. 2HCl + 2e- + 2H+ + 2HNO3 → Cl2 + 2H+ + 2e^- + 2NO2 + 2H2O

  8. 2HCl + 2HNO3 → Cl2 + 2NO2 + 2H2O

oxidation numbers in SO4²-

S + 4(-2) = -2

S - 8 = 2

S = +6

oxidation numbers CO3²-

C + 3(-2) = -2

C = +4

oxidation numbers CH4

C + 4(+1) = 0

C = -4

Cr2O7²- oxidation numbers

2 Cr + 7(-2) = -2

2Cr - 14 = -2

2Cr = +12

Cr = +6

H2O2 oxidation numbers

2(+1) + 2(-2)) dne 0

oxygen changes before hydrogen

2(+1) + 2O = 0

Oxygen = -1

equation for the combustion of methane

? CH4 + ? O2 → ? CO2 + ? H2O

1 CH4 + 2CO2 → 1CO2 + 2H2O

Balance SO2 + O2 → SO3

2SO2 + O2 → 2SO3

Is BaSO4 soluble or insoluble?

insoluble because although sulfates are soluble, barium sulfate is not

is CuCl2 soluble or insoluble?

soluble because chlorides are soluble unless they are Ag, Pb(II), or Hg(I)

is KNO3 soluble or insoluble?

soluble because K is an alkali metal and NO3 is nitrite, both soluble things

is AgI soluble or insoluble?

insoluble because Ag is insoluble

is NiCO3 soluble or insoluble?

insoluble because carbonates are insoluble unless they are with group I or NH4

Is Pb(NO3)2 soluble or insoluble

soluble because it is a nitrate

AgNO3(aq) + NaCl(aq) → ?

AgNO3(aq) + NaCl(aq) → NaNO3(aq) + AgCl(s)

K3PO4(aq) + MgCl2(aq) → ?

K3PO4(aq) + MgCl2(aq) → Mg(PO4)2(s) + KCl(aq)

2K3PO4(aq) + 3MgCl2(aq) → Mg(PO4)2(s) + 6KCl(aq)

H2(g) and O2(g) are combined to form H2O. What is the oxidizing agent and what is the reducing agent? What is the number of electrons transferred?

2H2(g) + O2(g) = 2H2O(l)

oxidizing agent: is reduced, gains electrons

oxidizing agent: oxygen (goes from 0 to -4)

reducing agent: is oxidized, loses electrons

reducing agent: hydrogen (goes from 0 to +4)

4 electrons were transferred

acid, base, or salt: HI

acid

acid, base, or salt: KNO3

salt (no OH or H)

acid, base, or salt: NH4Cl

salt

acid, base, or salt: BaF2

salt

acid, base, or salt: LiOH

base

strong/weak/non electrolyte differences

strong electrolyte: strong acids, strong bases, soluble ionic compounds; dissociates fully & conducts electricity

weak electrolyte: weak acids, weak bases, insoluble ionic compounds, partially dissociates & does not conduct electricity as well

nonelectrolyte: sugar &alcohol molecules, CHxyz, does not dissociate or conduct electicity

strong/weak/non electrolyte: HCN, Zn(NO3)2, & HNO3

HCN: weak acid, weak electrolyte

Zn(NO3)2: soluble, strong electrolyte

HNO3: strong acid & soluble, strong electrolyte

determine the oxidation state of Zn in ZnSO4

• SO4: 2-

  • O = -2(4) = -8

  • -8 + S = -2

  • S = +6

• Zn + 6 - 8

• or Zn - 2

• Zn = +2

Complete the combustion of heptane: C7H16

C7H16 + O2 = CO2 + H2O

C7H16 + 11O2 → 7CO2 + H2O

Rank these in order of best oxidizing agent to best reducing agent: Mg²+, Ni²+, Fe²+, Ag+

• best oxidizing agent: reduced, more likely to gain electrons

• best reducing agent: oxidized, more likely to lose electrons

• Mg²+ = likely to gain = best reducing agent, further left

  Fe²+

  Ni²+

  Ag+ best oxidizing agent

What is the difference between the following types of reactions?

• combination

• decomposition

• combustions

• precipitation

• acid/base

• redox

• combination: two or more substances combine to form a single substance

  • A + B → AB

• decomposition: one compound breaks down into two or more simpler substances

  • AB → A + B

• combustions: one substance reacts with oxygen gas, releasing energy in the form of light and heat

  • when O2 (g) combusts with H2 (g), it forms water vapor H2O(g)

  • when O2 reacts with a hydrocarbon (C3H8 etc) it forms CO2 and H2O

  • marshmallow roasting on a stick

• precipitation: cations and anions form a solid in an aqueous solution

• acid/base:

  • there will be proton movement

  • acid + base = salt + water

  • NH3(g) + H2O(l) <=> NH4+(aq) + OH-(aq)

• redox: electron movement

Solubility rules

1. Most nitrate (NO3)- salts are soluble

2. Most salts containing the alkali metal ions (Li+, Na+, K+, Cs+, Rb+) and the ammonium ion (NH4+) are soluble

3. Most chloride, bromide, and iodide salts are soluble; most halogens are soluble

   • exceptions: Ag+, Pb^2+, Hg2²+

4. Most sulfates are soluble

   • exceptions: BaSO4, PbSO4, Hg2SO4, CaSO4

5. Most hydroxides are insoluble and form a solid

   • exceptions: NaOH, KOH, first row

6. Sulfide, carbonate, chromate, and phosphate salts are insoluble and form a solid

7. Metals like to be solid

Most nitrate salts are…

(NO3-) Soluble

Most salts containing the alkali metal ions are…

(Li+, K+, Rb+, Cs+, Na+) Soluble

The Ammonium ion is…

(NH4+) Soluble

Most chloride, bromide, and iodide salts are…

This is because most _________ are…

Soluble

Most halogens are soluble

Exceptions to the chloride, bromide, and iodide solubility rule

Pb²+, Ag+, Hg2²+ are insoluble

Most sulfates are…

(SO4²-) Soluble

Exceptions to sulfate solubility rule

BaSO4, PbSO4, Hg2SO4, CaSO4 are insoluble

Most hydroxides are…

Insoluble

Exceptions to the hydroxide solubility rule

NaOH, KOH, first row are soluble

Sulfide, carbonate, chromate, and phosphate are…

Insoluble

Metals typically like to be…

Solids, insoluble