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