chapter 4 chemical reactions and stoichiometry. representing chemical reactions

Chapter 4Chemical Reactions and

Stoichiometry

Representing Chemical Reactions

• 1st Balance elements that occur in only one compound on each side

• Balance free elements last

• (Balance unchanged polyatomics as groups)

• Clear fractional coefficients by multiplication

• Never introduce extraneous atoms

• Never change chemical formulas

Balancing Chemical Reactions

Equations and the Mole Concept

Reaction Types

Combination Reaction

Decomposition Reaction

Combustion Reaction

What mass of water is produced when 33.2 g ethanol is burned completely?

You are a biologist studying the

mating habits of the Asian Civet cat.

You manage to isolate 62 mg of the Civet cat sex pheromone

Combustion analysis of the pheromone produces:138.0 mg CO2 and 49.4 mg H2O

Combustion Analysis

The Sex Pheromone For The Asian Civet Cat

We are making bagel pizzas (pepperoni, of course). For each pizza, we need 1 bagel, 1 oz. tomato sauce, 2 slices of cheese and 5 slices of pepperoni:

Limiting Reagents

We are making bagel pizzas (pepperoni, of course). For each pizza, we need 1 bagel, 1 oz. tomato sauce, 2 slices of cheese and 5 slices of pepperoni:

Limiting Reagents

We are making bagel pizzas (pepperoni, of course). For each pizza, we need 1 bagel, 1 oz. tomato sauce, 2 slices of cheese and 5 slices of pepperoni:

If we have 13 bagels, 20 oz. of tomato sauce, 26 slices of cheese and 60 slices of pepperoni, how many pizzas can we make?

Limiting Reagents

If we react 85.0 g of Zn metal with 35.2 g of HCl, what is the mass of H2 gas that is produced?

Theoretical Yield: The maximum amount of product that can be produced (usually reported in mol or g)

Actual Yield: Real (measured) amount of product that was produced/obtained (usually reported in mol or g)

Percent Yield: The ratio of actual yield to theoretical yield (reported as a percent, ALWAYS 100% or less)

Percent Yield = Actual Yield

Theoretical YieldX 100%

Actual & Theoretical Yield

We reacted 12.8 g of Al metal with excess HBr and obtained 77.9 g of AlBr3. What is our percent yield?

Molarity (M) = Volume of solution (L) Amount of solute (mol solute)

Units of Concentration: Molarity

Solute - lesser component of a mixture

Solvent - greater component of a mixture

Aqueous - dissolved in water

How many moles of HCl are in 1.00 L of muriatic acid (a.k.a. concentrated aqueous HCl: 37% HCl by mass, density = 1.200 g/mL)?

How many grams of AgNO3 do we need to make 500 ml of a 1.0 M AgNO3 solution?

What volume of a 3.0 M HCl solution is needed to react with 38.4 g of Zn metal?

When NaCl dissolves in water – what happens to the NaCl?

Compounds in Aqueous Solution

It separates into ions

A Strong electrolyte dissociates completely.Soluble ionic compounds – Good electrical conduction.

A Weak electrolyte partially dissociates.Weak (molecular) acids & bases – Fair conductor of

electricity.

A Non-electrolyte does not dissociate. Molecular/Covalent Compounds – Poor (non-)

conductor of electricity.

Compounds in Aqueous Solution

If we dissolve 8.0 g of potassium carbonate in water and the total solution volume is 100 mL, what is the concentration of potassium ions in solution? The concentration of carbonate? Is potassium carbonate a strong electrolyte?

Precipitation Reactions

A solid (the precipitate) forms when two solutions are mixed

Ion Exchange (Metathesis)

AgNO3 (aq) + NaI(aq) AgI(s) + NaNO3 (aq)

Why does a precipitate form when the following solutions are mixed?

What Ionic Compounds Are Soluble In Water?

Alkali metal ion and ammonium ion saltsLi+, Na+, K+, Rb+, Cs+, NH4

+

Nitrates, perchlorates and acetates

NO3- , ClO4

- , CH3CO2-

Chlorides, bromides and iodides Cl-, Br-, I- Except those of Pb+2, Ag+, and Hg2

+2

Sulfates SO4

–2 Except those of Sr+2, Ba+2, Pb+2 and Hg2+2

Ca(SO4) is slightly soluble.

What Ionic Compounds Are Insoluble In Water?

Hydroxides and sulfides HO–, S–2

Except alkali metal (group 1) and ammonium salts

Sulfides of alkaline earth metals (group 2) are soluble

Hydroxides of Sr2+ and Ca2+ are slightly soluble

Carbonates and phosphates CO3

–2, PO4–3

Except alkali metal and ammonium salts

Write The Net Ionic Equation For:

Ca(NO3)2 (aq) + Na3PO4 (aq)

The reaction of lithium carbonate with magnesium sulfide?

Net Ionic Equations

AgNO3 (aq) +NaI(aq) AgI(s) + NaNO3 (aq)

A net ionic equation contains only the ions undergoing a change during the reaction

Overall Equation:

Net Ionic Equations

Ag+ (aq) + I– (aq) AgI (s)

A net ionic equation contains only the ions undergoing a change during the reaction

Net Ionic Equation:

• Acids provide H+ (protons) in aqueous solution (Arrhenius 1884)

• Bases provide OH- in aqueous solution (Arrhenius 1884)

• Bases react to accept or consume H+ (Bronstead 1923)

Acids and Bases

HCl (aq) H+ (aq) + Cl–

(aq)

H2SO4 (aq) H+ (aq) + SO4

–2

(aq) NaOH (aq) Na+

(aq) + OH– (aq)

NH3 + H2O NH4+

(aq) + OH–(aq)

Acids and Bases

H2O ?

Is water an acid, a base, or neither?

• Acids provide H+ (protons) in aqueous solution (Arrhenius 1884)

• Bases provide OH- in aqueous solution (Arrhenius 1884)

• Bases react to accept or consume H+ (Bronstead 1923)

In pure water at 25 °C, the concentration of H+ ions is always 1X10-7 M

Autoionization of Water and pH

H2O H+ (aq) + OH–

(aq)

pH = -log[H+] = -log(1X10-7) = 7

Acids and Bases

Strong acids/bases – Acids and bases that dissociate/react completely in water to yield ions (including H+ or HO–) [Strong bases: hydroxides of group 1 & “heavy” group 2 metals (Ca, Sr, Ba)]

Weak acids/bases - Acids and bases that only partially dissociate/react in water to yield ions (including H+ or HO–)

Acids and Bases

HCl (aq) H+ (aq) + Cl–

(aq)

CH3 CO2H (aq) H+ (aq) + CH3 CO2

– (aq)

Strong acids/bases – Acids and bases that dissociate/react completely in water to yield ions (including H+ or HO–) [Strong bases: hydroxides of group 1 & “heavy” group 2 metals (Ca, Sr, Ba)]

Weak acids/bases - Acids and bases that only partially dissociate/react in water to yield ions (including H+ or HO–)

acetic acid

Weak acids/bases:

NaOH (aq) Na+ (aq) + OH–

(aq)

NH3 + H2O NH4+

(aq) + OH–(aq) ammonia

Strong acids/bases:

Acid/Base Neutralization

H+ (aq) + OH–

(aq) H2O

The reaction of H+ with HO– yields water:

Therefore:

H+ and HO– cannot “co-exist” beyond the amount of water autoionization

Acids and bases will react to consume each other

Acids and Bases

Monoprotic acids yield 1 H+ ion per formula unit: HCl, HClO4, HNO3

Diprotic acids yield 2 H+ ion per formula unit: H2SO4

Triprotic acids yield 3 H+ ion per formula unit: H3PO4

Acids and Bases

Monoprotic acids yield 1 H+ ion per formula unit: HCl, HClO4, HNO3

Diprotic acids yield 2 H+ ion per formula unit: H2SO4

Triprotic acids yield 3 H+ ion per formula unit: H3PO4

200 mL of aqueous 0.1 M H2SO4 is allowed to react completely with 7.4 g of solid NaOH. What ions remain in solution at the end of the reaction? If we assume that the total volume remains constant at 200 mL, what are the concentrations of the remaining ions?

• The above process is known as a titration – the careful addition of one solution to another until one component has exactly consumed another (at the Equivalence Point)

• An indicator is a substance that undergoes an observable (color) change near or at an equivalence point

Acids and Bases

A student carefully adds 0.05 M potassium hydroxide to 50 mL of a hydrobromic acid solution with an unknown concentration. If it takes 17.8 mL of the potassium hydroxide solution to turn the indicator (phenolphthalein) slightly pink, what is the concentration of the hydrobromic acid solution?

Acids and Bases

A student carefully adds 0.05 M potassium hydroxide to 50 mL of a hydrobromic acid solution with an unknown concentration. If it takes 17.8 mL of the potassium hydroxide solution to turn the indicator (phenolphthalein) slightly pink, what is the concentration of the hydrobromic acid solution?

Oxidation/Reduction Reactions (Redox)

In an oxidation-reduction (redox) reaction, some atoms undergo changes in oxidation state:

charges change due to the exchange of electrons

Oxidation State

• Oxidation state describes the charge an atom would possess if all bonds in the compound were ionic

• Oxidation states refer to real charges for ions

• Oxidation states do not refer to real charges in covalent (molecular) compounds

Oxidation state is a term used by chemists to give a general idea of how electron rich/poor an atom is

1) All free elements = 0

2) The sum of all atomic oxidation states in a compound = the total charge on the compound

3) For monoatomic ions, the oxidation state = charge on the ion(Group 1 elements = +1, Group 2 elements = +2, etc.)

4) Fluorine = –1

5) Hydrogen = +1

6) Oxygen = –2

7) Halogens = –1, Group 16 = –2, Group 15 = –3

Determination of Oxidation States

1. H2S

2. S8

3. ClO4–

4. Na2S

5. O3

6. SO4–2

7. Fe2S3

8. ClO2

Determination of Oxidation States

During an Oxidation

The oxidation state of a species increases (becomes more positive)

Electrons are lost by the atom

During a Reduction

The oxidation state of a species decreases (becomes more negative)

Electrons are gained by the atom

Oxidation and reduction always occur together.

Ag+(aq) + Cu(s) Ag(s) + Cu2+

(aq)

Zn + HCl H2 + ZnCl2

Redox Vocabulary

An oxidizing agent (oxidant) oxidizes something it reacts with

Contains an element whose oxidation state decreases (is reduced) during the redox reaction

A reducing agent (reductant) reduces something it reacts with

Contains an element whose oxidation state increases (is oxidized) during the redox reaction

Redox Vocabulary

In which of the following reactions do atoms undergo oxidation state changes?

Hydrogen peroxide, H2O2, is a versatile chemical. Its uses include bleaching wood pulp and fabrics and substituting for chlorine in water purification. One reason for its versatility is that it can be either an oxidizing or a reducing agent.

Identifying Oxidizing and Reducing Agents

H2O2 + Fe+2(aq) H2O + Fe+3

(aq)

H2O2 + MnO4–

(aq) Mn+2(aq) + O2

2 H2O2 (aq) 2 H2O (l) + O2 (g)

Writing and Balancing Redox Reactions

Cu(s) + Ag+(aq) Cu+2

(aq) + Ag(s)

Is this reaction balanced?

Na2SO3 (aq) + KMnO4

(aq) Na2SO4

(aq) + K+

(aq) + Mn+2(aq)

Balancing Redox Reactions

CN– (aq) + MnO4–

(aq) MnO2 (s) + OCN– (aq)

• Write the equations for the half-reactions.– Balance all atoms except H and O (balance H and O

also if they undergo redox)– Add e- based on oxidation state changes– Balance oxygen atoms using H2O– Balance hydrogen atoms using H+

• Equalize the number of electrons.• Add the half reactions.• If in base, add hydroxide to neutralize H+ and cancel water

molecules that appear on both sides of the equation.• Check the balance.

Balancing Redox Reactions: Half-Reactions

The Activity Series

We can measure (electrochemically) how easily an element will accept electrons (reduce) or release electrons (oxidize).

By putting these measurements in order of reduction potential, we generate an activity series