Physical and Chemical Change Law of Conservation of Mass Balancing Equations

   Chemical reactions are used in many ways in daily life. A chemical reaction is the process by which a chemical change happens.

All chemical reactions are also accompanied by changes in energy

 Some chemical reactions

absorb

energy, such as in the chemical reactions that cook food.

Other chemical reactions release energy in the form of heat , light and/or sound , such as the burning of wood in a campfire

Chemical reactions happen at different rates . Some chemical reactions are fast , such as when rocket fuel burns.

Other chemical reactions happen

slowly

, such as the formation of rust on a corroding bicycle chain.

The chemical reactions in your own

body

, which are keeping you alive, are among the

fastest

chemical reactions known.

Scientists are constantly working to find new kinds of

chemical reactions

in order to produce new substances with useful

properties

.

All chemical reactions involve the conversion of starting materials, called

reactants

, into new substances, called

products

. The products have different

properties

than the reactants.

 A description of a substance that does not involve forming a new substance.

Examples:  Colour   Texture State    Density Solubility Melting point

 A description of what a substance does as it changes into one or more new substances.

Examples:  Combustibility    Corrosion Reaction with acid Bleaching ability

  Properties are descriptions similar to an adjective: describes what the substance is like.

Change are descriptions similar to a verb: describes what the substance is doing

A

physical change

is the change in the state or form of a substance that does not change the original substance. A physical change can result in new physical properties but not new chemical properties.

Classes of physical change: 1.

Change in state (includes dissolving) 2.

Change in form Example:   Evaporation Cutting paper in half

A

chemical change

is the transformation of one or more substances into

new substances

with

new properties

Presence of a new

colour

Formation of a

precipitate

Release of

heat or light

Production of

gas or bubbles

Example  Reactants: Solid magnesium metal placed into a solution of hydrochloric acid  Clue: bubbles / gases  Product: hydrogen gas and magnesium chloride

A chemical reaction is often described by writing a

chemical equation

A chemical equation uses either

words

or

symbols

and formulas to describe the changes that occur during a chemical

reaction

.

Every chemical equation must have:  reactant   product arrow directing reactant to product

For example, the chemical reaction between solid magnesium metal and hydrochloric acid is: word equation: magnesium + hydrochloric acid  magnesium chloride + hydrogen formula equation: Mg(s) + 2HCl(aq)  MgCl 2 (aq) + H 2 (g)

The chemical formulas in a chemical equation will often include: A. the

state of matter

of each substance  (s) =

solid

 (l) =

liquid

 (g) =

gas

 (aq) =

aqueous

(substance is dissolved in water)

word equation: magnesium + hydrochloric acid  magnesium chloride + hydrogen formula equation: Mg(s) + 2HCl(aq)  MgCl 2 (aq) + H 2 (g) Notice that

hydrogen

is expressed in the formula equation as

H 2

. Recall that pure hydrogen exists as a

diatomic molecule

. (You will need to know which elements exist as molecules when writing formula equations)

B. one or more

coefficients

 A coefficient is an integer that is placed

in front

of the symbol of an element to show the

ratios

of the different substances that are present in the chemical reaction

formula equation: Mg(s) + 2HCl(aq)  MgCl 2 (aq) + H 2 (g) For example, in the formula equation above, a coefficient of

2

is in front of the formula

HCl

. This means that

Mg

and

HCl

combine in a ration of

1:2

In a chemical reaction, the mass of the products always equals the mass of the reactants.

In other words, the mass is

conserved.

   No atoms are

destroyed

and no new atoms are

produced

during a chemical reaction.

Instead, the atoms in the reactants are simply

rearranged

to form the products Chemical

bonds

between atoms are

broken

and new ones are

formed

, and the atom simply reconnect in

new ways

The rearrangement of atoms that occurs during a chemical reaction can be illustrated using models or diagrams.

For example: word equation: hydrogen + oxygen  formula equation: 2H 2 (g) + O 2 (g)  water 2H 2 O(l)

In this equation, there are

equal

numbers of hydrogen atoms (

4

) and equal numbers of oxygen atoms (

2

) on both the reactants side and the products side.

formula equation: 2H 2 (g) + O 2 (g)  2H 2 O(l)

When the number of each kind of atom is the

same

in the reactants and products, the equation is said to be

balanced

.

formula equation: 2H 2 (g) + O 2 (g)  2H 2 O(l)

A chemical reaction that is complete except for

coefficients

is called an

unbalanced

equation or

skeleton

equation.

To balance a chemical equation, begin by

counting

the number of

atoms

of each

element

in the skeleton equation.

    Balance by placing

coefficients

chemical formulas. in front of the Use only whole numbers as coefficients.

Check that the coefficients in the equation are the

lowest common factor

.

Never change a subscript in a formula to help make atoms balance!

Balance atoms of elements in any

complicated

looking formulas first and balance atoms of

pure elements

last.

H 2

  Hydrogen atoms and/or oxygen atoms will often appear in many or all of the formulas of the reactants and products. When this is the case, balance other elements first, balance hydrogen second last and oxygen last.

You may be able to treat

polyatomic unit

. ions as a Example: If NO 3 appears in the reactants and products of a skeleton equation, count the number of NO 3 -

groups

rather than the number of N and O

atoms

separately.

Example 1: Balance the following chemical equation: AlBr 3 (s) + Cl 2 (g)  AlCl 3 (s) + Br 2 (g) 1.) Count the number of atoms in the reactants and products:

Example 1:

2

AlBr 3 (s) + Cl 2 (g)  AlCl 3 (s) +

3

Br 2 (g) Balance the number of bromine atoms by adding a coefficient of 2 in front of AlBr 3 the atoms again: and a coefficient of 3 in front of Br 2 . Count

Example 1:

2

AlBr 3 (s) + Cl 2 (g)  AlCl 3 (s) +

3

Br 2 (g) The number of aluminum atoms is no longer equal.

Example 1:

2

AlBr 3 (s) + Cl 2 (g) 

2

AlCl 3 (s) +

3

Br 2 (g) Balance the number of aluminum atoms by adding a coefficient of 2 in front of AlCl 3 . Count the atoms again:

Example 1:

2

AlBr 3 (s) + Cl 2 (g) 

2

AlCl 3 (s) +

3

Br 2 (g) The number of chlorine atoms is no longer balanced.

Example 1:

2

AlBr 3 (s) +

3

Cl 2 (g) 

2

AlCl 3 (s) +

3

Br 2 (g) Balance the number of chlorine atoms by adding a coefficient of 3 in front of Cl 2 . Count the atoms again:

Example 1:

2

AlBr 3 (s) +

3

Cl 2 (g) 

2

AlCl 3 (s) +

3

Br 2 (g) The equation is balanced!

Try it!

Balance the following chemical equations: 1.

2.

3.

Al + F 2  AlF 3 Ca + H 2 O  Ca(OH) 2 CaCl 2 + Na 3 PO 4  Ca 3 + H 2 (PO 4 ) 2 + NaCl

Try it!

Balance the following chemical equations: 1.

2.

3.

2

Al +

3

F 2  Ca +

2

H 2 O 

2

AlF 3 Ca(OH) 2

3

CaCl 2 +

2

Na 3 PO 4  + H 2 Ca 3 (PO 4 ) 2 +

6

NaCl