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 jet of 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