Describing
Chemical
Reactions
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 gas2. formation of new precipitate (solid)3. release (heat or light) or absorption
of energy4. 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”
exothermic reaction– energy is released (burning)
endothermic reaction– energy is absorbed kept in the bonds (cold packs)
Energy is either released or absorbed during a chemical reaction
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?
Balancing Chemical Equations 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
frame + wheel + handlebar + pedal → bike balanced equation
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
How about these? ?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