Reactants: Zn + I2
Product: ZnI2

used to convey as much info. as possible about
what happens in a chemical reaction
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Word Equations

write out what chemicals are reacting
hydrogen peroxide → water + oxygen
 hydrogen + oxygen → water
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Chemical Equations

show the chemical formulas of the chemicals
reacting
 H2O2 (aq) → H2O(l) + O2(g)
 H2(g) + O2(g) → H2O (l)
(s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous
solution (dissolved in water)
 the above are often referred to as skeletal equations
because they are not mathematically balanced


chemical equations show the conversion of
reactants (the molecules shown on the left of
the arrow) into products (the molecules
shown on the right of the arrow).
 a (+) sign separates molecules on the
same side
 the arrow is read as “yields”
 example
C + O2  CO2
 this reads “carbon plus oxygen react to
yield carbon dioxide”

a catalyst is a substance that speeds up the
reaction but is not used up in the reaction
MnO4
H2(g) + O2(g)
H2O (l)
 this is written small over the yield arrow



each side of the equation must have the
same number of atoms of each element
bicycle example



frame + wheel + handlebar + pedal → bike
frame + 2 wheel + handlebar + 2 pedal → bike
tricycle example


frame + wheels + handlebar + tire → tricycle
frame + 3 wheel + handlebar + 3 tire → tricycle

when balancing a chemical reaction you
may add coefficients in front of the
compounds to balance the reaction
coefficients represent the number of atoms
(or moles) of that compound
 sometimes easier if you balance O and H at
the end


you may NOT change the subscripts

changing the subscripts changes the
compound

ex: H2O (water) cannot be changed into H2O2
(hydrogen peroxide) in order to help you balance
the equation
 H2(g) + O2(g) → H2O (l)


subscripts tell
you how many
atoms of a
particular
element are in a
compound.
coefficients tells
you about the
quantity, or
number, of
molecules of the
compound

this is NOT balanced
1. A solution of sodium iodide is added to a solution of
potassium nitrate yields a potassium iodide precipitate
and a sodium nitrate solution.
NaI (aq) + KNO3 (aq)  KI (s) + NaNO3 (aq)
already balanced
2. Magnesium metal burns in oxygen gas with a bright white
light to make a white powder called magnesium oxide.
Mg (s) + O2 (g)  MgO (s) + heat
2Mg (s) + O2 (g)  2MgO (s) + heat
3. Gaseous hydrogen (dihydride) and gaseous oxygen
(dioxide) react explosively to form water vapor.
H2 (g) + O2 (g)  H2O (g) + heat
2H2(g) + O2(g)  2H2O (g) + heat
___
2 Al(s) + ___
3 Br2(l) ---> ___ Al2Br6(s)
?
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
Al(s) + O2(g)  Al2O3(s)
4Al(s) + 3O2(g)  2Al2O3(s)
NaCl (aq) + AgNO3 (aq)  AgCl (s) + NaNO3 (aq)
balanced
Na2SiO3 + HF  H2SiF6 + NaF + H2O
Na2SiO3 + 8HF  H2SiF6 + 2NaF + 3H2O