chemistry - chapter 8: writing equations

evidence of a chemical reaction

- production of gas
- evolution of heat and light
- formation of precipitate (solid)
- change in color

types of chemical equations

word equation, skeleton equation, balanced equation

word equation

reactants and products are written in words.
ex) Hydrogen + Oxygen → Water

skeleton equation

an equation that shows only the formulas of the reactants and products, and does not balance them.
ex) H₂ + O₂ → H₂O

balanced equation

a chemical equation in which mass is conserved; each side of the equation has the same number of atoms of each element.
ex) 2H₂ + O₂ → 2H₂O

how to balance a chemical equation

1. write formulas for reactants on the left and products on the right
2. count the number of atoms of each element/polyatomic ion on each side
3. balance the elements one at a time using COEFFICIENTS only--you cannot change the subscript numbers
4. make sure all the coefficients are in the lowest possible ratio

characteristics of a chemical equation

1. The equation must represent known facts
2. The equation must contain the correct formulas for the reactants and products
3. The law of conservation of mass must be satisfied

ΔH in an equation

the reaction is exothermic if ΔH is negative
the reaction is endothermic if ΔH is positive

five types of chemical reactions

- combination/synthesis
- decomposition
- single-replacement
- double-replacement
- combustion

combination/synthesis reaction

A+X→AX
Two or more substances react to form a single substance. Give off energy (exothermic)
Ex) S + O₂ → SO₂

decomposition reaction

AX→A+X
A single compound is broken down into two or more simpler substances. Often require energy (endothermic)
- there are 5 types of decomposition reactions

metallic chlorate decomposition

metallic chlorates decompose into the *metallic chloride* and *oxygen gas*.
Ex) KClO₃ → KCl + O₂

metallic carbonate decomposition

metallic carbonates decompose into the *metallic oxides* and *carbon dioxide*.
Ex) CaCO₃ → CaO + CO₂

metallic hydroxide decomposition

metallic hydroxides decompose into a *metallic oxide* and *water*.
Ex) NaOH → Na₂O + H₂O

oxyacid (acid containing oxygen) decomposition

oxyacids decompose into *nonmetallic oxides* and *water*.
Ex) H₂SO₄ → SO₃ + H₂O

peroxide decomposition

peroxides decompose into the *oxide* and *oxygen gas*.

single replacement reaction

AX+B → AB+X or AX+Y → XY+A
one element replaces one other element. Nonmetals replace nonmetals and metals replace metals.
there are 4 types of single displacement reactions

4 types of single displacement reactions

1. replacement of the metal by a more active metal
2. replacement of hydrogen in an acid by an active metal
3. the replacement of hydrogen in water by a very active metal
4. the replacement of a halogen by a more reactive halogen

double replacement reactions

the exchange of positive ions between two compounds.
Ex) AgNO₃ + NaCl → AgCl + NaNO₃

requirements for the products of double replacement reactions

always involves the formation of either a precipitate or a liquid (water). if two aqueous products are formed, the reaction does not take place.

aqueous/soluble vs solid/insoluble

if a compound is aqueous (represented by ₍ₐq₎), it dissolves in water.
if a compound is solid (represented by ₍ₛ₎), it does not dissolve in water.

3 different ways of writing double replacement equations

1. molecular equation
2. complete ionic equation
3. net ionic equation

molecular equation (double replacement)

reactants and products are in molecule form.
Ex) AgNO₃₍ₐq₎ + NaCl₍ₐq₎ → AgCl₍ₛ₎ + NaNO₃₍ₐq₎

complete ionic equation (double replacement)

better represents the actual forms of reactants and products. all *aqueous* substances are broken down into their ions. reveals spectator ions.
Ex) Na⁺ + Cl⁻ + Ag⁺ + NO₃⁻ → Na⁺ + NO₃⁻ + AgCl₍ₛ₎

spectator ions

ions that do not participate in a reaction. these are the ions that are on both sides of a double replacement equation when the equation is written in its complete form. they are removed when writing the net ionic equation.

net ionic equation (double replacement)

includes only the components involved in a chemical reaction, with spectator ions removed.
Ex) Cl⁻ + Ag⁺ → AgCl₍ₛ₎

combustion reactions of hydrocarbons

involve a hydrocarbon (carbon-hydrogen compound) reacting with oxygen. 2 possible types: complete combustion and incomplete combustion.

complete combustion

hydrocarbon + oxygen → carbon dioxide (CO₂) + water (H₂O)

incomplete combustion

hydrocarbon + oxygen → carbon monoxide (CO) + water (H₂O)