Chapter 4

Chemical Reactions

Chemistry B11

Chemical Reactions

Chemical change = Chemical reaction

Substance(s) is used up (disappear)

New substance(s) is formed.

Different physical and chemical properties.

Chemical Reactions

A + B C + D

Reactants Products

Chemical Reactions

Chemical Equation

Chemical Reactions

A + B AB1. Synthesis reaction (combination)

2H2 + O2 2H2O

AB A + B2. Decomposition (analysis)

2NaCl 2Na + Cl2

A + BC AC + B3. Single replacement reaction

Fe + CuSO4 FeSO4 + Cu

AB + CD AD + CB4. Double replacement reaction

NaCl + AgNO3 NaNO3 + AgCl

5. Combustion

C3H8 + 5O2 3CO2 + 4H2O

Chemical Reactions

Solid (s)Liquid (l)Gas (g)

Aqueous (aq)

Ca(OH)2(s) + 2HCl(g) CaCl2(s) + H2O(l)

AB + xO2 yCO2 + zH2O

Why balancing?

Balance a chemical equation

Balance a chemical equation

Law of conservation of mass

Atoms are neither destroyed nor created.

They shift from one substance to another.

Balance a chemical equation

1. Begin with atoms that appear in only one compound on the left and right.

2. If an atom occurs as a free element, balance it last.

3. Change only coefficients (not formulas).

C3H8(g) + O2(g) CO2(g) + H2O(g)

last

Formula and Molecule

Ionic & covalent compounds Formula formula of NaCl

Covalent compounds Molecule molecule of H2O

Formula Weight of NaCl:

23 amu Na + 35.5 amu Cl = 58.5 amu NaCl

Molecular Weight of H2O:

2 (1 amu H) + 16 amu O = 18 amu H2O

Mole

Mole (mol): formula weight of a substance (in gram).

12g of C = 1 mol C 23g of Na = 1 mol Na

58.5 g of NaCl = 1 mol NaCl

18 g of H2O = 1 mol of H2O

Avogadro’s number (6.02×1023): number of formula units in one mole.

1 mole of apples = 6.02×1023 apples

1 mole of A atoms = 6.02×1023 atoms of A

1 mole of A molecules = 6.02×1023 molecules of A

1 mole of A ions = 6.02×1023 ions of A

Molar mass (g/mol): mass of 1 mole of substance (in gram)(Formula weight)

molar mass of Na = 23 g/molmolar mass of H2O = 18 g/mol

Relationships between amounts of substances in a chemical reaction.

Look at the Coefficients!

Stoichiometry

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

2 moles 2 moles 1 mole

2 2 1

2 liters 2 liters 1 liter

2 particles 2 particles 1 particle

2 grams 2 grams 1 gram

Amole

Bmolemass

volume volume

Particle(atom)

(molecule)(ion)

Particle(atom)

(molecule)(ion)

mass

CH4 + 2O2 CO2 + 2H2O

1 step: use coefficient in the balanced equation.

CH4 + 2O2 CO2 + 2H2O

10 cc O2 = ? cc CO2 10 cc O2 (1 cc CO2

2 cc O2

) = 5 cc CO2

23 mole CH4 = ? moles H2O 23 mole CH4 (2 moles H2O

1 mole CH4

) = 46 moles H2O

40 g CH4 = ? L CH4 40 g CH4 (1 mole CH4

16 g CH4

) = 56 L CH4

22.4 L CH4

1 mole CH4

)(

STP: 1 mole of substance (gas) = 22.4 L = 22400 cc (cm3 or mL)

32 g CH4 = ? moles CO2 32 g CH4 (1 mole CH4

16 g CH4

) = 2 mole CO2

1 mole CO2

1 mole CH4

)(

Limiting Reagents

N2(g) + O2(g) 2NO(g)

1 mole 1 mole 2 moles

1 mole 4 moles

0 mole 3 moles 2 moles

Before reaction:

After reaction:

Stoichiometry:

Limiting Reagents

N2(g) + O2(g) 2NO(g)

1 mole 1 mole 2 moles

1 mole 4 moles

0 mole 3 moles 2 moles

Before reaction:

After reaction:

Stoichiometry:

Used up first Left over

Limiting Reagents

N2(g) + O2(g) 2NO(g)

1 mole 1 mole 2 moles

1 mole 4 moles

Limiting reagent

0 mole 3 moles 2 moles

Before reaction:

After reaction:

Stoichiometry:

Used up first Left over

Limiting Reagents

Limiting reagents can control a reaction:

N2(g) + O2(g) 2NO(g)

Limiting reagent: is the reactant that is used up first.

Limiting Reagents

Example:

C(s) + O2(g) CO2(g) 12g of C64g of O2

? Limiting reagent? g of CO2 will be formed

Make sure that the chemical equation is balanced.

12g C (1 mole C

12g C) = 1 mole C

64g O2 (1 mole O2

32g O2

) = 2 mole O2

C is the limiting reagent.

C(s) + O2(g) CO2(g)

1 mol 1 mol 1 mol

Limiting Reagents

We should use the mass of the limiting reagent.(because it controls our reaction).

12g C (1 mole C

12g C)(

1 mole CO2

1 mole C)(

44g CO2

1 mole CO2

) = 44g CO2

? g of CO2 will be formed: C(s) + O2(g) CO2(g)A B

12g 64g ? g

Percent Yield

Percent yield = actual yield

theoretical yield× 100

actual yield: mass of product formed (experimental)

theoretical yield: mass of product that should form (according to stoichiometry)

N2(g) + O2(g) 2NO(g)

theoretical yield = 40g NOactual yield = 37g NO Percent yield =

37g NO

40g NO× 100 = 92.5%

7.5% error or lost

Aqueous Solution (ionic compounds)

NaCl(s) Na+(aq) + Cl-(aq) Dissociation (Ionization)

aqueous solution: solvent is water

H2O

AgNO3(s) Ag+(aq) + NO3-(aq)

H2O

NaCl AgNO3

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

H2O

Molecular equation:

2As3+(aq) + 3s2-(aq) As2S3(s)

total charge on left side = total charge on right side

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

Na+(aq) + Ag+(aq) + Cl-(aq) + NO3-(aq) AgCl(s) + Na+(aq) + NO3

-(aq)Ionic equation:

Net ionic equation:

Na+(aq) + Ag+(aq) + Cl-(aq) + NO3-(aq) AgCl(s) + Na+(aq) + NO3

-(aq)

Spectator ions

Ag+(aq) + Cl-(aq) AgCl(s)

Oxidation and Reduction reactions (redox)

Zn(s) Zn2+(aq) + 2e- Zn is oxidized (reducing agent)

Cu2+(aq) + 2e- Cu(s) Cu2+ is reduced (oxidizing agent)

oxidation: is the loss of electrons.

reduction: is the gain of electrons.

Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s) redox reaction

Oxidation and Reduction reactions (redox)

oxidation: is the gain of oxygen / loss of hydrogen.

reduction: is the loss of oxygen / gain of hydrogen.

CH4(s) + 2O2(g) CO2(g) + 2H2O(g) redox reaction

C gains O and loses His oxidized

(reducing agent)

O gains HIs reduced

(oxidizing agent)

single replacement reactions and combustion reactions redox reactions

double replacement reactions non redox

Heat of reaction

C3H8(s) + 5O2(g) 3CO2 + 4H2O + heat (energy)

2HgO(s) + heat (energy) 2Hg(l) + O2(g)

Endothermic reaction

Exothermic reaction

All combustion reactions are exothermic.