Chemical Reactions

Chemical reactions involve changes in matter—the making of new materials with new properties accompanied by energy changes.

Chemical Reactions are described using shorthand called chemical equations.

IntroductionChemical reactions occur when bonds between the outermost parts of atoms are formed or broken.

Four Indicators of a Chemical Reaction

1. Change in color

2. Release of a gas

3. Formation of a precipitate

4. Change in energy—of light or heat

The only absolute proof of a chemical reaction is that a new substance has been produced!

But…

Chemical Chemical EquationsEquations

A+B C+DREACTANTS PRODUCTS

Left of Arrow: Substances that existed before the reaction took place.

Right of Arrow: Substances that are produced in the reaction.

Three Types of Equations… 1.) Word Equations

Propane + Oxygen Carbon Dioxide + Water + Energy

Can see the products and the reactants

2.) Formula Equations C3H8 + O2 CO2 + H2O + energy Can see the formulas & how many atoms of each element

are reacting

3.) Formula Equations w/ physical state indicated H2(g) + O2(g) H2O(l) + energy Indicates state of matter in reaction

Symbols Used in a Chemical Equation

, reacts with

, yields

MnO2 catalyst was used (catalyst written above the arrow)

In a Chemical Equation…Symbols are used to show:

How many of the compound/elements? – Coefficient

How many of each element? – Chemical formula Individual elements – atoms Covalently bonded elements – molecules Ionic compounds – formula units or just units

In a Chemical Reaction…

Total mass stays the same Atoms are only rearranged, not changed into

different ones

4 H

2 O4 H

2 O

4 g 32 g

36 g

Mass is neither created nor destroyed—Law of the Conservation of Mass and Energy

So…how does the conservation of mass affect chemical equations?

The types and amount of atoms on the reactant side must be the same as on the product side.

The equation must be BALANCED.C + O2 → CO2

1 atom of carbon plus 2 atoms of oxygen yields 1 atom of carbon and 2 atoms of oxygen.

Balancing Equations

1. Write the unbalanced equation.

2. Count atoms on each side.

3. Calculate coefficients to make #s equal.

Coefficient subscript = # atoms

4. Reduce coefficients to lowest possible ratio.

5. Double check atom balance!!!

Helpful Guidelines1. Balance one element at a

time.2. Update ALL atom counts

after adding a coefficient.3. If elements appear more

than once per side, balance them last.

4. Balance polyatomic ions as single units.

“1 SO4” instead of “1 S” and “4 O”

More Helpful Guidelines5. Balance diatomic molecules

last.6. Remember: change

coefficients only—not subscripts.

Go in this order:MetalsNonmetals except H & OPolyatomicsHO

For example…For example…

One atom of solid zinc (reacts with) 2 molecules of aqueous hydrochloric acid (to produce)One unit of aqueous zinc chloride andOne molecule of hydrogen gas

• How many?• Of what?• In what state?

Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)

Try one backwards… One molecule of oxygen gas Reacts with Two molecules of hydrogen gas To produce Two molecules of liquid water.

O2 (g) + 2 H2 (g) → 2 H2O (l)Write the equation.

Try another one…

Two atoms of aluminum react with three units of aqueous copper (II) chloride to produce three atoms of copper and two units of aqueous aluminum chloride.

• How many?• Of what?• In what state?

Al 2 (s) + 3CuCl2(aq) 3 Cu(s) + 2AlCl3(aq)

Al + CuCl2 Cu + AlCl3

Al

Cu

Cl

1 1

1 1

2 3

Balancing ExampleAluminum and copper(II)chloride form copper and aluminum chloride.

3 2

6 / / 63 /

3

/ 32 /

2

/ 2

Try these examples… AgNO3 + Cu Cu(NO3)2 + Ag

2 AgNO3 + Cu Cu (NO3)2 + 2 Ag

Fe(OH)3 Fe2O3 + H2O2 Fe(OH)3 Fe2O3 + 3 H2O

NH3 + O2 NO + H2O4 NH3 + 5 O2 4 NO + 6 H2O

Try these examples…

NH3 + O2 NO2 + H2O4 NH3 + 7 O2 4 NO2 + 6 H2O

C2H4 + O2 CO2 + H2OC2H4 + 3 O2 2 CO2 + 2H2O