1. Herriman High Chemistry
Chapter 18 Section 1
Chemistry
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2. Herriman High Chemistry
Essential Questions
What is chemical kinetics?
What is collision theory and how does it relate to rate of reactions?
What is activation energy and how does it relate to rate of reaction?
What is a reaction rate and what affects reaction rates?
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3. Herriman High Chemistry
Chemical Kinetics
Different reactions occur at different rates
Chemical kinetics focuses on the speed or rate of reactions
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4. Herriman High Chemistry
Collision Theory
18.1
In chemistry, the rate of chemical change, or the reaction rate, is usually expressed as the amount of reactant changing per unit time.
A reaction rate is measure of how quickly reactants are converted into products.
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5. Herriman High Chemistry
Collision Theory
18.1
Rates of chemical reactions are often measured as a change in the number of moles during an interval of time.
As time passes, the amount of reactant (red squares) decreases and the amount of product (blue spheres) increases. Rates of chemical reactions are often measured as a change in the number of moles during an interval of time. Interpreting Diagrams Assuming equal time intervals between the boxes, how can you tell that the rate of conversion of reactant to product is not constant throughout this reaction?
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6. Herriman High Chemistry
Collision Theory
Atoms/molecules must collide in order to react
Only some collisions successfully lead to formation of products.
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7. Collision Theory (cont)
Particles must have both correct orientation and enough kinetic energy to break and reform bonds
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8. Effective Collision (cont)
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9. Herriman High Chemistry
Activation Energy
Minimum amount of energy required is called the activation energy
Once energy requirements are met the reaction proceeds
Activation energy = Ea
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10. Herriman High Chemistry
Activated Complex
Brief moment where particles are in a transition state
They are not reactants or products, but one big complex
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11. Herriman High Chemistry
Exothermic Reaction
Energy of reactants greater than energy of products
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12. Endothermic Reactions
Energy of reactants is less than energy of products
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13. Herriman High Chemistry
Rate of Reactions
What affects the rate of a reaction?
Temperature
Concentration
Particle size and states
Catalyst
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14. Particle Size and States
More surface area means a higher rate
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15. Herriman High Chemistry
Concentration
The greater the concentration the greater the reaction rate
Why?
There is more that can react and more collisions
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16. Herriman High Chemistry
Catalyst
Catalyst are agents that increase the rate of reaction
Catalyst are not reactants because they are still left at the end of a reaction like a spectator ion
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17. Herriman High Chemistry
Catalyst (cont)
Catalyst lowers Ea required to react by providing a different path for the reaction
The new path requires less energy
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18. Herriman High Chemistry
Catalyst (cont)
The lower the Ea the less energy required to react which makes it easier for it react and thus speeds up the reaction
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19. Herriman High Chemistry
Temperature
If you increase temperature the reaction will go faster
The molecules move faster and collide more often leading to more reactions
There is also more energy
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20. Herriman High Chemistry
Essential Questions
What is chemical kinetics?
What is collision theory and how does it relate to rate of reactions?
What is activation energy and how does it relate to rate of reaction?
What is a reaction rate and what affects reaction rates?
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21. Herriman High Chemistry
Chapter 18 Section 1
P 547 2, 4-5
P #43-45, 84, 86
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22. Herriman High Chemistry
Chapter 18
Section 3 - chemistry
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23. Herriman High Chemistry
Essential Questions
What is a reversible reaction?
What is a chemical equilibrium?
How does a reaction(s) come to equilibrium?
What does Le Chatelier’s Principle say?
What are ways to affect chemical equilibrium?
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24. Herriman High Chemistry
Reversible Reactions
What is a reversible reaction?
A reaction that can go backwards so that products are reforming as reactants.
Example:
Forward reaction NO2 (g) → N2O4 (g)
Reverse Reaction N2O4 (g) → 2 NO2 (g)
Write reversible RXN using a double headed arrow.
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25. Reversible Reactions (cont)
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26. Herriman High Chemistry
Chemical Equilibrium
Chemical equilibrium is when the rate of forward reaction equals rate of reverse reaction
In chemical equilibrium the concentration of reactants and products stays constant because they are being formed as soon as they are being consumed
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27. Herriman High Chemistry
Chemical Equilibrium
Any system involving a reversible reaction will reach equilibrium if left alone
Called a dynamic equilibrium because its always active
How is it active?
Both reactions are always going on
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28. Graphing equilibrium Concentrations
At first the concentration of the reactants decreases.
At first the concentration of the products increases
Eventually neither concentration changes because they have reached equilibrium.
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29. Graphing Equilibrium Rates
At first the reaction rate of the forward reaction decreases.
At first the reaction rate or the reverse reaction increases.
After some time they become equal to each other and do not change
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30. Le Chatelier’s Principle
How does a see-saw work?
Weight be placed on one end causes the see-saw to move as it tries to get balanced
How do you think that principle applies to a chemical equilibrium?
When there is a change in conditions, the chemical equilibrium will shift
Like a see-saw, the equilibrium will shift to maintain balance
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31. Le Chatelier’s Principle (cont)
Le Chatelier’s Principle states: When conditions of a chemical equilibrium are changed the equilibrium will shift towards the direction that will reduce that change
The only 3 ways you can affect the equilibrium is by changing the concentration of reactants/products, changing the pressure, or changing the temperature
Adding a catalyst has no impact on the equilibrium
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32. Changes in Concentration
If you change the amount of a product or reactant by adding or taking some away equilibrium will shift accordingly
What will happen if NO2 is added?
What will happen if N2O4 is added?
What will happen if N2O4 is taken away?
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33. Herriman High Chemistry
Changes in Pressure
For gases if you increase pressure, volume decreases
Increased pressure/decreased volume causes the equilibrium to shift to the side with fewest number of gas molecules because they take up less space
Decreased pressure/increased volume will have opposite effect
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34. Changes in Pressure (cont)
N2 (g) + 3H2 (g) ↔ 2NH3 (g)
If I wanted more ammonia (NH3) should I increase or decrease the pressure?
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35. Herriman High Chemistry
Examples
5 CO(g) + I2O5(s) <--> I2(g) CO2(g) ΔH = kJ
What will happen if you increase pressure?
2 NO2 (g) <--> N2O4 (g)
What will happen if you decrease pressure?
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36. Changes in Temperature
Depends on if reaction is Endothermic or Exothermic
Treat heat term as a reactant or product
Example: H2 (g) + I2 (g) ↔ 2 HI (g) + heat
What will happen if you increase the temperature?
If I wanted more HI, what would you change?
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37. Changes in Temperature (cont)
For endothermic reaction which direction will the equilibrium shift if heat is added?
To the right (products)
How about for an exothermic reaction?
To the left (reactants)
How would removing heat affect an exo?
How about endo?
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38. Herriman High Chemistry
Essential Questions
What is a reversible reaction?
What is a chemical equilibrium?
How does a reaction(s) come to equilibrium?
What does Le Chatelier’s Principle say?
What are ways to affect chemical equilibrium?
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39. Chapter 8 Sec 3 Assignment
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