Rates of Chemical Reactions

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Rates of Chemical Reactions

• Chapter 21 Section 4

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In a chemical reaction

• Atoms are not created or destroyed, but are
  rearranged
• (Thank you, John Dalton!!)
• Energy is required to make and to break chemical
  bonds
• Some chemical reactions require more energy to
  take place than others

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Activation Energy

• In order for a chemical reaction to take place,
  the reactants need to gain a certain amount of
  energy or become activated
• The energy needed for a reaction to move forward
  is called activation energy.

Activation Energy Animation
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Exothermic Reactions

• An exothermic reaction releases energy in the
  form of heat or light (feels warm).
• The products end up having less energy than the
  reactants because some of the original energy was
  lost

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Endothermic Reactions

• An endothermic reaction absorbs energy (feels
  cold).
• The products end up having more energy than the
  reactants because energy was absorbed

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Exothermic vs. Endothermic Energy Curves
Based on what you know about exothermic and
endothermic reactions, which graph represents an
exothermic reaction, and which represents an
endothermic reaction? Why?
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Exothermic vs. Endothermic Energy Curves
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How can we speed up a reaction?

• In order for a chemical reaction to take place,
  the reactants need to have the right amount of
  energy and orientation
• Collision Theory says the rate of a reaction is
  affected by four factors
• Concentration
• Surface area
• Temperature
• Catalysts

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How to Increase Reaction Rates (and get a date)
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Concentration

• How much substance do you have in a given area?
• The higher the concentration, the more collisions
  that will take place, the faster the rate of the
  reaction
• You can increase concentration by adding more
  particles or decreasing the volume of the
  container
• Example
• Walking in the school hallways during class vs.
  during passing time

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Surface Area

• The area surrounding a reactant that is available
  for contact with another reactant
• The higher the surface area, the more contact the
  reactants have with each other, the faster the
  rate of reaction
• Example
• Walking in the school hallways during passing
  time while holding hands with your significant
  other (you bump into more people than if you were
  by yourself).

Sawdust
Creamer Cannon
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Temperature

• An increase in temperature will increase the rate
  of a reaction.
• Temperature is a measure of the average kinetic
  energy of the particles the higher the
  temperature, the faster the particles are moving
  the more collisions which result in a reaction
• Example
• To prevent food from spoiling quickly, we
  refrigerate or freeze it to slow down the
  chemical reactions that make food go bad

Temperature and Reaction Rate Animation
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Catalysts

• Catalysts lower the activation energy needed to
  make a reaction occur.
• They lower the energy hill by helping the
  reactants line up in the correct orientation
• Catalysts do not participate in the actual
  chemical reaction, they are just bystanders (do
  not get used up)
• Examples
• Your body has special catalysts called enzymes
  that assist in almost all chemical reactions in
  your body
• Going to a packed concert and having the band
  play their best song (is like a catalyst to get
  people moving colliding)