Presentation on theme: "Energy Changes & Reactions Rates"— 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 reactantsForming 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 energyForming of chemical bonds releases energy.For combustion reactions Heat and light are given off because new chemical bonds formed
5 Chemical Bonds and Energy Exothermic and Endothermic ReactionsDuring a chemical reaction, energy is either released or absorbed.ExothermicReleases energy to its surroundings“Exo” means Energy “exits” from the reactionEnergy 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 reactionExample: Emergency Cold PacksEnergy 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 reactionExothermic reaction:Chemical energy of reactants = heat + chemical energy of products.Reactants Heat + ProductsEndothermic reaction:Chemical energy of reactants + heat = chemical energy of products.Reactants + Heat Products
11 Reaction Rates Reaction Rates tell you: the rate that reactants change into productsFast or slow, controlled or uncontrolled - total amount of energy released is the same in all cases
12 Reaction Rates What is a Rate? Reaction rates are often expressed as: Changes over time are expressed as rates.Reaction rates are often expressed as:How fast reactants are consumed <or>How fast products are formed <or>How fast energy is absorbed or released
13 Some Factors Affecting Chemical Reaction Rates: TemperatureSurface areaConcentrationStirringCatalysts
14 Factors Affecting Reaction Rates: General 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 changedVary conditions so collisions increase or decrease
15 Factors Affecting Reaction Rates: Temperature Increasing temperature increases reaction rateDecreasing temperature decreases reaction rateExample: Milk stored in a refrigerator slows down the reactions that cause the milk to spoil.WHY?Increased temperature particles moving fasterCollide more often Collide with more energy faster rate of reaction
16 Factors Affecting Reaction Rates: Surface Area For a given mass of material:smaller particle sizes greater surface areaIncreased surface increased collisions More particles reacting Faster reaction rateExamples:Crushed ice melts much more quickly than large block of iceSmall twigs and branches burn much more quickly than big logs
17 Factors Affecting Reaction Rates: Stirring Stirring reactants:Increases particles exposure to each other (moving faster – colliding more often)More exposure faster reaction rateExample: Clothes in washing machine
18 Factors Affecting Reaction Rates: Concentration Concentration = # of particles in a given volume.More particles in a given volume more opportunities for collisions reaction rate is fasterGreater the concentration faster the reaction rate
19 Factors Affecting Reaction Rates: Concentration Dye solution in left beaker is more concentrated than solution in right.Increasing concentration of dye increases rate of color change in the material.
20 Factors Affecting Reaction Rates: Gas Pressure For gases, concentration changes with pressure.Greater the pressure of a gaseous reactant Greater is its concentrationFaster reaction rate.
21 Factors Affecting Reaction Rates: Catalysts Catalyst: substance that affects the reaction rate without being used up in the reaction.Catalysts can:Speed up a reaction <or>Enable a reaction to occur at lower temperature.
22 Factors Affecting Reaction Rates: Catalysts 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.
23 Factors Affecting Reaction Rates: Catalysts Catalyst lowers energy required for effective collisions
24 Exit Slip Questions1. Label the following graphs as Endo or Exothermic and mark the change in energy:24
25 Exit Slip Questions2. A reaction rate is the rate at which reactants change into products over time. True False
26 Exit Slip Questions3. What are two things that might happen to a reaction if a catalyst is added to the reaction?26
27 Exit Slip Questions4. What impact will each of the following changes have on the rate of most chemical reactions (increase or decrease)?decreasing the temperaturegrinding a reactant into a fine powderstirring the reaction mixturedecreasing the concentration of one of the reactants