1. Chapter 2: Chemical Reactions Section 1: Observing Chemical Changes How can matter and changes in matter be described?
In terms of two kinds of properties- physical properties and chemical properties
Changes in matter can be described in terms of physical or chemical changes

2. What is a physical property?
A characteristic of a substance that can be observed without changing the substance into another substance
Examples: ice melting, color, hardness, texture, shine, etc.

3. What is a chemical property?
A characteristic of a substance that describes its ability to change into other substances
Examples: burning magnesium, rusting

4. What is the difference between a physical and a chemical reaction?
A change in matter that produces one or more new substances is a chemical change, or chemical reaction
Physical change examples: braiding your hair, squashing a marshmallow
Chemical change examples: burning of gasoline, burning a marshmallow

5. How can you tell when a chemical reaction occurs?
Chemical changes occur when bonds break and new bonds form
Chemical reactions involve two main kinds of changes that you can observe-formation of new substances and changes in energy

6. What are some changes in properties that indicate a chemical change?
Precipitate – a solid that forms from a solution during a chemical reaction
Color change may indicate a chemical reaction
Gas production – bubbles

7. How do you categorize changes in energy?
Endothermic – A reaction in which energy is absorbed
Examples: baking soda and vinegar gets cooler when combined
Exothermic – A reaction that releases energy in the form of heat
Examples: burning of airplane fuel

8. Section 2: Describing chemical reactions What information does a chemical equation contain?
Chemical equations use chemical formulas and other symbols instead of words to summarize a reaction
Reactants – substances you have at the beginning
Products – new substances produced when the reaction is complete

9. What does the principle of conservation of mass state?
That in a chemical reaction, the total mass of the reactants must equal the total mass of the products
Matter is neither created nor destroyed

10. What does open or closed system mean?
Open system – matter can enter from or escape to the surroundings
Closed system – matter is not allowed to enter or leave

11. What must a balanced chemical equation show?
The same number of each type of atom on both sides of the equation
Represents the conservation of mass
Coefficients – a number placed in front of a chemical formula in an equation that tells you how many atoms or molecules of a reactant or product take place in the reaction

12. How do you balance chemical equations?
Count the # of atoms of each element in the reactants & products
ID element that is not equal on both sides
Add coefficient to the front of the formula that will make the # = on both sides for that element

13. Sample Problems Sample Problem: Multiply HBr x 2 to correct (balance)
Reactants
Products
1 Zn
1 H
2 H
1 Br
2 Br
Sample Problem:
Zn + HBr  H2 + ZnBr2
Multiply HBr x 2 to correct (balance)
Zn + 2HBr  H2 + ZnBr2
Reactants
Products
1 Zn
2 H
2 Br

14. Try these!! Na + Cl2  NaCl Ca + Cl2  CaCl2 H2O  H2 + O2
N2 + H2  NH3
Al2O3  Al + O2
P4 + O2  P4O6
Fe + H2O  Fe3O4 + H2 2 2 3 2 4 3 2 3 3 4 4

15. What are the three categories of chemical reactions?
Synthesis
Decomposition
Replacement

16. What is a synthesis reaction?
When two or more substances combine to make a more complex substance
A + B  AB
2H2 + O2 2H2O
Example: hydrogen and oxygen to make water

17. What is a Decomposition reaction?
Breaking down compounds into simpler products
AB  A + B
2H2O2  2H2O + O2
Example: Hydrogen peroxide decomposes into water and oxygen gas

18. What is a replacement reaction?
When one element replaces another in a compound, or when two elements in different compounds trade places
Example: copper metal obtained by heating copper oxide with carbon
Single (one element replaces another)
AB + C  AC + B
Zn + 2HCl  ZnCl2 + H2
or double (elements appear to trade places with another compound)
AB + CD  AC + BD
NaCl + AgF  NaF +AgCl

19. Section 3: Controlling Chemical Reactions How is activation energy related to chemical reactions?
The minimum amount of energy needed to start a chemical reaction
All chemical reactions need a certain amount of activation energy to get started

20. What factors affect the rate of a chemical reaction?
Surface area – the greater the surface area that faster the reaction (ex. Chewing)
Temperature – raising the temperature causes particles to move faster and therefore have more energy; they also come into contact more often; lowering temperature slows things down
Concentration – amount of substance in a given volume; increased concentration-increased reaction
Catalysts – increases the rate of a reaction by decreasing the energy needed to start
Enzymes: biological catalysts
Inhibitors – material used to decrease the rate of reaction (ex. preservatives in food)