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Chemical Reactions

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  1. Chemical Reactions 8.5 Students know: a) Reactant atoms/molecules interact to form products with different chemical properties b) The number of atoms (mass) remains the same (conservation of matter) c) Chemical reactions give off or absorb heat (energy)

  2. Journal: • What are the reactants of a chemical reaction? • What are the products? • What is a chemical reaction? • How would you know a chemical reaction has taken place?

  3. Describing chemical reactions • Chemical equation: shows a chemical reaction using chemical formulas. • Reactant(s) Product(s) • Reactant(s): the substance(s) with which you start. • Product(s): the substance(s) that are created by the reaction • The arrow meansyieldsin a chemical equation

  4. How do you know a chemical reaction has taken place? • A chemical reaction occurs when bonds are broken and rearranged - process gives off certain signs as this happens • Signs of a chemical reaction (signs that a new compound(s) has formed or is forming): • Formation of a precipitate • Bubbles: gas formation • Temperature change

  5. LAW: Conservation of matter • In a chemical reaction, no matter is lost. Matter can be converted into different phases/types - but is NOT LOST! • Translates: Chemical equations are balanced……..

  6. What goes in must come out! • The number of atoms of each type in the reactants will be the same in the products after a chemical reaction has occurred. • Bonds are broken during a chemical reactions and the atoms may be rearranged in the products. • Now for the math…….

  7. Balancing equations • The number of atoms on each side of an equation are equal when an equation is balanced. • Chemical equations can be a real challenge to balance - so a few clues: • Subscript: H2O is 2 H + 1 O • Coefficient:2(H2O) is 2(2H + 1O) = 4H + 2O

  8. Balancing equations • DO NOT CHANGE A SUBSCRIPT when balancing an equation as you will be changing the actual chemical formula of the reactant or product! • YOU MAY CHANGE COEFFICIENTS to balance equations - Sometimes more than 1 molecule of a particular reactant or product is necessary or produced when a reaction occurs….. 2 2 (H2) + 02 (H2O)

  9. Steps in balancing: • Write out the chemical equation • Count the number of atoms of each element on each side of the equation • Determine which atom numbers are unequal and use coefficients to balance • Count the number of atoms on each side again and make more coefficient changes if necessary

  10. Practice: • What is meant by the Law of Conservation of Matter? • Balance the following equation: CO2 + H2O C6H12O6 + O2

  11. Step 1: Count atoms: CO2 + H2O C6H12O6 + O2 C = 1 O = 3 H = 2 C = 6 O = 8 H = 12

  12. Step 2: Balance Carbon….. 6 CO2 + H2O C6H12O6 + O2 C = 6 O = 8 H = 12 C = 6 O = 13 H = 2

  13. Step 3: Balance Hydrogen… 6CO2 + H2O C6H12O6 + O2 6 C = 6 O = 18 H = 12 C = 6 O = 8 H = 12

  14. Step 4: What about Oxygen?…. 6CO2 + 6H2O C6H12O6 + O2 6 C = 6 O = 18 H = 12 C = 6 O = 18 H = 12 Bravo!!! Does anyone recognize this equation?

  15. PHOTOSYNTHESIS!! UV Light (energy) 6CO2 + 6H2O C6H12O6 + 6O2

  16. Types of chemical reactions: • Synthesis reaction: 2 or more reactants combine to form a product: O2 + 2H2 2H20 • Decomposition reaction: A reactant breaks down into the atoms/molecules that make it: 2H20 O2 + 2H2

  17. Types of chemical reactions: • Replacement (substitution) reaction: two or more atoms/molecules trade places in compounds: 2K + MgBr2 2KBr + Mg

  18. Energy in reactions: • Activation energy: The minimum amount of energy that all chemical reactions require to get going…(see cartoon p. 235 of text) • Example: for O2 and H2 to make water, it takes a spark (energy) and then the reaction spreads as the energy is transmitted through the remaining O2 and H2 atoms -

  19. Energy in reactions: • All reactions have a specific activation energy • Beyond this, some require more energy and some need to give it off…. • The energy is kinetic - motion of the particles. We use temperature as a measure of this energy. • Some reactions give off heat - some take it (feel cold)

  20. “Endo” versus “Exo”: • ENDO - take inside/ “IN” • EXO - give off • Thermic/thermal - having to do with heat • Endothermic reactions: require more energy to maintain the reaction after the initial activation energy…reactions get colder as use energy up • Exothermic reactions: require activation energy and then must get rid of energy….reactions give off heat

  21. Exothermic reaction Activation energy ENERGY Heat released Reactants Products TIME

  22. Endothermic reaction Activation energy ENERGY Heat absorbed Reactants Products TIME

  23. Rates of chemical reactions: • Chemical reactions occur at different rates • Various factors affect rate of reaction: • Surface area: amount of reactants exposed to reaction (use smaller pieces to speed up?) • Temperature: increase/decrease energy to cause more contact • Concentration: increase amounts of reactants per volume • Catalysts: Speed up reaction but are not part of product - ex: enzymes in biological systems • Inhibitors: Substance that slow down reactions - can become part of final product - ex: food preservatives

  24. Add catalyst Original Reaction Add Inhibitor ENERGY Reactants Products TIME