When chemicals react, they break apart and form new chemicals

Chemical Reactions

Physical: no new substances are formed• Melting, boiling, breaking, cutting,

ripping, dissolving

Chemical:

• totally new substances are formed

Types of Changes

• Bubbling – gas formed

• Turns cloudy – precipitate formed

• Color change

• Smell

• Temperature change• Exothermic – heat given off => the reaction

gets hot• Endothermic – heat taken in => the reaction

gets cold

Evidence of a chemical change

Chemical Equation

2H2 + O2 → 2H2O

Before After

Reactants

Products

Coefficients: # of molecules/compounds

Subscripts: # of atoms/ions in the formula

Example: 2Li3N = Li3N + Li3N

6 Li atoms and 2 N atoms

Example:Be(NO3)2 = Be(NO3)

(NO3)

1 Be atom, 2 N atoms, 6 O atoms

The Law of conservation of mass states that matter cannot be created or destroyed in any chemical reaction The atoms in the reactants are rearranged to form new compounds, but none of the atoms disappear, and no new atoms are formed.

HH

HH O O

Reactants Products

A chemical equation identifies the starting and ending chemicals as reactants and products:

reactants products

Formation of water

2H2 + O2 2H20

A chemical equation is balanced when it reflects the conservation of mass.

Graphic: http://www.chemistry.ohio-state.edu/betha/nealChemBal/

The same number of each kind of atom must be on the left side of the arrow as are on the right side when an equation is balanced.

Al Al

Al Al

O O

O OO O

Al AlOO O

Al AlOO O

Oxygen, as a product, must be multiplied by 2. A coefficient is placed in front of the compounds that contains O.

Oxygen atoms are =When a coefficient is added you must

multiply all atoms in the compound by this

number. Redo the inventory

After redoing the inventory, add coefficients to balanced other atoms. With each additional coefficient the inventory must be updated.

Oxygen atoms are≠

Hydogen atoms are =

2 H atoms 2 H atoms

2 O atoms 1 O atom

Hydogen atoms are ≠Oxygen atoms are =

First: Inventory the atoms on both sides of the yield arrow

Balancing an equation:

H2 + O2 H2O

HH

HO O O

H

Second: Determine which atoms are not equal and add coefficients to balance these atoms.

2

4 H atoms

2 O atoms

2

4 H atoms

2 O atoms

Hydogen atoms are =

This is now a balanced equation

So: Chemical equations are balanced, when the numbers and kinds of atoms on each side of the reaction arrow are equal.

HH

HH O O

Reactants Products

HH

HH

OO

4 Hydrogen 4 Hydrogen

2 Oxygen 2 Oxygen

Because atoms are conserved, the mass of

the reactants in the chemical reaction is

equal to the mass of the products.

H2 + F2 2 HF

2g + 38g = 2( 20g)

40g = 40g

40 g 40 g

Reactants Products

H H F F H F H F

The Law of Conservation of Mass allows the calculation of the reactants or products.

If 27.0 g of mercuric oxide (the red solid compound) is heated and completely decomposes to give the elements oxygen and mercury. 2.0 g of oxygen are produced. How many grams of mercury are produced?

mercuric oxide mercury + oxygenHgO Hg + O2

Reactants Products

27.0 g HgO

? g Hg

2.0 g O2

27 g HgO = 2 g O2 + ? Hg =25 g Hg

Which of the following chemical reactions indicate that mass in conserved?A. Mg + Cl2 2 MgCl

B. 2 Ca + O2 2 CaO

C. Zn + S 2 ZnS

D. C + O2 2 CO

The correct answer is B

CaCa

OO

OOCa

Ca

Because atoms must be equal on each side of the equation:

Mg + HCl → MgCl2 + H2

When the above equation is balanced, the coefficient for magnesium is —

A 0

B 1

C 2

D 4

1 Mg + 2 HCl → MgCl2 + H2

The correct answer is B

Suppose 100 g of iron metal rusts. We weigh the rust and find that the rust has a mass of 143 g. What mass of oxygen reacted with the iron?

A. 243 g B. 57 g C. 100 gD 43 g

43 g Oxygen Choice D

Iron + Oxygen Rust100 g + ?g 143g

What mass of

oxygen is needed?

Mass of the reactants = Mass of the products

(also # of atoms before = # of atoms after)

2H2 + O2 → 2H2O 2 x 2 H atoms + 2 O atoms = 2 (2 H atoms + 1 O atom)

4 x 1 = 4 amu + 2 x 16 = 16 amu = 2(2x1 amu + 16 amu)

36 = 36 amu

Conservation of Mass

Open vs. Closed systems

Na + N2 → Na3N

MgCl2 + Li2O → MgO + LiCl

Practice