1. Chemical Reactions Section 1: Observing Chemical Changes How can matter and changes in matter be described? 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  Burning Magnesium

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 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

11. 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

12. Sample Problems Sample Problem: Zn + HBr  H 2 + ZnBr 2 Multiply HBr x 2 to correct (balance) Zn + 2HBr  H 2 + ZnBr 2 ReactantsProducts 1 Zn 1 H 2 H 1 Br 2 Br ReactantsProducts 1 Zn 2 H 2 Br

13. Try these!!  Na + Cl 2  NaCl  Ca + Cl 2  CaCl 2  H 2 O  H 2 + O 2  N 2 + H 2  NH 3  Al 2 O 3  Al + O 2  P 4 + O 2  P 4 O 6  Fe + H 2 O  Fe 3 O 4 + H 2 2 2 2 3 3 2 2 4 3 3 2 4 4

14. 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

15. 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)