Chemistry 6.0

CHEMICAL EQUATIONS &

REACTIONS

I. Chemical ReactionsA. Definition: a process by which 1 or more

substances, called reactants, are changed into 1 or more substances, called products, with different physical & chemical properties.

B. Evidence of a Chemical Reaction

1. Color change2. Formation of a precipitate, ppt3. Release of a gas4. Energy change – heat, light, sound5. Odor change

C. Reactions are started by the addition of energy

II. Chemical Equation

A. Form1. Reactant + Reactant Product + Product2. Symbols: (s), (l), (g), (aq)

NR methanol

Interpretation of a Balanced Equation

2Mg(s) + O2(g) 2MgO(s)

2 atoms of solid magnesium react with 1 molecule of oxygen gas to form

2 formula units of solid magnesium oxide

OR2 moles of solid magnesium react with

1 moles of oxygen gas to form 2 moles of solid magnesium oxide

B. Energy & Chemical Equations

1. Exothermic reactions – release energy; energy a product

H2O(g) H2O(l) + 40.7 kJ

2. Endothermic reactions – absorbs energy; energy a reactant

H2O(s) + 6.0 kJ H2O(l)

C. Characteristics of A Balanced Chemical Equations

1. The equation must represent known facts. All substances have been identified.

2. The equation must contain the correct symbols and/or formulas for the reactants and products

3. Can be either a word equation or a formula equation

4. The law of conservation of mass must be satisfied. This provides the basis for balancing chemical equations. 1st formulated by Antoine Lavoisier

TOTAL MASS REACTANTS = TOTAL MASS PRODUCTS

Number of atoms of EACH element is the SAME on both sides of the equation.

D. Balancing Chemical Equations

1. Balance using coefficients after correct formulas are written.

Coefficients are usually the smallest whole number – required when interpreted at the molecular level

2. Balance atoms one at a time3. Balance the atoms that are combined and appear

only once on each side.4. Balance polyatomics that appear on both sides5. Balance H and O atoms last

NEVER CHANGE SUBSCRIPTS!!!**Count atoms to be sure that the

equation is balanced**

BALANCING Examples

1. sodium + chlorine sodium chloride

2. CH4 (g) + O2 (g) CO2 (g) + H2O(l)

3. K(s) + H2O(l) KOH(aq) + H2(g)

4. AgNO3(aq) + Cu(s) Cu(NO3)2(aq) + Ag(s)

Synthesis Reactions• Two or more substances combine to form a more

complex product.A + B → AB (only ONE PRODUCT)

• A.K.A. Direct Combination Reactions, or composition reactions

• Ex. Fe + S → FeS

• Ex. CaO + H2O → Ca(OH)2

Synthesis Reaction

Sodium Metal plus Chlorine Gas Video

2 Na + Cl2 2 NaCl

Synthesis Reaction

Decomposition Reactions

• Single Reactant breaks down to into a simpler substance.

AB → A + B (only ONE REACTANT)

• The opposite of a synthesis reaction.

• Ex. 2HgO → 2Hg + O2

• Ex. CaCO3 → CaO + CO2

Single Replacement Reaction

• Atoms of one element replace atoms of another element in a compound.

A + BX → AX + B

• A more active element will replace a less active element. (See activity series)

• Ex. Fe + CuSO4 → FeSO4 + Cu

• Ex. Mg + CuSO4 → MgSO4 + Cu

Double-Replacement Reactions

• Atoms or ions from 2 different compounds replace each other.

AX + BY → AY + BX

• Ex. CaCO3 + 2HCl → CaCl2 + H2CO3

Combustion Reactions• One substance reacts with oxygen to produce oxide

compounds.

• Occurs when burning.

• Combustion reactions are often classified as synthesis reactions.

• These reactions are usually exothermic, releasing a large amount of energy as light, heat, or sound.

Combustion Reactions (cont.)• When a hydrocarbon is involved in a combustion

reaction, H2O and CO2 are the products.

• Ex. CH4 + 2O2 → CO2 + 2H2O + 803 kJ

• Ex. S + O2 → SO2

Combustion Reaction

5 Types of Chemical Reactions Video

III. Classifying Chemical Reactions

Pattern for prediction based on the kind of reactants

A. Combustion or Burning – complete combustion always produces carbon dioxide and water!

1. HydrocarbonsCxHy + O2 CO2 + H2O

2. AlcoholsCxHyOH + O2 CO2 + H2O

3. SugarsC6H12O6 + O2 CO2 + H2OC12H22O11 + O2 CO2 + H2O

B. Synthesis or Composition

2/more reactants 1 product1. Element + Element Compound

A + B AB2 Na + Cl2 2 NaCl

4 Al + 3 O2 2 Al2O3

2. Compound + Compound Compound

EXAMPLE 1: metal oxide + carbon dioxide a carbonateCaO + CO2 CaCO3

EXAMPLE 2: metal oxide + water a base (hydroxide) Na2O + H2O 2 NaOH

EXAMPLE 3: metal oxide + sulfur trioxide metal sulfateNa2O + SO3 Na2SO4

EXAMPLE 4: metal oxide + sulfur dioxide metal sulfiteNa2O + SO2 Na2SO3

EXAMPLE 5: nonmetal oxide + water an acidSO3 + H2O H2SO4

Decomposition - Binary Compounds

1. Binary Compound 2 elements AB A + B

2 H2O 2 H2 + O2

2 HgO 2 Hg + O2

Decomposition - Ternary Compounds

2. Ternary Compound Compound + Element/Compound a. metal carbonate metal oxide + carbon dioxide

CaCO3 CaO + CO2

b. metal hydroxide metal oxide + water(Except Group IA metals) Mg(OH)2 MgO + H2O

c. metal chlorate metal chloride + oxygen 2KClO3 2KCl + 3O2

d. metal nitrate metal nitrite + oxygen

2NaNO3 2NaNO2 + O2

e. acids nonmetal oxide + water (HINT: MUST USE CHARGE OF H and O) H2CO3 CO2 + H2O

f. Other 2H2O2 2H2O + O2

Single Replacement or Single Displacement

Element + Compound New Compound + New Element

1. Metals

A + BC AC + B

Active metals displace less active metals or hydrogen from their compounds in aqueous solution. Refer to the Activity Series.

a. Zn + CuSO4 ZnSO4 + Cu

b. metal + H2O metal hydroxide + H2

Metals include the alkali metals and calcium.

2Na + 2H2O 2NaOH + H2

Reactivity or Activity of Metals

• The reactivity of a metal is based on its ability to replace another in a compound.

• If a single replacement reaction occurs, the metal that “cuts in” is MORE reactive than the one that was removed or replaced.

• An activity series of metals is a listing that ranks metals according to their reactivity. – The most active metal is at the TOP of the list– The least active metal is at the BOTTOM of

the list

The ACTIVITY SERIES is listed below:lithiumpotassiumbariumstrontiumcalciumsodiummagnesiumaluminummanganesezincironcadmiumcobaltnickeltinleadhydrogencoppersilvermercurygold

The most active metal is LITHIUM

The least active metal is GOLD

Which is more active nickel or iron? IRON

3CuCl2 + 2Al 2AlCl3 + 3Cu

http://www.youtube.com/watch?v=QQ0eLyBzKYs

Single Replacement or Single Displacement

Element + Compound New Compound + New Element

1. Metals

A + BC AC + B

Active metals displace less active metals or hydrogen from their compounds in aqueous solution. Refer to the Activity Series.

a. Zn + CuSO4 ZnSO4 + Cu

b. metal + H2O metal hydroxide + H2

Metals include the alkali metals and calcium.

2Na + 2H2O 2NaOH + H2

Single Replacement or Single Displacement

2. Nonmetals D + EF ED + F Cl2 + 2NaBr 2NaCl + Br2

Many nonmetals displace less active nonmetals from combination with a metal

or other cation. Order of decreasing activity is

F2 Cl2 Br2 I2

Double Replacement or Double Displacement or Metathesis:

Compound + Compound New Compound + New Compound

AB + CD AD + CB

AgNO3 + NaCl AgCl + NaNO3

Special Notes:

If NH4OH is one of the products, it breaks down into NH3 and H2O.

If H2CO3 is one of the products, it breaks down into CO2 and H2O.

Double Replacement Reactions and Precipitates

• If the reactants are both aqueous in a double replacement reaction, a precipitate may form.

• Use solubility rules to determine the identity of the precipitate.

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

Exothermic Reactions

• Release heat into the surroundings

• Heat is a product of the reaction

• Combustion reactions are exothermic

• C3H8 + 5O2 → 3CO2 + 4H2O + 2043kJ

Endothermic Reactions

• Heat is absorbed by the reactants and stored in the chemical bonds of the product.

• Heat acts as a reactant.

• C + H2O + 113kJ → CO + H2

• Motorcycle Helmet

Molar Heat Capacity• The heat absorbed or released during a reaction

depends on a difference in a quantity called enthalpy. (Total energy content of a sample.)

• The symbol for enthalpy is H.• When reactions take place at standard temperature

and pressure, q = H.• Stand. temp. = 25°C Stand. Pres. = 1 atm• Purest form of a substance = most stable form• Enthalpy change at STP denoted H°

Enthalpy Change ∆H = H products – H reactants

Type of reaction

∆H H products H reactants

Exothermic

Endothermic

-

+ > <

><

Exothermic Reaction

Endothermic Reaction

• What is the energy of the reactants?• What is the energy of the products?• Is the forward reaction exothermic or endothermic?• What is the ΔH for the forward reaction?• What is the ΔH for the reverse reaction?

150 kJ

50 kJ

exothermic-100 kJ

100 kJ

kJ

Hess’s Law

• Hess’s Law states that if a series of reactions are added together, the enthalpy change of the net reaction will be the sum of the enthalpy changes of the individual steps.

Steps for using Hess’s Law

• Identify the compounds• Locate the compounds on the periodic table• Write a reaction from the table.• Write the appropriate “sub equation.”– If needed, multiply equation and enthalpy change.– If you reverse the reaction, change sign of enthalpy

change.• Add equations.• Add enthalpy changes.

Bond Energy• Bond energy is the strength of a chemical bond between

atoms, expressed as the amount of energy required to break it apart. (Unit kJ/mol)

• It is as if the bonded atoms were glued together: the stronger the glue is, the more energy would be needed to break them apart. A higher bond energy, therefore, means a stronger bond.

• Ionic bonds are stronger than covalent bonds. Among covalent bonds, triple bonds are stronger than double bonds and double are stronger than single bonds.

Bond Energy for Carbon Bonds

• Bond lengths (Å):Single > double > triple 1.54 1.34 1.20

• Bond Energy (kJ/mol) Single < double < triple 346 612 835

Using Bond Energy in Calculations

• On the REACTANT side, determine the number and type of each bond. – Determine the total amount of energy needed to break

them. (endothermic – positive sign)• On the PRODUCT side, determine the number and

type of each bond.– Determine the total amount of energy needed for bonds

to form. (exothermic – negative sign)• Calculate the enthalpy change for the entire

reaction.– ∆H = reactants + products