Equations & Reactions

8.1 Describing Chemical ReactionsA. Chemical Changes and Reactions

1. New substances are produced.2. Chemical reaction – chemical bonds

between atoms or ions break, and new bonds form between atoms or ions.

B. Evidence of a Chemical Reaction1. color change2. formation of a precipitate3. temperature change4. formation of a gas

C. Mechanics of a Chemical Reaction1. Starting Materials – reactants2. Ending Materials - products3. reactants → products

Arrow = yields or produces4. Many reactions occur to complete a set of

valence electrons.5. Symbols above the yield sign represent

conditions necessary for a reaction to proceed. Ex)

= delta = heat

= electrolysis

Δ

elec

6. Some reactions occur spontaneously.7. Symbols represent the state of the

reactants and products.

Liquid = l Gas = g Solid = s Crystal = cr

Aqueous = aq (solids in water solution)

DEMO

Ex) Al(s) + CuCl2(aq) → AlCl3(aq) + Cu(s)

Ex) 2Al(s) + 3CuCl2(aq) → 2AlCl3(aq) + 3Cu(s)

8. Complete chemical equations include the subscript to indicate the physical state of each substance.

9. Diatomic molecules – certain elements exist in nature as diatomic molecules (X2)

List them: N2 O2 F2 Cl2 Br2 I2 H2

Natural States of the Elements• Diatomic Molecules

Nitrogen gas containsN2 molecules.

Oxygen gas containsO2 molecules.

Natural States of the Diatomic Elements

8.2 Balancing Equations

A. Equations in Chemistry1. Chemical equation: an expression that uses

symbols and formulas to describe a chemical

reaction.2. + means “reacts with”3. → means produces (Called the yield sign)

B. Balancing Chemical Equations

1. Conservation of mass leads to balancing equations – the number of atoms of each element must be the same before & after the reaction.

2. The Law of Conservation of Mass also states that the total mass before and after the reaction must be the same. You cannot lose or gain mass.

3. Therefore the MASS OF THE PRODUCTS = MASS OF REACTANTS

4. Subscript – indicates number of atoms of an element present in a compound.

5. Coefficient – indicates the number of atoms or molecules involved in the reaction.

6. Steps to Balance Equations:

A. Write equation with symbols.B. Count # of atoms on each side of the

reaction.C. Balance atoms using coefficients. D. General Rule: Balance all elements first.

Then, balance C, H, and O. E. NEVER CHANGE

SUBSCRIPTS!!!!

3 H2 + N2 → 2 NH3

4 P + 5 O2 → 2 P2O5

2 NaNO3 → 2 NaNO2 + O2

2 C8H18 + 25 O2 → 16 CO2 + 18 H2O

8.3 Classifying Chemical Reactions

A. Synthesis Reactions (direct combination)1. Two or more elements or compounds combine

to form a more complex product. A + B → AB

2. Ex. Fe + S → FeSCaO + H2O → Ca(OH)2

sodium + chlorine → sodium chloride

2 Na + Cl2 → 2 NaCl

Synthesis Reaction

Sodium Metal plus Chlorine Gas Video

2 Na + Cl2 2 NaCl

Synthesis Reaction

B. Decomposition Reactions (analysis)

1. A single reactant breaks down into simpler substances.

AB → A + B2. The opposite of a synthesis reaction.3. Ex. 2 HgO → 2 Hg + O2

CaCO3 → CaO + CO2

Decomposition Reaction

CuCO3(s) CO2(g) + CuO(s)

What is one of the products?

C. Single Replacement Reactions

1. Atoms of an uncombined element replace atoms of another element in a compound.

A + BX → AX + B2. A more active element will replace a less

active element. (See activity series.)

3CuCl2 + 2Al 2AlCl3 + 3Cu

3. An Activity Series is a way of ranking elements (usually metals) in order from greatest to least reactivity. It can be used to predict whether a reaction will occur or not.

Fe + CuSO4 → FeSO4 + Cu FeSO4 + Cu → No Reaction

Mg + CuSO4 → MgSO4 + Cu

MgSO4 + Cu → No Reaction

D. Double-Replacement Reactions

Atoms or ions from 2 different compounds replace each other.

AX + BY → AY + BX

Ex. CaCO3 + 2HCl → CaCl2 + H2CO3

Ex. NaOH + HCl → NaCl + HOH

E. Combustion Reactions

1. One substance reacts with oxygen, O2 to produce oxide compounds.

2. Occurs during burning.3. Some combustion reactions can

often be classified as synthesis reactions.

Ex) S + O2 → SO2

Hydrogen Burning Video

H2 + 2 O2 2 H2O

Synthesis Reaction

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

5. When a hydrocarbon (compound containing H & C) is involved in a combustion reaction, CO2 and H2O are always the products.

6. Ex. __CH4 + __O2 → __CO2 + __H2O + 803 kJ

C2H6 + O2 → CO2 + H2O + heat

2 2

2 7 4 6

Combustion Reaction

Combustion Reaction

5 Types of Chemical Reactions Video