1. Section 2 – pg 224 Describing Chemical Reactions
Chapter 6
Section 2 – pg 224
Describing Chemical Reactions

2. What are Chemical Equations?
Pg 225
What are Chemical Equations?
Chemical equations use chemical formulas and other symbols instead of words to summarize a reaction
Uses formulas to represent the substances involved in a reaction
The formula tells you the ratio of atoms

3. Structure of an Equation
Pg 225
Structure of an Equation
A chemical equation tells you the substances you start with in a reaction (reactants) and the substances you get at the end (products)

4. Conservation of Matter
Pg 226
Conservation of Matter
Discovered by French chemist
Antoine Lavoisier in 1774
Conservation of Matter Principle: during a chemical reaction, matter is neither created nor destroyed
The total mass of the reactants must equal the total mass of the products

5. Pg 226
Conservation of Atoms
In chemical reactions, the number of atoms stays the same no matter how they are arranged. So, their total mass stays the same.
All the atoms present at the start of the reaction are present at the end of the reaction

6. Open and Closed SystemsPg
Open and Closed Systems
Open System: matter can enter from or escape to the surroundings
Any reaction that takes place in a open container
Closed System: matter is not allowed to enter or leave
Any reaction that takes place in a closed container
Closed Open

7. Balancing Chemical Equations
Pg 228
Balancing Chemical Equations
To describe a reaction accurately, a chemical equation must show the same number of each type of atom on both sides of the equation

9. 1. Write the Equation H2 + O2 = H2O
Pg 228
1. Write the Equation
To begin, write the correct formula for both reactants and products
Place reactants on the left side of the arrow, separated by a plus sign, then write the products on the right side
H2 + O2 = H2O

10. Pg 228
2. Count the Atoms
Count up the number of atoms of each element on each side of the equation
Are they equal?
H2 + O2 = H2O
2 Hydrogen ≠ 2 Hydrogen
2 Oxygen Oxygen

11. 3. Use Coefficients to Balance Atoms
Pg 228
3. Use Coefficients to Balance Atoms
Coefficient: a number placed in front of a chemical formula in an equation
You can balance an unbalanced equation by adding coefficients
H2 + O2 = H2O
2 Hydrogen ≠ 2 Hydrogen
2 Oxygen Oxygen
2H2 + O2 = 2H2O
4 Hydrogen = 4 Hydrogen
2 Oxygen Oxygen

12. Pg 229
4. Look Back and Check
Check that the total number of atoms stays the same before and after the reaction

13. Classifying Chemical Reactions
Pg 230
Classifying Chemical Reactions
The general types of chemical reactions are synthesis, decomposition, and replacement

14. Pg 230
Synthesis
Synthesis: when two or more elements or compounds combine to make a more complex substance
Like the combination of hydrogen and oxygen to make water

15. Pg 230
Decomposition
Decomposition: occurs when compounds break down into simpler products

16. Pg 230
Replacement
Replacement: when one element replaces another in a compound, or when two elements in different compounds trade places
Things get jumbled up
Single Replacement: one element replaces another
Double replacement: one element trades places with another

17. Chapter 6 Section 2 Homework – pg 231

18. 1A. What do the formulas, arrow, and plus signs in a chemical equation tell you?

19. 1B. How are reactants and products treated the same in a chemical reaction? How are they treated differently?

20. 2A. How does the idea of atoms explain the principle of conservation of matter?

21. 2B. If the total mass of the products of a reaction is 250g, what was the total mass of the reactants?

22. 3A. What are three types of chemical reactions?

23. 3B. What is the smallest possible number of products in a decomposition reaction?

24. 3C. Classify the following reaction: P4O10 + 6H2O -> 4H3PO4

25. Extra Credit: Balance the equations 4. Fe2O3 + C -> Fe + CO2 5
Extra Credit: Balance the equations 4. Fe2O3 + C -> Fe + CO2 5. SO2 + O2 -> SO3