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