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Chemical reactions
 

Chemical reactions

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Science on the WAY

Science on the WAY

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    Chemical reactions Chemical reactions Presentation Transcript

    • 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
    • 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
    • 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 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
    • 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
      2 Br 1 Br 2 H 1 H 1 Zn 1 Zn Products Reactants 2 Br 2 Br 2 H 2 H 1 Zn 1 Zn Products Reactants
    • 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
    • What are the three categories of chemical reactions?
      • Synthesis
      • Decomposition
      • Replacement
    • What is a synthesis reaction?
      • When two or more substances combine to make a more complex substance
        • A + B  AB
        • 2H 2 + O 2  2H 2 O
      • Example: hydrogen and oxygen to make water
    • What is a Decomposition reaction?
      • Breaking down compounds into simpler products
        • AB  A + B
        • 2H 2 O 2  2H 2 O + O 2
      • Example: Hydrogen peroxide decomposes into water and oxygen gas
    • 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  ZnCl 2 + H 2
      • or double (elements appear to trade places with another compound )
        • AB + CD  AC + BD
        • NaCl + AgF  NaF +AgCl
    • 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)