
a chemical reaction is the process of breaking
of chemical bonds (ionic or covalent) in one
or more substances, and then reforming of
new bonds to create new substances

indicators of chemical change may be:
1.
formation of new gas
2.
formation of new precipitate (solid)
3.
release (heat or light) or absorption
of energy
4.
color change

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”
Energy is either released or absorbed
during a chemical reaction
 exothermic reaction– energy is released
(burning)

endothermic reaction– energy is absorbed
kept in the bonds (cold packs)

some very important chemical reactions

burning of fossil fuels release LOTS of
energy in the form of heat

photosynthesis

rusting of metals

oxygen is chemically added to the metal

Conservation of mass
matter cannot be created nor destroyed in
chemical reactions
 all of the atoms that started the reaction
must be present after the reaction is over


chemical equations convey as much info. as
possible about what happens in a chemical
reaction during a chemical change
What is wrong with this?


each side of the equation must have the same
number of atoms of each elements (atoms
cannot disappear or be created)
bicycle example

skeletal equation


balanced equation


frame + wheel + handlebar + pedal → bike
frame + 2 wheels + handlebar + 2 pedals → bike
tricycle example

skeletal equation


frame + wheels + handlebar + bell → tricycle
balanced equation

frame + 3 wheels + handlebar + bell → tricycle
NOT BALANCED!

this is NOT balanced

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
?KCl + ? H2SO4  ? HCl + ? K2SO4
2KCl + H2SO4  2HCl + K2SO4
NaCl + AgNO3  AgCl + NaNO3
balanced
__ Al + __ O2  __ Al2O3
4Al + 3O2  2Al2O3
__ Na2SiO3 + __ HF  __ H2SiF6 + __ NaF + __ H2O
Na2SiO3 + 8HF  H2SiF6 + 2NaF + 3H2O

balanced equation for lab tomorrow or
Friday


C6H8O7 + 3NaHCO3 → 3H2O + 3CO2 + Na3C6H5O7
word equation for lab

citric acid + sodium bicarbonate ---> water +
carbon dioxide + sodium citrate