1. Chemical Reactions

2. What Are Chemical Reactions
Chemical Reactions occur when two or more substances chemically combine (react) to produce a new substance with different properties than the original substances
-- in other words, chemical reactions cause chemical changes
Chemical reactions can occur in the bonding of two elements to form a compound, or they can occur between two compounds to form one or more new compounds

3. Evidence for Chemical Reactions
There are many pieces of evidence that you can look for in order to see if a chemical reaction has occurred when two substances are mixed:
-- formation of gas bubbles
-- color change
-- formation of a precipitate
-- production of new materials with different properties
-- changes in energy/temperature

4. Formation of Gas Bubbles
Formation of gas bubbles usually occur when two liquids react
-- you cannot see the gas in solids or gases
Gas bubbles are usually not enough information alone to indicate a chemical reaction
-- for example, when water is boiled, it produces bubbles, but this is only a physical change
Usually, bubbles and a second characteristic are used to detect the presence of a reaction

5. Color Changes Color changes usually are a good indicator of a reaction
-- often the new substance may have a different color than the original substance
Many times we add a compound called an indicator to detect a reaction
-- an indicator is a compound that will change color under certain conditions
-- for example, the indicator phenolphthalein turns colorless in acids and pink in bases
-- if the indicator turns color, the pH or temperature has changed, indicating a reaction occurred

6. Precipitate
A precipitate is a solid that falls out of solution during a chemical reaction
-- happens when two liquids are combined
Precipitates can be many colors depending on the type of compound used
Precipitates are GREAT indicators of a reaction occurrence

7. Production of New Materials
If chemical reactions aren’t always obvious from color changes, bubbles, or precipitates, we can always test the new substances for new properties
-- for example, marble becomes crumbly when reacted with acid rain
-- also, iron loses is magnetic properties when reacted with sulfur
-- water is a liquid, different than hydrogen and oxygen gases

8. Changes in Energy/Temperature
Some reactions absorb energy and some reactions release energy
-- this energy is noticed by us in the form of heat, which we notice as a temperature change
Based on whether the reaction absorbs or releases energy, the reaction can be classified as endothermic or exothermic

9. Endothermic Reactions
Endothermic Reactions are reactions that absorb heat from the environment
-- they do this because heat is needed to cause the reaction to occur
When heat is removed from the environment, the reaction feels colder
-- this is how those crunchy cold-packs work!

10. Exothermic Reactions
Exothermic Reactions are reactions that release heat to their environment
-- this reaction is so reactive that it gives off heat readily
We feel the results of exothermic reactions as heat
Many reactions we are familiar with are exothermic:
-- burning of fossil fuels
-- disposable heat packs

11. Bonds and Chemical Reactions
In order to make new substances from original substances, you have to break bonds and make bonds
-- breaking bonds of old substance releases energy
-- making bonds of new substances requires energy
For a reaction to be complete, all atoms of one substance must come in contact will all of the atoms of the other substance
The stronger the chemical bonds in a substance, the less likely a substance is to react chemically

12. Writing Chemical Reactions

13. Chemical Equations
When a chemical reaction occurs, we write it as a chemical equation
There are two parts to a chemical equation:
The reactants are located on the left side of the reaction arrow and consist of the atoms or compounds that react together
The products are location on the right side of the reaction arrow and consist of the atoms or compounds produced by the chemical reaction

14. Identifying Reactants and Products
What are the reactants and the products of the following chemical reactions?
1) 4 Al (s) + 3 O2 (g) 2 Al2O3
Reactants:
Products:
2) Zn2SO NaNO3 2 ZnNO Na2SO4
Aluminum and Oxygen
Aluminum Oxide
Zinc (I) Sulfate and Sodium Nitrate
Zinc (I) Nitrate and Sodium Sulfate

15. The Numbers in a Chemical Equation
When we write a chemical equations, there are two different numbers that are extremely important, subscripts and coefficients
Subscripts are the small numbers located on the right of each atom in a compound that tell you how many of each type of atom are in the compound
Ex: H2SO4 has 2 Hydrogen atoms, 1 Sulfur atom (we never write a subscript of 1!), and 4 oxygen atoms
If there is a polyatomic ion in parenthesis, we need to multiply the subscript outside the parenthesis by the subscript on each atom in the polyatomic ion
Ex: Mg(NO3)2 has 1 magnesium atom, 2 (or 2 x 1) nitrogen atoms, and 6 (or 2 x 3) oxygen atoms

16. Coefficients
Coefficients are the big numbers to the left of each compound in a chemical reaction.
-- coefficients tell you how many molecules of each compound take place in the reaction
-- only change coefficients when balancing an equation. If you change the subscripts then you are completely changing what the reaction is saying.
For example, in the equation:
4 Al O Al2O3
I have: 4 molecules (or atoms) of solid aluminum
3 molecules of oxygen gas (O2)
2 molecules of aluminum oxide (Al2O3)

17. Practice With Subscripts and Coefficients
In the reaction:
Zn2SO NaNO ZnNO Na2SO4
In the reactants:
How many atoms of zinc do I have?
How many atoms of sulfur do I have?
How many atoms of sodium do I have?
How many atoms of oxygen do I have?
How many atoms of nitrogen do I have? 2 1 2 10 2

18. Chemical Reactions and Mass
REMEMBER, mass is neither gained nor lost during a chemical reaction. . .
THEREFORE:
THE NUMBER OF EACH TYPE OF ATOM IN THE REACTANTS EQUALS THE NUMBER OF EACH TYPE OF ATOM IN THE PRODUCTS
So, if there are 12 total oxygen molecules in the products, there must be 12 total oxygen molecules in the reactants and vice versa

19. Practice With Balancing Equations
In the reaction:
Zn2SO4 + NaNO ZnNO Na2SO4 2 2
Make a table with it’s middle line between the reactants and the products and label the columns: reactants on the left and products on the right.
Reactants
Products
That “2” coefficient also effects the N and the O in that compound
Next write the numbers of each element you have on each side of the equation. Zn S O Na N
Remember to multiply coefficients by the subscripts to get the total number of atoms in the compound
Next step, figure out what coefficients you’ll need to make the amount of atoms on each side (R and P) the same.
Set up two different OXYGEN rows since it appears in two different compounds in the equation 2 2 1 4 3 Zn S O Na N 1 4 2 3
Last step, double check that all of your atoms are = on both sides of the equation; here, add the O atoms on each side and the total will be the same. 2 2 2 6 6

20. Types of Chemical Reactions
For our class, there are only three types of chemical reactions that we need to worry about.

21. Type #1: Synthesis Reactions
Synthesis reactions occur when 2 or more substances combine to form a larger, more complex compound
-- also called a combination reaction
-- also called an addition reaction
THERE WILL ONLY BE 1 COMPOUND AS A PRODUCT
Mostly follows the equation:
A + B AB +

22. Type #2: Decomposition Reactions
Decomposition reactions are the opposite of synthesis reactions
Large compounds break down into smaller compounds or elements
THERE IS ALWAYS ONLY 1 REACTANT
Mostly follow the formula:
AB A + B +

23. Type #3: Replacement Reactions
In a replacement reaction, one element replaces another element in a compound
Mostly follows the formula:
A + BC AC + B + +

24. Type #3: Replacement Reactions
There can also be double replacement reactions
Always occurs between two ionic compounds
-- the ions “switch partners”
-- the cation of one compound partners with the anion of the other compound
Always follows the general formula:
AB + CD AD + CB + +