Unit 2 – Lecture 8

Chemical EquationsA shorthand way to describe a chemical

reaction using chemical symbols and formulae

Three Parts to a Chemical EquationReactants

substances present before the reactionYield Arrow

indicates the direction of a reactionsome rxns are reversible [double-sided

arrow]Products

substances present after the reaction

Chem. Equations – cont’dChemical symbols give a “before-and-after”

picture of a chemical reaction

Reactants Products

MgO + C CO + Mg

magnesium oxide to form carbon monoxidereacts with carbon and magnesium

or… magnesium oxide reacting with carbon yields carbon monoxide and magnesium

Chemical Equations – cont’dCoefficient – (large) # in front of an element

applies to all elements in the group to the right of it

ex: 4C 4 carbon atoms

ex: 5KI 5 potassium atoms & 5 iodine atoms

ex: 4SO4

4 Sulfur atoms & 16 Oxygen atoms (4x4)

Chemical Equations – cont’dSubscript – (small) # to the lower right of an

elementapplies only to the element it is attached toif outside of parentheses [(XY)3]

applies to everything inside of parenthesesex: H2

2 hydrogen atomsex: CO2

1 carbon atom, 2 oxygen atomsex: (SO4)3

3 Sulfur atoms (1x3), 12 Oxygen atoms (4x3)

Chemical Equations - cont’dTry this:Al2(SO4)3

Aluminum?2 atoms

Sulfur?3 atoms

Oxygen?12 atoms (4x3)

Balancing Chem EquationsLaw of Conservation of Matter

matter is neither created nor destroyed in a chemical reaction, it is conserved

biology: energy is lost, matter is RECYCLED(changed from one thing to another)

ie, if you begin a reaction with 5 atoms of Hydrogen, you must end up with 5 atoms of Hydrogen

Balancing – cont’dBalanced = same # of each element on each

side

Al + S Al2S3

2Al + 3S Al2S3

Unbalanced

Balanced

Balanced or Unbalanced?C3H8 + 5O2 4H2O + 3CO2

Carbon? = 3 vs 3

Hydrogen? = 8 vs 8

Oxygen = 10 vs 10 (4 + 6)

Is it balanced?

Yes.

Balanced or Unbalanced?Fe3O4 + H2 Fe + H2O

Iron = 3 vs 1

Oxygen = 4 vs 1

Hydrogen = 2 vs 2

Is it balanced?

No.So, now what?

Balancing a Chemical ReactionSet up two columns: labeled “reactants”

&“products.” Identify & record reacting elements on the left.Record elements in the SAME ORDER on the right.Count & Record # of each reacting element.Count & Record # of each element produced.Unequal? Balance Coefficients. [can’t change subscripts]

you can treat Polyatomic Ions as one if they appear on both sides of the equation like (SO4) in parentheses

change your greatest inequality firstleave solitary elements for last

Practice Fe3O4 + H2 Fe

+ H2O

PracticeOn your worksheet, complete the two

problems on the front of your sheet.

____H2 + ____O2 ____H2O

____Cl2 + ____NaBr ____Br2 + ____NaCl

2 1 2

1 2 1 2

HomeworkPage 6 –

Balancing Chemical Equations w/s1-9 on-levelall for honors