Reaction Typesand

Chemical Equations

Chemical Reactions

Describing a Chemical Reaction

Indications of a Chemical Reaction

Evolution of heat, light, and/or sound

Production of a gas

Formation of a precipitate

Color change

Signs of Chemical Reactions

There are five main signs that indicate a chemical reaction has taken place:

change in color change in odor production of newgases or vapor

input or releaseof energy

rele

ase

inpu

t

formation of precipitate

Chemical Reactions

The starting substances turn into different substances by rearranging the atoms.

Mass is conserved in chemical reactions.

Starting substances = reactants Ending substances = products

5 Major Types of Chemical Reactions

Synthesis two or more substances combine to

form one substance A + B → AB

Decomposition one substance breaks down into 2 or

more substancesAB → A + B

Replacement/Displacement

Single Replacement one element switches places with an

element in a compoundA + BC → AC + B

Double Replacement two elements in two compounds

exchange placesAB + CD → AD + CB

Combustion of a Hydrocarbon

Combustion is the burning of a substance in the presence of oxygen.

This reaction is very specific. O2 should always be a reactant. CO2 and H2O should always be products.

CH4 + 2O2 → CO2 + 2H2O

Chemical Equations

A chemical reaction can be described by a chemical equation.

As mass is conserved in a chemical reaction, the amount of each element involved must be equal on both sides.

Example:

2 2 22 2H O H O

The Law of Conservation of Mass

Mass is neither created nor destroyed during a chemical reaction. Mass is conserved.

The law was first shown to be true by French scientist Antoine Lavosier (1743-1794) by making careful measurements with sensitive analytical balance.

Equation: Massreactants = Massproducts

Skeleton Equation

Uses chemical formulas, but is not balanced

Na(s) + H2O(l) → NaOH(aq) + H2(g)

Subscripts in parentheses are states of matter.

s = solid l = liquid g = gasaq = aqueous solution

Balancing Chemical Equations Add coefficients in front of the chemical

formulas. The coefficients act as multipliers.

Never change the subscripts when balancing and equation!

__ Na(s) + __ H2O(l) → __ NaOH(aq) + __H2(g)

Laws about the Composition of Pure Substances

Law of Definite Composition A given compound always contains the

same, fixed ratio of elements.

Law of Multiple Proportions Elements can combine in different

ratios to form different compounds. If two elements form more than one

compound between them, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers.

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Pure Substances

For example…

Two different compounds, each has a definite composition.

Carbon, C Oxygen, O Carbon monoxide, CO

Carbon, C Oxygen, O Oxygen, O Carbon dioxide, CO2

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Example Problem

If 39.098 g of potassium reacts completely with 35.453 g of chlorine, what mass of potassium chloride is formed?

Example Problem

Determine if compound I and II are the same compound.

Compound I total mass = 76.00 gmass of Fe = 53.46 g mass of O = 22.54 g

Compound II total mass = 56.00 gmass of Fe = 43.53 g mass of O = 12.47 g