Chemical reactions occur when bonds (between the electrons of atoms) are formed or broken

Chemical reactions involve changes in the chemical composition of matter

the making of new materials with new properties energy changes:

Bond breaking absorbs Energy (endothermic process) Bond making releases Energy (exothermic process)

Symbols represent elements Formulas describe compounds Chemical equations describe a chemical

reaction

Balancing EquationsBalancing Equations

Chemical Equations

A chemical equation is written as an expression similar to a mathematic equation that can be compared to a recipe that a chemist follows in order to produce desired results.

Chemical EquationsChemical EquationsTheir Job: Depict the kind of Their Job: Depict the kind of

reactantsreactants and and productsproducts and and their relative amounts in a reaction.their relative amounts in a reaction.

44 Al Al (s)(s) + + 33 OO2 (g)2 (g) ---> ---> 22 AlAl22OO3 (s)3 (s)

The The numbers in the frontnumbers in the front are called are called

stoichiometric coefficientsstoichiometric coefficients

The letters (s), (g), and (l) are the The letters (s), (g), and (l) are the physical states of compounds.physical states of compounds.

Because of the principle of the Because of the principle of the conservation of matterconservation of matter

(matter can not be (matter can not be created or destroyed) created or destroyed) an an

equationequation must be must be balancedbalanced..

It must have the same number of It must have the same number of atoms of the same kind on both atoms of the same kind on both sides.sides.

Law of Conservation of Energy MUST ALSO BE FOLLOWED!

Energy changes are written in (endo-/ exothermic reactions)

Lavoisier, 1788Lavoisier, 1788

Chemical EquationsChemical Equations

Chemical Equations All chemical equations have reactants and

products. We express a chemical equation as follows:

Reactants Products The arrow is equivalent to an “=“ math. When

we describe the equation we use the word “yields” or “produces” instead of equals

Example

C + O2 CO2

This reads “carbon plus oxygen react to yield carbon dioxide”

Balancing a Chemical Equation

A chemical equation is balanced when the ions or atoms found on the reactant side of the equation equals that found on the product side.

The arrow can be considered the balance point.

Solid (s)Liquid (l)Gas (g)Aqueous solution (aq) (dissolved in water)

Catalyst H2SO4 or Pt

Escaping gas ()Change of temperature/ heat energy

( or + 3kJ or – 3kJ) *there is no subtraction…a negative sign means released/exothermic

Symbols Used in EquationsSymbols Used in Equations

Chemical Reactions and Chemical Equations

Reactants Products

Driving forces:

a) Color change

b) Formation of a solid/precipitate

c) Evolution of a gas

d) Evolution or absorption of heat

When balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but you may notnot change the subscripts.Changing the subscripts changes the

compound. Subscripts are determined by the valence electrons (charges for ionic or sharing for covalent)

Think back to naming compounds/ determining formulas. NaCl exists, because Na is + and Cl is -, but NaCl2 does NOT exist since you would not have a neutral compound! You can’t just add a number to a formula to balance an equation.

Balancing EquationsBalancing Equations

Subscripts vs. Subscripts vs. CoefficientsCoefficients

The subscripts The subscripts tell you how tell you how many atoms of many atoms of a particular a particular element are in a element are in a compound. The compound. The coefficient tells coefficient tells you about the you about the quantity, or quantity, or number, of number, of molecules of molecules of the compound.the compound.

Chemical EquationsChemical EquationsChemical EquationsChemical Equations

4 Al(s) + 3 O4 Al(s) + 3 O22(g) ---> 2 Al(g) ---> 2 Al22OO33(s)(s)

This equation meansThis equation means

4 Al atoms + 3 O4 Al atoms + 3 O22 molecules molecules ---produces--->---produces--->

2 molecules of Al2 molecules of Al22OO33

AND/ORAND/OR

4 moles of Al + 3 moles of O4 moles of Al + 3 moles of O22 --- ---produces--->produces--->

2 moles of Al2 moles of Al22OO33

There are four basic steps to balancing a chemical equation.1. Write the correct formula for the reactants and the

products. DO NOT TRY TO BALANCE IT YET! You must write the correct formulas first. **And most importantly, once you write them correctly DO NOT

CHANGE THE FORMULAS!2. Find the number of atoms for each element on the left

side. Compare those against the number of the atoms of the same element on the right side.

3. Determine where to place coefficients in front of formulas so that the left side has the same number of atoms as the right side for EACH element in order to balance the equation.

4. Check your answer to see if: The numbers of atoms on both sides of the equation

are now balanced. The coefficients are in the lowest possible whole

number ratios. (reduced)

Steps to Balancing Steps to Balancing EquationsEquations

Some Suggestions to Help Some Suggestions to Help YouYou

Some helpful hints for balancing equations: Take one element at a time, working left to right except

for H and O. Metals, then nonmetals are a good way, too. Save H for next to last, and O until last.

IF everything balances except for O, and there is no way to balance O with a whole number, double all the coefficients and try again. (Because O is diatomic as an element)

(Shortcut) Polyatomic ions that appear on both sides of the equation should be balanced as independent units

Balancing Balancing EquationsEquationsBalancing Balancing EquationsEquations

___ H___ H22(g) + ___ O(g) + ___ O22(g) ---> ___ H(g) ---> ___ H22O(l)O(l)2 2

What Happened to the Other Oxygen Atom?????

This equation is not balanced!

Two hydrogen atoms from a hydrogen molecule (H2) combine with one of the oxygen atoms from an oxygen molecule (O2) to form H2O. Then, the remaining oxygen atom combines with two more hydrogen atoms (from another H2 molecule) to make a second H2O molecule.

Balance this equation!

Na + Cl2 NaCl Na- 1 Na-1

Cl- 2 Cl-1

**note that the number of sodiums balance but the chlorine does not. We will have to use coefficients in order to balance this equation.

Inserting subscripts

Na + Cl2 2 NaCl

Na- 1 Na- 1 2

Cl- 2 Cl- 1 2

** Now the chlorine balances but the sodium does not! So we go back and balance the sodium.

Finally balanced!

2Na + Cl2 2 NaCl

Na- 1 2 Na-1 2

Cl- 2 Cl-1 2

**Since the number of each element on the reactant side and the product side of the equation are equal, the equation is balanced.

Balancing Balancing Equations EquationsBalancing Balancing Equations Equations

___ Al(s) + ___ Br___ Al(s) + ___ Br22(l) ---> ___ Al(l) ---> ___ Al22BrBr66(s)(s)2 3

Balancing Balancing EquationsEquationsBalancing Balancing EquationsEquations

Sodium phosphate + iron (III) oxide Sodium phosphate + iron (III) oxide sodium oxide + iron (III) phosphatesodium oxide + iron (III) phosphate

NaNa33POPO44 + Fe + Fe22OO33 ----> ---->

NaNa22O + FePOO + FePO44 22

22

33