Chemical reactions


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Chemical reactions

  1. 1. Chapter 2: Chemical Reactions Section 1: Observing Chemical Changes How can matter and changes in matter be described? <ul><li>In terms of two kinds of properties- physical properties and chemical properties </li></ul><ul><li>Changes in matter can be described in terms of physical or chemical changes </li></ul>
  2. 2. What is a physical property? <ul><li>A characteristic of a substance that can be observed without changing the substance into another substance </li></ul><ul><li>Examples: ice melting, color , hardness, texture, shine, etc. </li></ul>
  3. 3. What is a chemical property? <ul><li>A characteristic of a substance that describes its ability to change into other substances </li></ul><ul><li>Examples: burning magnesium, rusting </li></ul>
  4. 4. What is the difference between a physical and a chemical reaction? <ul><li>A change in matter that produces one or more new substances is a chemical change , or chemical reaction </li></ul><ul><li>Physical change examples: braiding your hair, squashing a marshmallow </li></ul><ul><li>Chemical change examples: burning of gasoline, burning a marshmallow </li></ul>
  5. 5. How can you tell when a chemical reaction occurs? <ul><li>Chemical changes occur when bonds break and new bonds form </li></ul><ul><li>Chemical reactions involve two main kinds of changes that you can observe-formation of new substances and changes in energy </li></ul>
  6. 6. What are some changes in properties that indicate a chemical change? <ul><li>Precipitate – a solid that forms from a solution during a chemical reaction </li></ul><ul><li>Color change may indicate a chemical reaction </li></ul><ul><li>Gas production – bubbles </li></ul>
  7. 7. How do you categorize changes in energy? <ul><li>Endothermic – A reaction in which energy is absorbed </li></ul><ul><ul><li>Examples: baking soda and vinegar gets cooler when combined </li></ul></ul><ul><li>Exothermic – A reaction that releases energy in the form of heat </li></ul><ul><ul><li>Examples: burning of airplane fuel </li></ul></ul>
  8. 8. Section 2: Describing chemical reactions What information does a chemical equation contain? <ul><li>Chemical equations use chemical formulas and other symbols instead of words to summarize a reaction </li></ul><ul><li>Reactants – substances you have at the beginning </li></ul><ul><li>Products – new substances produced when the reaction is complete </li></ul>
  9. 9. What does the principle of conservation of mass state? <ul><li>That in a chemical reaction, the total mass of the reactants must equal the total mass of the products </li></ul><ul><li>Matter is neither created nor destroyed </li></ul>
  10. 10. What does open or closed system mean? <ul><li>Open system – matter can enter from or escape to the surroundings </li></ul><ul><li>Closed system – matter is not allowed to enter or leave </li></ul>
  11. 11. What must a balanced chemical equation show? <ul><li>The same number of each type of atom on both sides of the equation </li></ul><ul><li>Represents the conservation of mass </li></ul><ul><li>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 </li></ul>
  12. 12. How do you balance chemical equations? <ul><li>Count the # of atoms of each element in the reactants & products </li></ul><ul><li>ID element that is not equal on both sides </li></ul><ul><li>Add coefficient to the front of the formula that will make the # = on both sides for that element </li></ul>
  13. 13. Sample Problems <ul><li>Sample Problem: </li></ul><ul><li>Zn + HBr  H 2 + ZnBr 2 </li></ul><ul><li>Multiply HBr x 2 to correct (balance) </li></ul><ul><li>Zn + 2HBr  H 2 + ZnBr 2 </li></ul>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. 14. Try these!! <ul><li>Na + Cl 2  NaCl </li></ul><ul><li>Ca + Cl 2  CaCl 2 </li></ul><ul><li>H 2 O  H 2 + O 2 </li></ul><ul><li>N 2 + H 2  NH 3 </li></ul><ul><li>Al 2 O 3  Al + O 2 </li></ul><ul><li>P 4 + O 2  P 4 O 6 </li></ul><ul><li>Fe + H 2 O  Fe 3 O 4 + H 2 </li></ul>2 2 2 3 3 2 2 4 3 3 2 4 4
  15. 15. What are the three categories of chemical reactions? <ul><li>Synthesis </li></ul><ul><li>Decomposition </li></ul><ul><li>Replacement </li></ul>
  16. 16. What is a synthesis reaction? <ul><li>When two or more substances combine to make a more complex substance </li></ul><ul><ul><li>A + B  AB </li></ul></ul><ul><ul><li>2H 2 + O 2  2H 2 O </li></ul></ul><ul><li>Example: hydrogen and oxygen to make water </li></ul>
  17. 17. What is a Decomposition reaction? <ul><li>Breaking down compounds into simpler products </li></ul><ul><ul><li>AB  A + B </li></ul></ul><ul><ul><li>2H 2 O 2  2H 2 O + O 2 </li></ul></ul><ul><li>Example: Hydrogen peroxide decomposes into water and oxygen gas </li></ul>
  18. 18. What is a replacement reaction? <ul><li>When one element replaces another in a compound, or when two elements in different compounds trade places </li></ul><ul><li>Example: copper metal obtained by heating copper oxide with carbon </li></ul><ul><li>Single (one element replaces another ) </li></ul><ul><ul><li>AB + C  AC + B </li></ul></ul><ul><ul><li>Zn + 2HCl  ZnCl 2 + H 2 </li></ul></ul><ul><li>or double (elements appear to trade places with another compound ) </li></ul><ul><ul><li>AB + CD  AC + BD </li></ul></ul><ul><ul><li>NaCl + AgF  NaF +AgCl </li></ul></ul>
  19. 19. Section 3: Controlling Chemical Reactions How is activation energy related to chemical reactions? <ul><li>The minimum amount of energy needed to start a chemical reaction </li></ul><ul><li>All chemical reactions need a certain amount of activation energy to get started </li></ul>
  20. 20. What factors affect the rate of a chemical reaction? <ul><li>Surface area – the greater the surface area that faster the reaction (ex. Chewing) </li></ul><ul><li>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 </li></ul><ul><li>Concentration – amount of substance in a given volume ; increased concentration-increased reaction </li></ul><ul><li>Catalysts – increases the rate of a reaction by decreasing the energy needed to start </li></ul><ul><ul><li>Enzymes: biological catalysts </li></ul></ul><ul><li>Inhibitors – material used to decrease the rate of reaction (ex. preservatives in food) </li></ul>