Chemical Reactions
Section 1 Chemical Changes
A. Describing chemical reactions- change of one
or more substances into new substances
1. Reactants are substances that combine or
change
2. New substances that are produced are
called products.
B. Conservation of Mass- a law which states
that, in a chemical reaction, matter is not
created or destroyed; it stays the same
1. Antoine Lavoisier- experimented with
mercury(II) oxide and heat
2. Found mass of products (liquid mercury
and oxygen gas) equaled mass of reactants
C. Writing equations- a chemical equation uses
chemical formulas and symbols to describe a
chemical reaction and the product(s) it
produces
1. Chemical formula expresses the relationship
between elements in the compounds and
molecules they make up
2. Coefficients- numbers which represent the
number of units of each substance in a
reaction
3. Knowing coefficients of chemical reactions
allows chemists to use the correct amounts of
reactants to predict the amounts of products
4. Subscripts- numbers which represent the
number of atoms in a molecule of a particular
element.
5. Symbols used to show state of reactants:
(s)solid, (aq) aqueous, (g)gas, (clear) liquid.
When a chemical has been dissolved in water, this is
denoted by writing (aq) after the chemical name.
D. Metals react with atmosphere in different
ways.
Section 2 Chemical Equations
A. Checking for balance- law of conservation of
mass requirement
1. A balanced chemical reaction- both sides of
equation have the same number of atoms of
each element
2. Choosing coefficients- becomes easier with
practice; trial and error at first
B. Writing balanced chemical reactions- a four
step process
1. Describe the reaction in words.
2. Write the equation using formulas and
symbols.
3. Check for balance.
4. Add coefficients where needed for balance.
Section 3 Classifying Chemical Reactions
A. Synthesis reaction- two or more
substances form a new substance; A + B --->
C
+
->
B. One substance breaks down into two or
more substances in a decomposition reaction
AB ---> A + B
The starting compound is ammonium dichromate. When heated, it begins to decompose into
nitrogen gas, water vapor and powdered chromium (III) oxide. It looks like a volcano with ash
being spread all over the place.
http://www.youtube.com/watch?v=jowikn6tsyY
->
+
C. Single-displacement reaction- one element
replaces another one in a compound:
A + BC ---> AC + B or D + BC ---> BD + C
A + BC --> AC + B
+
->
+
D. A double-displacement reaction results if a
precipitate, water, or a gas forms when two
ionic compounds in solution are combined; AB
+ CD --> AD + CB
AB + CD --> AD + CB
+
->
+
Combustion- O2 as a reactant and CO2 and
H2O as the product
MgCO3 + 2HCl --> MgCl2 + H2O + CO2
Section 4 Chemical Reactions and Energy
A. Chemical reactions involve energy
exchange.
Reaction of sulfuric acid and sugar.
1. Breaking chemical bonds requires energy.
Dr. Pyenta melted some potassium chlorate in a test tube over a Bunsen burner in a hood. Then he dropped in a sugar cube. The result was a considerable je
flame that burned for 30 seconds or more.
2. Forming chemical bonds releases energy.
B. More energy out
1. Exergonic reactions- energy required to break
bonds is less than the energy released from
new bonds; energy given off is usually light
2. Exothermic reactions- energy given off in the
form of heat
C. More energy in
1. Energonic reactions- more energy is
required to break bonds than to form new
ones; need energy for the reaction to occur
2. If energy needed is heat; the reaction is
endothermic.
vinegar & baking soda
3. A catalyst speeds up a chemical reaction
without itself being permanently changed.
Enzymes speed up
chemical reactions
4. An inhibitor prevents or slows a chemical
reaction or interferes with a catalyst's action