1. Chapter 6: Intro to Chemical Reactions
Think of some things in our everyday lives that could possibly be a result of chemicals reacting.
Chemical Reaction: The process by which the atoms of one or more substances are re-arranged to form different substances.
1. Also called a “chemical change”
2. they often give a visual signal
3. they are represented by a chemical equation
The word “reaction” is commonly abbreviated to “rxn”

2. Evidence for a Chemical Reaction
A temperature change has occurred.
1. Some absorb heat
a. Would “sense” a coolness in temperature
2. Some release heat (like a forest fire)
A color change is seen
1. a nail placed outside will change color from silver to orange/brown (rust)
An odor is emmitted (like “rotten egg” smell)
Gas bubbles appear
A solid appears when liquids are mixed together

3. Sect. 6.2: Representing a chemical reaction thru “Chemical Equations”
Chemical reactions (changes) always involve the rearrangement of the ways in which atoms are grouped.
1. This “rearrangement” is represented in a statement called a chemical equation.
Chemical equation: a statement, using chemical formulas and an arrow symbol, to represent the rearrangement of atoms that has taken place during a chemical reaction.

4. Chemical Equations
A chemical equation has two parts: reactants & products
1. reactants: the starting substances in a chemical equation
a. Before the reaction takes place
b. Are located to the left of the arrow ()
2. products: the substances that are formed during the rxn
a. Are located to the right of the arrow ()
3. The reactants and the products are separated by an arrow , which indicates the direction in which the reaction is taking place
a. The arrow is read as “yields” or “reacts to produce”
4. If there are multiple reactants or multiple products, they are separated from each other by + signs:
EX: reactant 1 + reactant 2  product 1 + product 2

5. Chemical Equations
Symbols are also used to show the physical states of the reactants and products:
1. solid = (s)
2. liquid = (l)
3. gas = (g)
4. aqueous = (aq)
a. Indicates that a substance is dissolved in water
Symbols are written to the right of the reactant & product chemical formulas (with parenthesis)
Remember: H2, N2, O2, F2, Cl2, Br2, and I2 (diatomic)

6. Chemical Equations
Chemical equations do not express numerical equalities (like = 2)
1. reactants are ingredients that are “used up” as products are formed.
2. EX:
CH4 + O2  CO2 + H2O
All chemical equations obey the “law of conservation of matter/mass”.
1. atoms are neither created or destroyed during a chemical reaction.
a. All atoms present within the reactants must be present in the products.

7. Sect. 6.3: Balancing Chemical Equations
When we make sure that all of the atoms present in the reactants are the same as what is present in the products, we are going through a process known as “Balancing the Chemical Equation” for a chemical reaction.
Examine the reaction:
CH4 + O2  CO2 + H2O
1. are the number of hydrogen & oxygen atoms equal on both sides of the equation?
a. Answer: NO!

8. Balancing Chemical Equations
In order to “balance” the chemical equation
CH4 + O2  CO2 + H2O, we must place whole numbers in front of the compounds, called “coefficients”.
Coefficient: the smallest integers that can be used to balance a chemical equation.
1. a coefficient, placed in front of one or more of the reactants or products, will multiply the number of each of the atom(s) present.
2. coefficients will create the “balance”, so that there will be the same # of the atoms present in both the reactants and the products.

9. Chemical Equations EX: CH4 + 2O2  CO2 + 2H2O 1 carbon 1 carbon
4 hydrogen 4 hydrogen
4 oxygen 4 oxygen
1. placing coefficient of 2 in front of the O2 & H2O now will show an equal number of oxygen, hydrogen & carbon atoms present on both sides of the equation.
2. This equation now demonstrates “balance”, and also obeys the Law of Conservation of Mass.

10. Balancing Chemical Equations: The Steps:
Step 1: Obtain the reactants, products and their physical states from the written description of the reaction
1. Write appropriate chemical formulas
Step 2: Write the unbalanced chemical equation (called a “skeletal equation”). **Go to questions pg 171
Step 3: Balance the skeletal equation.
1. Count the number of atoms of each element within each compound of the reactant(s) and compare those numbers to the products.
2. Place the smallest number coefficients in front of the reactant/product formulas to show balance
3. Always start with the most complex formula

11. Balancing Chemical Equations: The Steps:
Step 4: Check to see if you used the correct coefficients, by re-adding all atom types of the reactants & products.
1. make sure the coefficients used are the smallest integers possible to have balance.
EX: Solid potassium reacts with water to form hydrogen gas and aqueous potassium hydroxide.
1. K(s) + H2O(l)  H2(g) + KOH(aq)
a. Count all atoms on both sides of arrow.

12. Balancing Chemical Equations
K(s) + H2O(l)  H2(g) + KOH(aq)
1-K K
2-H  3-H
1-O O
2. Add coefficients to balance. Most times, this will take several steps to accomplish.
2K(s) + 2H2O(l)  H2(g) + 2KOH(aq)
2-K K
4-H  4-H
2-O O

13. Balancing Chemical Equations
EX: Solid manganese (IV) oxide is reacted with solid carbon to produce solid manganese and carbon dioxide gas.
MnO2(s) + C(s)  Mn(s) + CO2(g)
1-Mn Mn
2-O  2-O
1-C C
2. This skeletal equation is already balanced. Do Not Do The Following:
2MnO2(s) + 2C(s)  2Mn(s) + 2CO2(g)
**though it is balanced, it is not the smallest possible integers. In this case, the smallest integer is “1”.

14. Balancing Chemical Equations
EX: In solution, lead (II) nitrate reacts with potassium chromate to produce solid lead (II) chromate and aqueous potassium nitrate.
1. Write out skeletal (unbalanced) equation for the reaction:
Pb(NO3)2(aq) + K2CrO4(aq)  PbCrO4(s) + KNO3(aq)
2. Balance equation. In this case, you can save time by counting the unchanged polyatomic ions as a unit:
1-Pb Pb
2-NO  1-NO3
2-K K
1-CrO CrO4

15. Balancing Chemical Equations
Add coefficients to balance equation:
Pb(NO3)2 (aq) + K2CrO4(aq)  PbCrO4(s) + 2KNO3(aq)
Check your work:
1-Pb Pb
2-NO  2-NO3
2-K K
1-CrO CrO4
5. Equation is balanced…. Your work is done!

16. Balancing Chemical Equations from Word Problems
The steps:
1. Identify from the word problem which substances are reactants and which are products.
a. Words like react, decomposes, breaks down, “is burned”, and others will follow the reactants.
b. Words like “to produce”, forms, yields, “converted to” will be just before the products.
2. Write out the chemical formulas for each of the substances, indicating their physical states:
(s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous

17. Balancing Chemical Equations from Word Problems
Place reactants to left, then an arrow, and products to the right, producing an unbalanced equation.
a. Separate multiple reactants/products with + sign.
Balance the equation using coefficients.

18. Balancing Chemical Equations from Word Problems
Example: Solid iron (II) oxide when heated together with solid carbon will be converted to molten iron metal and carbon dioxide gas. Write the balanced chemical equation showing this process.
1. Reactants: solid iron (II) oxide & solid carbon
Products: molten iron metal
carbon dioxide gas
2. Unbalanced: FeO(s) + C(s)  Fe(l) + CO2(g)

19. Balancing Chemical Equations from Word Problems
Balance the equation:
2FeO(s) + C(s)  2Fe(l) + CO2(g)