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Energy Changes & Reactions Rates Unit 7 - Chapter 7.3 & 7.4.

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Presentation on theme: "Energy Changes & Reactions Rates Unit 7 - Chapter 7.3 & 7.4."— Presentation transcript:

1 Energy Changes & Reactions Rates Unit 7 - Chapter 7.3 & 7.4

2 Chemical Bonds and Energy  Chemical reactions:  Breaking chemical bonds in the reactants  Forming new chemical bonds in the products.  Chemical energy: Energy stored in the chemical bonds of a substance.  True for both reactants and products

3 Chemical Bonds and Energy  Energy changes in reactions  determined by changes in chemical bonding.  Breaking chemical bonds requires energy  Forming of chemical bonds releases energy.  For combustion reactions  Heat and light are given off because new chemical bonds formed

4 Chemical Bonds and Energy

5  Exothermic and Endothermic Reactions  During a chemical reaction, energy is either released or absorbed.  Exothermic  Releases energy to its surroundings  “Exo” means Energy “exits” from the reaction  Energy required to break reactant bonds  less than energy released as products formed

6 Chemical Bonds and Energy  Exothermic reaction: chemical energy of reactants is greater than chemical energy of products.

7 Chemical Bonds and Energy  Chemical energy peaks before reactants change into products.  Peak Height = energy required to break the chemical bonds of the reactants.  Particles must collide with enough energy to break these bonds, or the reaction will not occur.

8 Chemical Bonds and Energy  Endothermic - Absorbs energy from surroundings  “Endo” means Energy goes “into” the reaction  Example: Emergency Cold Packs  Energy required to break reactant bonds  greater than energy released by formation of products.

9 Chemical Bonds and Energy  Endothermic reaction - energy of products is greater than energy of reactants.

10 Chemical Bonds and Energy  Conservation of Energy:  Total Energy is same before and after a reaction  Exothermic reaction:  Chemical energy of reactants = heat + chemical energy of products.  Reactants  Heat + Products  Endothermic reaction:  Chemical energy of reactants + heat = chemical energy of products.  Reactants + Heat  Products

11  Reaction Rates tell you:  the rate that reactants change into products  Fast or slow, controlled or uncontrolled - total amount of energy released is the same in all cases Reaction Rates

12  What is a Rate?  Changes over time are expressed as rates.  Reaction rates are often expressed as:  How fast reactants are consumed  How fast products are formed  How fast energy is absorbed or released Reaction Rates

13 Some Factors Affecting Chemical Reaction Rates:  Temperature  Surface area  Concentration  Stirring  Catalysts

14  Reaction rate depends on how often reactant particles collide. If collisions are more frequent  reaction rate increases. If collisions are less frequent  reaction rate decreases.  Reaction rates can be changed Vary conditions so collisions increase or decrease Factors Affecting Reaction Rates: General

15  Increasing temperature increases reaction rate  Decreasing temperature decreases reaction rate  Example: Milk stored in a refrigerator slows down the reactions that cause the milk to spoil. WHY?  Increased temperature  particles moving faster  Collide more often  Collide with more energy  faster rate of reaction Factors Affecting Reaction Rates: Temperature

16  For a given mass of material: smaller particle sizes  greater surface area  Increased surface  increased collisions  More particles reacting  Faster reaction rate  Examples:  Crushed ice melts much more quickly than large block of ice  Small twigs and branches burn much more quickly than big logs Factors Affecting Reaction Rates: Surface Area

17  Stirring reactants:  Increases particles exposure to each other (moving faster – colliding more often)  More exposure  faster reaction rate  Example: Clothes in washing machine Factors Affecting Reaction Rates: Stirring

18  Concentration = # of particles in a given volume.  More particles in a given volume  more opportunities for collisions  reaction rate is faster  Greater the concentration  faster the reaction rate Factors Affecting Reaction Rates: Concentration

19  Dye solution in left beaker is more concentrated than solution in right.  Increasing concentration of dye increases rate of color change in the material. Factors Affecting Reaction Rates: Concentration

20  For gases, concentration changes with pressure.  Greater the pressure of a gaseous reactant  Greater is its concentration  Faster reaction rate. Factors Affecting Reaction Rates: Gas Pressure

21  Catalyst: substance that affects the reaction rate without being used up in the reaction.  Catalysts can:  Speed up a reaction  Enable a reaction to occur at lower temperature. Factors Affecting Reaction Rates: Catalysts

22 Ex: One of the ways to make sulfuric acid is to react sulfur dioxide with oxygen to form sulfur trioxide.  This reaction happens very slowly without a catalyst  [ vanadium(V) oxide is a good catalyst for this reaction]  The catalyst is neither a reactant nor a product, so it is written over the arrow. Factors Affecting Reaction Rates: Catalysts

23  Catalyst lowers energy required for effective collisions Factors Affecting Reaction Rates: Catalysts

24 1. Label the following graphs as Endo or Exothermic and mark the change in energy: Exit Slip Questions

25 2. A reaction rate is the rate at which reactants change into products over time. True False

26 3. What are two things that might happen to a reaction if a catalyst is added to the reaction? Exit Slip Questions

27 4. What impact will each of the following changes have on the rate of most chemical reactions (increase or decrease)? 3)decreasing the temperature 4)grinding a reactant into a fine powder 5)stirring the reaction mixture 6)decreasing the concentration of one of the reactants Exit Slip Questions


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