Chapter 2: Chemical Reactions Section 1: Observing Chemical Changes How can matter and changes in matter be described?

1. Chapter 2: Chemical ReactionsSection 1: Observing Chemical ChangesHow 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 reactionsWhat 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: Zn + HBr  H2 + ZnBr2 Multiply HBr x 2 to correct (balance) Zn + 2HBr  H2 + ZnBr2

14. Try these!! 2 2 • 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 ReactionsHow 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)