"Things turn out best for those who make the best of the way things turn out."Jack Buck

CHEMICAL EQUATIONS

IN CHEMISTRY, WE ARE CONTINUALLY CONCERNED WITH CHEMICAL REACTIONS – COMBINING DIFFERENT SUBSTANCES TO MAKE A NEW POLYMER OR A NEW MEDICINE OR COMBUSTING A FUEL TO GENERATE ENERGY.

WE USE CHEMICAL REACTIONS CONTINUALLY IN OUR DAILY LIVES, EVEN COOKING FOOD.

WHEN WE TALK ABOUT CHEMICAL REACTIONS, WE ARE TALKING ABOUT CHANGE.

A CHEMICAL EQUATION DESCRIBES THAT CHEMICAL CHANGE (CHEMICAL REACTION).

2 H2 + O2 2 H2O

THE SUBSTANCES ON THE LEFT SIDE OF THE ARROW ARE THE REACTANTS, AND THE SUBSTANCES ON THE RIGHT SIDE OF THE ARROW ARE THE PRODUCTS.

THE ARROW CAN BE READ AS “YIELDS” OR “PRODUCES”.

SO, A CHEMICAL EQUATION TELLS US WHAT SUBSTANCES ARE REACTING AND WHAT SUBSTANCES ARE BEING PRODUCED.

THE COEFFICIENTS TELL US IN WHAT RATIO THE SUBSTANCES ARE REACTING ARE BEING PRODUCED.

2 H2 + O2 2 H2O

THIS EQUATION COULD BE INTERPRETED AS 2 MOLECULES OF HYDROGEN REACT WITH ONE MOLECULE OF OXYGEN TO PRODUCE 2 MOLECULES OF WATER. OR, 2 MOLES OF HYDROGEN REACT WITH ONE MOLE OF OXYGEN TO PRODUCE 2 MOLES OF WATER.

TO BE CORRECT, A CHEMICAL EQUATION MUST BE BALANCED, THAT IS, IT MUST HAVE THE SAME NUMBER OF ATOMS OF EACH KIND ON ONE SIDE OF THE EQUATION AS THE OTHER.IN CHEMICAL CHANGES, ATOMS ARE NEITHER CREATED OR DESTROYED. THE LAW OF CONSERVATION OF MASS MUST HOLD.

2 H2 + O2 ---> 2 H2O

IN 2H2 THERE ARE 4 HYDROGEN ATOMS.IN 2H2O THERE ARE 4 HYDROGEN ATOMS.IN O2 THERE ARE 2 OXYGEN ATOMS.IN 2H2O THERE ARE 2 OXYGEN ATOMS.

OR, IN TERMS OF MASS

2 H2 + O2 ---> 2 H2O

2 x (2 x 1.01 g) + (2 x 16.0 g) = 2 (2 x 1.01 g + 2 x 16.0 g)

4.04 g hydrogen plus 32.0 g oxygen yields 36.04 g water

SO,

2 H2 + O2 ---> 2 H2O 2 molecules 1 molecule 2 molecules

2 moles 1 mole 2 moles 2(6.02 x 1023 molecules) (6.02 x 1023 molecules) 2(6.02 x 1023 molecules) 4.04 g 32 g 36.04 g

A BALANCED CHEMICAL EQUATION CAN BE INTERPRED SEVERAL DIFFERENT WAYS, AND IT GIVES US A TREMENDOUS AMOUNT OF INFORMATION.

NOTE: IF THERE ARE THE SAME NUMBER OF ATOMS OF EACH KIND ON BOTH SIDES OF THE EQUATION, THE MASS MUST ALSO BE THE SAME.

SO, THE LAW OF CONSERVATION OF MASS HOLDS.

QUICK REVIEW: HOW MANY ATOMS OF EACH KIND ARE IN

3 Ca(NO3)2

3 Ca(NO3)2

WE COULD BE TALKING ABOUT 3 MOLECULES OR 3 MOLES OF Ca(NO3)2, BUT LET’S ASSUME WE ARE TALKING ABOUT MOLECULES (or formula units, since it is ionic) HERE.

TAKE Ca. THERE ARE 3Ca(NO3)2 SO THERE ARE 3 Ca ATOMS

TAKE N. THERE IS ONE N IN NO3, THERE ARE TWO NO3, AND THERE ARE 3 Ca(NO3)2 SO THERE ARE 6 N ATOMS.

TAKE O. THERE ARE 3 O IN NO3, THERE ARE 2 NO3, AND THERE ARE 3 Ca(NO3)2, SO THERE ARE 18 O ATOMS.

BALANCE THE FOLLOWING EQUATIONS:

Zn + HCl ---> ZnCl2 + H2

KClO3 ---> KCl + O2

S8 + F2 ---> SF6

Fe + O2 ---> Fe2O3

C2H6 + O2 ---> CO2 + H2O

SnO2(s) + 2 H2(g) → Sn(s) + 2 H2O(g)

YOU CAN INCLUDE ADDITIONAL INFORMATION IN A CHEMICAL EQUATION SUCH AS THE PHYSICAL STATE OF THE REACTANTS AND PRODUCTS.

THIS IS DONE IN PARENTHESIS AFTER THE FORMULA FOR THE SUBSTANCE. HERE, s IS SOLID, l IS LIQUID, AND g IS GAS.

WE CAN CLASSIFY REACTIONS INTO SEVERAL DIFFERENT TYPES.

THE SIMPLEST TYPE WOULD BE A COMBINATION REACTION – A REACTION IN WHICH TWO OR MORE SUBSTANCES COMBINE TO FORM A SINGLE SUBSTANCE, FOR EXAMPLE:

2 Mg + O2 2MgO magnesium oxide

Fe (s) + S (s) FeS (s) iron (II) sulfide

THE REVERSE OF A COMBINATION REACTION WOULD BE A DECOMPOSITION REACTION, WHERE A SUBSTANCE IS DECOMPOSED INTO TWO OR MORE REACTANTS, FOR EXAMPLE:

2HgO (s) 2 Hg (l) + O2 (g)

IN A SINGLE REPLACEMENT REACTION, ONE ELEMENT REPLACES ANOTHER ELEMENT IN A COMPOUND, FOR EXAMPLE:

2 Na + 2 H2O 2 NaOH + H2

Cu + AgNO3 CuNO3 + Ag

IF ONE METAL DISPLACES ANOTHER IN A COMPOUND DEPENDS ON THE RELAIVE ACTIVITY OF THE TWO METALS. THERE IS AN ACTIVITY SERIES OF METALS THAT COMPARES THE RELATIVE ACTIVITY. A REACTIVE METAL WILL DISPLACE ANY METAL BELOW IT IN THE SERIES.

AN INTERESTING SINGLE DISPLACEMENT REACTION IS THE THERMITE REACTION:

2Al + Fe2O3 Al2O3 + 2Fe + (lots of heat)

A DOUBLE DISPLACEMENT REACTION INVOLVES THE EXCHANGE OF POSITIVE IONS BETWEEN TWO COMPOUNDS.

THIS OFTEN OCCURS IN SOLUTIONS WHEN THE MIXING OF TWO IONIC SOLUTIONS RESULTS IN THE FORMATION OF A SOLID (A PRECIPITATE).

NaCl (aq) + AgNO3 (aq) NaNO3 (aq) + AgCl (s)

Na2S (aq) + Cd(NO3)2 (aq) CdS (s) + 2NaNO3

A COMBUSTION REACTION INVOLVES THE REACTION OF A SUBSTANCE WITH OXYGEN ACCOMPANIED BY THE PRODUCTION OF HEAT AND LIGHT.

2 Mg (s) + O2 (g) 2MgO (s)

CH4 (g) + 2O2 (g) 2H2O (g) + CO2 (g)

NOTE: ALL CHEMICAL REACTIONS FOLLOW THE LAW OF CONSERVATION OF MASS, WHICH SAYS THAT IN A CHEMICAL REACTION MATTER IS NEITHER CREATED OR DESTROYED.

IN OTHER WORDS, YOU HAVE THE SAME MASS OF PRODUCTS AS YOU HAVE REACTANTS – THE SAME MASS YOU END UP WITH AS THE MASS YOU START WITH.

IF THIS IS TRUE, YOU HAVE THE SAME NUMBER AND TYPE ATOMS AS PRODUCTS AS YOU HAD AS REACTANTS.

THE EQUATION MUST BE BALANCED

BALANCE EACH EQUATION, AND IDENTIFY ITS TYPE:

a.Hf + N2 Hf3N4

b.Mg + H2SO4 MgSO4 + H2

c.C2H6 + O2 CO2 + H2O

d.Pb(NO3)2 + NaI PbI2 + NaNO3

e.Fe + O2 Fe2O3

GO TO: http://www.sciencegeek.net/Chemistry/taters/EquationBalancing.htm

COMPLETE THE EXERCISE AND THEN CHECK YOUR ANSWERS.