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Chemical reactions

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Science on the WAY

Science on the WAY

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  • 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:
    • 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
  • 14. 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
  • 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
      • 2H 2 + O 2  2H 2 O
    • Example: hydrogen and oxygen to make water
  • 17. 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
  • 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  ZnCl 2 + H 2
    • 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)