CH 4 Reactions in Aqueous Solutions

CH 4 Reactions in Aqueous Solutions

Types of Equations Used to Describe Reactions in Solution

1. Molecular: overall reaction stoichiometry- not actual forms

MgSO4 (aq) + Na2CO3 (aq) --> MgCO3 (s) + Na2SO4 (aq)

2. Complete Ionic: reactants and products that are strong electrolytes are represented as ions.

Mg2+ + SO42- + 2Na+ + CO3

2- --> MgCO3 (s) + 2Na+ + CO32-

3. Net Ionic: includes only those solution components undergoing a change. Spectator ions not included.

Mg2+ + CO32- --> MgCO3 (s)

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Types of Chemical Reactions Combination (Synthesis) reaction A + B ABDecomposition reactions AB A + BDisplacement reactions AB + C AC + BMetathetical (change of position)

reactions (double-replacement reactions)

AB + CD AD + CBCombustion reactions reactions with oxygen CxHy + nO2 xCO2 + (y/2) H2O04/21/23 3

Combination Reactions (Synthesis): A + B → C

Metals + Oxygen:– Lithium + oxygen →– Magnesium + oxygen →– Gold + oxygen →– Platinum + oxygen →

Remember the diatomicsMetals with multiple charges: choose the one with higher charge; Cu+2 and not Cu+1

Nonmetals + Oxygen (Redox?)Excess carbon with oxygen →Limited amount of carbon with excess of oxygen →Phosphorus + excess oxygen →Phosphorus with limited amount of oxygen →

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Metals + nonmetals (Redox?)Cesium metal + iodine →Zinc + sulfur →Magnesium + nitrogen →

Metal Oxides (most are solid) + Water: (Redox?)

Magnesium oxide + water →Lithium oxide + water →Aluminum oxide + water → Iron(III) oxide + water →

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Nonmetal Oxides + Water : (Redox?)

solid calcium oxide + water →solid lithium oxide + water →

Can be Redox: 2NO2(g) + H2O (l) → HNO3 (aq) + HNO2(aq)

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Metal Oxides + Nonmetal Oxides (Redox?)calcium oxide + silicon dioxide →lithium oxide + tetra phosphorus deca oxide →

Notes: The more electropositive (most metallic) element is always

written firstP4O10; CaO; H2O, CO2

Check Periodic Table

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Decomposition Reactions : C → A + B Reverse of combination (synthesis)

Metallic oxides metal + oxygen

Nonmetallic oxides nonmetal + oxygen

Hydroxide metal oxide + water

Acid nonmetallic oxide + water

Which are Redox and which are not?

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Decomposition Reactions (Special Cases)

Metal carbonates metallic oxide + CO2

Metal bicarbonates: metal oxide + CO2(g) + H2O (l)

Metal sulfite metallic oxide + SO2

Metal chlorate metal chloride + oxygen (O2)

Binary compounds elements

Electrolysis of molten salts (ionic compounds) elements

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Decomposition Reactions : (Special Cases)

Decomposition of peroxides: peroxide water + oxygen (O2)

Ammonium compounds acid + ammonia; the acid may decompose

(NH4)2CO3 (s) 2NH3(g) + CO2(g) + H2O(l)

NH4NO2 (s) N2(g) + 2H2O (l)

NH4NO3(s) N2O (g) + 2H2O(l)

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Types of Equations Used to Describe Reactions in Solution

1. Molecular: overall reaction stoichiometry- not actual forms

MgSO4 (aq) + Na2CO3 (aq) --> MgCO3 (s) + Na2SO4 (aq)

2. Complete Ionic: reactants and products that are strong electrolytes are represented as ions.

Mg2+ + SO42- + 2Na+ + CO3

2- --> MgCO3 (s) + 2Na+ + CO32-

3. Net Ionic: includes only those solution components undergoing a change. Spectator ions not included.

Mg2+ + CO32- --> MgCO3 (s)

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Writing Equations

Write a balanced molecular, ionic and net ionic equations for the following reactions:

1. Solution of silver nitrate was added to a solution of sodium chromate

2. A piece of solid zinc was placed in a solution of Copper(II) chloride

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Single Replacement or Displacement Reactions

A0 + B+C- A+C- + B- ( metals)

A0 + B+C- B+A- + C0 (halogens)

All are Redox

Active metal replaces less active metalActive metal replaces H in water or acidsNonmetal replaces less active nonmetal

Activity series – used to predict RxStandard Reduction Potential Chart and SHE

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Single Replacement Reactions If a < reactive element is combined with a > reactive element in compound form → no Rx

1. Zinc metal reacts with copper (II) sulfate in water solutionMolecular equation:Net Ionic equation :

Redox?

2. zinc metal reacts with hydrochloric acid

3. aluminum metal reacts with sulfuric acid

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Single Replacement Reactions

Write formula and net ionic equations:

sodium metal reacts with cold a water

aluminum reacts with steam

magnesium reacts with hot water

Which metals will replace hydrogen from cold water?

Which metals will replace hydrogen from hot water?

Which metals will replace hydrogen from steam?

Activity series of metals:

http://www.chem.vt.edu/RVGS/ACT/notes/activity_series.html04/21/23 15

http://www.chem.vt.edu/RVGS/ACT/notes/activity_series.html

Single Replacement Reactions: Halogen Displacement

Write molecular and net ionic equations:

Chlorine gas reacts with aqueous solution with sodium bromide

Activity series:F2 > Cl2 > Br2 > I2

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Double Replacement Reactions orMetathetical Reactions

A+B- + C+D- A+ D- + C+B-

Reactions occur to completion when:Precipitate is producedGas is producedMolecular substance such as H2O, CO2, NH3,

SO2 are produced

Redox or NonRedox ?04/21/23 17


Write the molecular complete ionic

net ionic forms

Aqueous nickel (II) chloride reacts with aqueous sodium hydroxide

Aqueous sodium sulfide reacts with lead (II) nitrate

Aqueous potassium carbonate reacts with barium chloride

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Predict whether a reaction will occur in each of the following case. If so, write a net ionic equation for the reaction. If no reaction occurs, write NR after arrow.

Al2(SO4)3 + NaOH

K2SO4(aq)+FeBr3(aq)

CdCl2(aq) + (NH4)2S(aq)

Double Replacement: Gas FormationCommon gases formed in DR Rx

S2- + acid → H2S (g)+ salt

CO32- + acid → CO2 (g)+ H2O + salt

SO3- + acid → SO2 (g)+ H2O + salt

NH4+ + OH- + Δ → NH3 g) + H2O + salt

1. Sodium carbonate reacts with hydrochloric acid2. Ammonium chloride reacts with sodium hydroxide3. Magnesium nitride reacts with water 4. Calcium sulfite reacts with hydrobromic acid5. Sodium chloride + sulfuric acid6. Sodium sulfide reacts with hydrochloric acid

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Selective Precipitation

Precipitation reactions allow us to target specific substances, and separate and recover them from a solution.

Example:A solution contains Ca2+, Cu2+, and Pb2+. What

anions can we add, and in what order , to separate and recover each cation?

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Combustion

Write the products and balance the following combustion reaction:

C6H12O6 (s) + O2 →C3H8O3 + O2 →CH3OH + O2

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Acids and Bases: Arrhenius

Acid– Any substance that releases H+ ion in aqueous

solution

Base– Any substance that releases OH- ion in aqueous

solution

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Brønsted-Lowery Acid-Base Definitions

An acid is a substance that donates a proton (H+) to a base

A base is a substance that accepts a proton (H+) from an acid

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conjugate base

conjugate. acid

Brønsted-Lowery Acid-Base Definitions

An acid is a substance that donates a proton (H+) to a base

A base is a substance that accepts a proton (H+) from an acid

Acid-base reactions can be reversible:reactants products or products reactants

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Conjugate acid: ____________

Conjugate base: _________________

Compounds that act as Brönsted Acid and Base

Write equations for the following reactions. Identify the acid, base, conjugated acid and conjugated base:

1. HSO4-(aq) + H2O(l) →

2. HSO4-(aq) + H2O(l) →

3. H2O(l) + H2O(l) →4. HCO3

-1 (aq) + H2O(l)

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Important Acids and Bases

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Strong Acids:

HCl hydrochloric

HBr hydrobromic

HI hydroiodic

HNO3 nitric

H2SO4 sulfuric

HClO4 perchloric

Weak Acid:

CH3CO2H acetic

Strong Bases:

NaOH sodium hydroxide

KOH potassium hydroxide

Ca(OH)2 calcium hydroxide

Weak Base:

NH3 ammonia

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Know the strong Know the strong acids & bases!acids & bases!

Ca(OH)2, Ba(OH)2 and Sr(OH)2

Group IIA, heavy metals)

Ca(OH)2, Ba(OH)2 and Sr(OH)2

Group IIA, heavy metals)

3.2

STRONG acids in water: 100% of acid molecules form ions:

HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq)

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H3O+ is

hydronium ion

WEAK acids in water: ~5% or less of acid molecules form ions

(acetic, H3PO4, H2CO3)

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Polyprotic Acids: multiple acidic H atoms

H2SO4 H+ + HSO4-

HSO4- H+ + SO4

2-

Not all H’s are acidic: CH3CO2H

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If H3PO4 reacts as an acid, which of the following can it not make?

1. H4PO4+

2. H2PO4-

3. HPO42-

4. PO43-

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Reactions Involving Weak Bases

HCl(aq) + NH3(aq) NH4+(aq) + Cl-(aq)

Net-Ionic Equation:

NH3(aq) + H+(aq) NH4+(aq)

Spectator Ion?

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Acid-Base Reactions: Neutralization

The “driving force” is the formation of water.The “driving force” is the formation of water.NaOH(aq) + HCl(aq) NaOH(aq) + HCl(aq) → → NaCl(aq) + HNaCl(aq) + H22O(liq)O(liq)

Net ionic equationNet ionic equation OHOH--(aq) + H(aq) + H++(aq) (aq) →→ H H22O(liq)O(liq)

““Spectator Ions”? __________________________Spectator Ions”? __________________________

This applies to ALL reactions of STRONG acids This applies to ALL reactions of STRONG acids

and bases.and bases.

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Acid-Base Neutralization RxPolyprotic acids

H2SO4 (SA)or H3PO4 (WA)

H2SO4 : First H+ is ionized completely

H2SO4 → H+ + HSO4 -

1. If base is excess: all H+ form H2O

2. If equimolar acid + base: only 1 H+ ionizes3. Acidic anhydrides (NMO) + Basic anhydrides (MO) :

react with H2O before acid or base

CH3CO2H(aq) + NaOH(aq)

Choose the correct answer:1. CH3CO2H2

+(aq) + NaO(aq)2. CH3CO2

-(aq) + H2O(l) + Na+(aq)3. CH4(g) + CO2(g) + H2O(l)

Complete Ionic equation:

CH3COOH(aq) + OH-(aq) → CH3COO-(aq) + H2O(l)

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HCN(aq) + NH3(aq)

Answer?1. NH4

+(aq) + CN-(aq)

2. H2CN+(aq) + NH2-(aq)

3. C2N2(s) + 3 H2(g)

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Hydrolysis Rx- Reverse NeutralizationSalt + H2O → molecular species

Formation of a weak acid and/or weak baseNH4

+ + Cl- + H2O → H+ + Cl- + NH4OH

NH4Cl : salt from SA (HCl) + WB (NH3 )

Forms acidic solution due to: NH4+> OH-

*Salts of SA + WB → Acidic Solution*Salts of SA + WB → Basic Solution

*Salts of SA + SB → Neutral Solution*Salts of WA + SB → ?? Check Ka and Kb

1. Aqueous potassium fluoride undergoes hydrolysis when placed in water.

2. Sodium chloride and water are mixed together.

3. Ammonium fluoride and water are mixed together.

Oxidation-Reduction Reactions

Redox reactions: involve a transfer of electrons.

Assigning oxidation states to an element in a molecule:

K2CrO4 LiSCN

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LEO GER: Leo the Lion Says GEROIL RIG

Loss of Electrons is OxidationGain of Electrons is Reduction

Oxidation Involves LossReduction Involves Gain

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Determination of Oxidation States

Fe2O3 + 2Al → Al2O3 + 2Fe

Iron (III) gains 3 electrons to become elemental iron.

Elemental aluminum lost 3 electrons to become the aluminum ion.

Write the half reactions:

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N2 H4 + N2O4 → N2 + H2O

The combustion of hydrazine with dinitrogen tetroxide helps to keep the space shuttle in Earth Orbit.

Is it a Redox reaction? Explain.

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Fe2O3 + 2Al → Al2O3 + 2Fe

Iron (III) ion gained electrons. It has been reduced.

The aluminum lost electrons. It has been oxidized.

The oxidizing agent is the species that is reduced (Iron (III)).

The reducing agent is the species that is oxidized (aluminum).

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Rules for Assigning Oxidation States (OS)1. OS of an atom in an element is 0.

Na (s), O2 (g)2. OS of a monatomic ion is the same as its charge.

Na+ OS = +1, Cl- OS = -13. In its covalent compounds with nonmetals, hydrogen is

assigned an OS of +1. HCl, NH3, H2O.

4. Oxygen is assigned an OS of -2 in its covalent compounds.

CO, CO2, SO2, SO3

The exception to this rules occurs in peroxides (compounds contains the O2

2- group), where each oxygen is assigned an OS of -1.

H2O204/21/23 46

5. In binary compounds the element with the greater attraction for the electrons in the bond is assigned a negative OS equal to its charge in its ionic compounds.

HF, NH3, H2S, HI

6. The sum of the oxidation states must be zero for an electrically neutral compound and must be equal to the overall charge for an ionic species.

NH4+, CO3

2-

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Which Atoms Undergo Redox?

2H2 (g) + O2 (g) → 2H2O (g)

Zn (s) + Cu2+(aq) → Zn2+ (aq) + Cu(s)

2AgCl (s) + H2 (g) → 2H+ (aq) + 2Ag(s) + 2Cl- (aq)

2MnO4- (aq) + 16H+ (aq) + 5C2O4

2- (aq) → 2Mn2+(aq) + 10 CO2 (g) + 8 H2O (l)

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Methods for Balancing Redox Reactions

1. Oxidation states methodCdS + I2 + HCl → CdCl2 + HI + S

(1, 1, 2, 1, 2, 1)Cl2 + Ca(OH)2 → CaCl2 + Ca(ClO3) + H2O

( 6, 6, 5, 1, 6)

2. Half reaction method

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Balancing a Redox Equation by the Oxidation States Method

1. Assign the oxidation states of all atoms.2. Decide which element is oxidized and determine the

increase in oxidation state.3. Decide which element is reduced and determine the

decrease in oxidation state.4. Choose coefficients for the species containing the atom

oxidized and the atom reduced such that the total increase in oxidation state equals the total decrease in oxidation state.

5. Balance the remainder of the equation by inspection.

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The Half-Reaction or Ion-electron Method for Balancing Redox Reactions in Acidic Solutions

1. Split Rx into Half Reactions 2 parts of a REDOX RX- pick 1 to start then repeat oxidation - RA

reduction – OA2.Balance # of atoms first– not O or H3.Balance O by + H2O to side deficient in O

4.+ H+ to balance H5.Mass balance achieved

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6. Balance charges with e-7. Be sure # e- lost = #e- gained8. Cancel common terms on opposite side of →9. Σ the half Rx10. Check to make sure the Rx balances

Balance the equation in acid solution using the half reaction method

Cu(s) + HNO3 (aq) --> Cu2+ (aq) + NO(g)

The Half-Reaction or Ion-electron Method for Balancing Redox Reactions in Basic Solutions

1. Follow the same procedure as in acidic solutions2. +OH- to both sides to cancel out the H+

3. Make H2O from the OH- and H+

4. Cancel out H2O that is common to both sides

5. Be sure #e-lost = #e- gained6. Cancel common terms on opposite side of →7. Σ the half Rx8. Check to make sure the Rx balances

Balancing by Half Reaction Method

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Balance in acidic environment:

Cr2O72- (aq) + NO (g) → Cr +3 (aq) + NO3

- (aq)

Balance in basic environment:

Cr2O72- (aq) + NO (g) → Cr +3 (aq) + NO3

- (aq)

Cr2O72-(aq) + 2NO (g) + 6H+ (aq) → 2Cr3+ (aq) + 2NO3- (aq) + 3H2O (l)

Cr2O72-(aq) + 2NO (g) + 3 H2O --> 2Cr3+ (aq) + 2NO3- (aq) + 6OH- (aq)

Predicting Types of Redox RxSimple Redox

Hydrogen displacement Metal displacementHalogen displacement Combustion

DecompositionOxoanions

Table of Common Oxidizing and Reducing Agents

Atypical Redox Rx

1. Hydrogen + MO (hot) → M + HOH

2. MS + O2 → MO + SO2

3. Cl2 (g) + NaOH(dilute) → NaClO + NaCl + HOH

4. Cu + H2SO4 (conc) → CuSO4 + SO2 + HOH

5. Cu + HNO3 (dilute) → CuNO3 + NO + HOH

6. Cu + HNO3 (conc) → CuNO3 + NO2 + HOH

Disproportionation Reactions

Simultaneous oxidation and reduction of one species.

3NO2 + H2O → 2H+ + 2NO3- + NO

2H2O2(aq) → 2H2O(l) + O2(g)

Cl2(g) + 2OH-(aq) → ClO-(aq) + Cl-(aq) + H2O(l)

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CH 4 Reactions in Aqueous Solutions