chapter 7.4. * reaction rates tell you: the rate that reactants change into products * fast or...
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Chapter 7.4
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*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
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* Changes over time are expressed as rates. Examples:
*changes in location are often expressed as speed: *mph (miles per hour) kph (kilometers per hour)
*changes in volume are often expressed as flow:*gps (gallons per second) lps (liters per second)
* 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
Reaction Rates
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Some Factors Affecting Chemical Reaction Rates:
*Temperature
*Surface area
*Concentration
*Stirring
*Catalysts
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* Reaction rate depends on how often reactant particles collide. •If collision 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
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* Increasing temperature increases reaction rate*Generally (but not always)
* 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
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* 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
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* Stirring reactants:*Increases particles exposure to each other
*More exposure faster reaction rate
* Example: Clothes in washing machine
*Particles of detergent react with particles of the stains on clothes
*A washing machine speeds up the reaction by stirring the contents back and forth.
Factors Affecting Reaction Rates: Stirring
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* 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
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* 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
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* For gases, concentration changes with pressure.
* Greater the pressure of a gaseous reactant
Greater is its concentrationFaster reaction rate.
Factors Affecting Reaction Rates: Gas Pressure
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* 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.
Factors Affecting Reaction Rates: Catalysts
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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
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* Catalyst lowers energy required for effective collisions
Factors Affecting Reaction Rates: Catalysts
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Review - Factors Affecting Chemical Reaction Rates:
Exit Slips Next Slide
*Temperature
*Surface area
*Concentration
*Stirring
*Catalysts
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Exit Slip Questions
1. A reaction rate is the rate at which reactants change into products over time.
TrueFalse
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2. What are two things that might happen to a reaction if a catalyst is added to the reaction?
Exit Slip Questions
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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