Unit 5 - Chemical reactions and balancing equations

Physical Properties

Physical Properties

Can be observed or measured without changing the composition of the matter

Chemical Properties

a substance’s ability to undergo changes that transform it into a different substance

Chemical reactions

The process by which substances interact to form new substances with different compositions

Chemical change

A change in matter on a molecular level that produces one or more new substances

Unexpected color change

New molecules created in a chemical reaction radiate light differently, producing new colors

Rusting

A chemical reaction that causes rust called oxidation

Combustion reaction

A chemical reaction between substances, usually a hydrocarbon with oxygen, to produce CO₂, water (H₂O), light, and heat

Precipitate forms

A chemical reaction that causes a solid suddenly appears in a solution and settles to the bottom, collects on another object, or makes the solution cloudy

Bubbles form

When gases produced in a chemical reaction are released

Temperature change

When the temp. increases, energy was released during a chemical reaction.

When the temp. decreases, energy was absorbed during a chemical reaction

Change in Odor

when two or more compounds or elements are mixed and a scent or odor is present, a chemical reaction has taken place

chemical equation

A representation of a chemical reaction using symbols and chemical formulas

reactants

The starting substances that are combined to make products

products

the new substances which result from the chemical reaction

Chemical Formula

• represents the composition of a molecule or compound. 

• It lets you know how many of each atom there are. 

Reaction Symbols

The small symbols in the parentheses (s, l , g, aq)  next to each chemical formula indicate the state of matter of each substance in the reaction

Subscripts

Tells you the amount of atoms of an element in each molecule 

H₂O ~ There are two hydrogen atoms in a water molecule 

Coefficients

tells you how many molecules of a substance there are 

3H₂O ~ There are three water molecules, this means there are 6 total hydrogen atoms and 3 total oxygen atoms

The Law of Conservation of Mass

Mass is neither created nor destroyed during chemical or physical reactions

Balancing Equations

The Law of Conservation of Mass is applied by balancing the number & type of atoms on either side of the equation

A balanced chemical equation has the same number of each type of atom on the product side and the reactant side

We change the coefficients of the molecules.

Synthesis reactions

Two or more substances combine to form a new compound

A + B → AB

Forming bonds releases energy (Exothermic)

Decomposition reaction

A chemical reaction in which a single compound is broken down into 2+ smaller compounds

AB  —energy→ A + B

Breaking bonds requires energy

Single replacement/Single displacement reaction

When one element replaces a similar element in a compound

AX + B → BX + A

Double Displacement Reaction

Ions from two compounds in a solution exchange places to produce two new compounds

AB + CD → AD + CB

Signs of a Double Displacement Reaction

• a precipitate that settles out of the solution, 

• a gas that bubbles out of the solution, 

• or a molecular compound (like water)

Chemical energy

energy stored in the chemical bonds of a substance

Chemical reactions always involve energy changes

Making and breaking bonds involve energy changes

Activation Energy

The energy required to break the bonds in the reactants for a chemical reaction to occur.

Must supply energy to break bonds!

Endothermic

more thermal energy is ABSORBED  than released.

Endo = into

Surroundings feel COLD

REACTANTS  + ENERGY → PRODUCTS

REACTANTS + HEAT → PRODUCTS

Exothermic

more thermal energy is RELEASED than absorbed

Exo = exit

Surroundings feel HOT

REACTANTS → PRODUCTS + ENERGY

REACTANTS → PRODUCTS + HEAT