What is a physical change?

In a physical change,

• identity and composition substance do not change.

• state can change or material can be torn into smaller pieces.

What is a chemical change?

In a chemical change,

• reacting substances form new substances with different compositions and properties.

• chemical reaction takes place.

What is a chemical reaction?In a chemical reaction • chemical change produces one

or more new substances• change in composition of one

or more substances• old bonds are broken and new

bonds are formed• atoms in reactants are

rearranged to form one or more different substances.

• Fe and O2 form rust (Fe2O3).

4

Law of Conservation of Mass

Indicates chemical reaction• matter cannot be created nor

destroyed.• no change in total mass occurs in a

reaction.• mass of products is equal to mass of

reactants.

What are chemical equations?

chemical equation chemical formulas of reactants on left of arrow and products on right. atoms are not gained or lost

Reactants Product

C(s)O2 (g)

CO2 (g)

Symbols• states of reactants.• states of products.• reaction conditions• number of reactant

atoms is equal to the number of product atoms

Let’s look at a balanced chemical equation!In a balanced chemical equation,• same number of each type of atom on reactant

side and on product side of a balanced equation.• numbers called coefficients used in front of one

or more formulas.

Al + S Al2S3 Not Balanced 2Al + 3S Al2S3 Balanced

2 Al = 2 Al 3 S = 3 S

balance following equation, Fe3O4(s) + H2(g) Fe(s) + H2O(l)

• work on one element at a time.• use only coefficients in front of formulas.• do not change any subscripts.

Fe: Fe3O4(s) + H2(g) 3Fe(s) +

H2O(l)

O: Fe3O4(s) + H2(g) 3Fe(s) +

4H2O(l)

H: Fe3O4(s) + 4H2(g) 3Fe(s) +

4H2O(l)

Steps in Balancing an Equation

1. Need equation with correct formulas. NH3(g) + O2(g) NO(g) +

H2O(g)

2. Determine if equation is balanced.above, not all atoms are balanced.

3. Balance using coefficients in front of formulas. 4NH3(g) + 5O2(g) 4NO(g) +

6H2O(g)

4. Check atoms (elements) equal both sides. 4 N (4 x 1 N) = 4 N (4 x 1 N) 12 H (4 x 3 H) = 12 H (6 x 2 H) 10 O (5 x 2 O) = 10 O (4 O + 6 O)

Balancing Chemical Equations

Balancing with polyatomic ions

MgCl2(aq) + Na3PO4(aq) NaCl(aq) + Mg3(PO4)2(s)

Balance PO43- as a unit

MgCl2(aq) + 2Na3PO4(aq) NaCl(aq) + Mg3(PO4)2(s) 2 PO4

3- = 2 PO4

3-

Balance Mg and Cl3MgCl2(aq) + 2Na3PO4(aq) 6NaCl(aq) + Mg3(PO4)2(s)

3 Mg2+ = 3 Mg2+

6 Na+ = 6 Na+

6 Cl- = 6 Cl-

What is a mole?

A collection term statesa specific number of items.

• 1 dozen donuts = 12 donuts

• 1 ream of paper = 500 sheets

• 1 case = 24 cans

A mole: collection that contains

• same number of particles as there are carbon atoms in 12.0 g of carbon.

• 6.02 x 1023 atoms of an element (Avogadro’s number) 1 mole element Number of Atoms

1 mole C = 6.02 x 1023 C atoms1 mole Na = 6.02 x 1023 Na atoms1 mole Au = 6.02 x 1023 Au atoms

A mole • of covalent compound has Avogadro’s number of molecules.

1 mole CO2 = 6.02 x 1023 CO2 molecules

1 mole H2O = 6.02 x 1023 H2O molecules

• of ionic compound contains Avogadro’s number of formula units.1 mole NaCl = 6.02 x 1023 NaCl formula units1 mole K2SO4 = 6.02 x 1023 K2SO4 formula units

A mole can be expressed as:

Using Avogadro’s numberAvogadro’s number is used to convertmoles of a substance to particles

1 mole = 6.02 x 1023 particlesConversion Factors:6.02 x 1023 particles and 1 mole

1 mole 6.02 x 1023 particles

How many Cu atoms are in 0.50 mole Cu?

0.50 mole Cu x 6.02 x 1023 Cu atoms 1 mole Cu

= 3.0 x 1023 Cu atoms

Using Avogadro’s number

Avogadro’s number is used to convertparticles of a substance to moles

Conversion Factors:6.02 x 1023 particles and 1 mole 1 mole 6.02 x 1023 particles

How many moles of CO2 are in 2.50 x 1024 molecules CO2?

2.50 x 1024 molecules CO2 x 1 mole CO2

6.02 x 1023 molecules CO2

= 4.15 mole CO2

What is molar mass?

molar mass

• mass of one mole of an element or compound.

• atomic mass expressed in grams.

Molar mass of a compound

molar mass of a compound is sum of molar masses of elements in formula.

Example: Calculate molar mass of CaCl2.

Element Number of Moles

Atomic Mass Total Mass

Ca 1 40.1 g/mole 40.1 g

Cl 2 35.5 g/mole 71.0 g

CaCl2 111.1 g

Molar Mass of K3PO4

Calculate molar mass of K3PO4.

Element

Number of Moles

Atomic Mass

Total Mass in K3PO4

K 3 39.1 g/mole 117.3 g

P 1 31.0 g/mole 31.0 g

O 4 16.0 g/mole 64.0 g

K3PO4 212.3 g

Let’s calculate molar mass of :

S, NaCl, Fe,

Collision theory of reactions

chemical reaction occurs when• collisions between molecules have sufficient energy to

break bonds in the reactants. • bonds between atoms of the reactants (N2 and O2) are

broken and new bonds (NO) can form.

What is activation energy?

• minimum energy needed for a reaction to take place

• collision provides energy equal to or greater than activation energy, product can form

C + 2 H2 CH4 + 18 kcal

In an exothermic reaction,

• heat is released.• energy of products is

less than energy of reactants.

• heat is a product.

There are exothermic reactions

There are endothermic reactions

In an endothermic reaction

• Heat is absorbed.• The energy of the

products is greater than the energy of the reactants.

• Heat is a reactant (added).

N2 + O2 + 43.3 kcal 2NO

Reaction Rate and Catalysts

Reaction rate:• speed reactant is used up• speed at which product

formsCatalyst • increases rate of a reaction.• lowers energy of activation. • is not used up during reaction• increases when temperature rises because

reacting molecules move faster providing more colliding molecules with energy of activation