Experience Chemistry Lesson 12.1-Rates of Reaction

​Lesson 12.1: Rates of Reaction

​Expressing Rates of Change

• Rate: The ratio between two related quantities expressed in different units

  • Many describe how much something changes over time (ex: Speed is distance/time)

• Reaction Rate: The rate at which the reactants of a chemical reaction form the product​

  • Written in terms of a concentration [X]/t

  • Rate can be negative for a reactant or positive for a product

Experience Chemistry | Lesson 12.1

​Expressing Rates of Change

Experience Chemistry | Lesson 12.1

​Expressing Rates of Change

• To keep things simple, chemists use a single rate of reaction that is always positive based on the stoichiometry of the reaction

  • Take a typical reaction with reactants A & B and Product C

    • xA + yB → zC

Experience Chemistry | Lesson 12.1

​Calculating Reaction Rates

• Hydrogen and Iodine gas react to form hydrogen iodide, as shown in the equation H₂ + I₂ → 2HI. Suppose you are given a time interval from t₁=10 sec and t₂=20 sec where the change in H₂ concentration from 0.210 mol/L to 0.185 mol/L. Calculate the average reaction rate in that period

  • Step 1: Identify your knonws

    • [H₁] =

    • ​[H₂] =

    • t₁=

    • t₂=

    • x=, y=, z=

Experience Chemistry | Lesson 12.1

​Calculating Reaction Rates

• Hydrogen and Iodine gas react to form hydrogen iodide, as shown in the equation H₂ + I₂ → 2HI. Suppose you are given a time interval from t₁=10 sec and t₂=20 sec where the change in H₂ concentration from 0.210 mol/L to 0.185 mol/L. Calculate the average reaction rate in that period

  • Step 1: Identify your knonws

    • [H₁] =0.210 mol/L

    • ​[H₂] =0.185 mol/L

    • t₁=10

    • t₂=20

    • x=1, y=1, z=2

Experience Chemistry | Lesson 12.1

​Calculating Reaction Rates

• Hydrogen and Iodine gas react to form hydrogen iodide, as shown in the equation H₂ + I₂ → 2HI. Suppose you are given a time interval from t₁=10 sec and t₂=20 sec where the change in H₂ concentration from 0.210 mol/L to 0.185 mol/L. Calculate the average reaction rate in that period.

  • Step 2: Substitute your knowns into the equation

    • ΔH=0.185-0.210=-0.025

    • Δt=20-10=10

    • x=1, y=1, z=2

Experience Chemistry | Lesson 12.1

​Calculating Reaction Rates

• For the same reaction H₂ + I₂ → 2HI. You are given a product concentration of 0.180 mol/L at a time of 15 seconds and a concentration of 0.205 mol/L at a time of 20 seconds. Calculate the average reaction rate for the time interval.

  • Step 1: Identify your knowns

    • [HI₁] =

    • ​[HI₂] =

    • t₁=

    • t₂=

    • x=, y=, z=

Experience Chemistry | Lesson 12.1

​Calculating Reaction Rates

• For the same reaction H₂ + I₂ → 2HI. You are given a product concentration of 0.180 mol/L at a time of 15 seconds and a concentration of 0.205 mol/L at a time of 20 seconds. Calculate the average reaction rate for the time interval.

  • Step 2: Substitute your knowns into the equation

    • ΔHI=0.205-0.0180= 0.025

    • Δt=20-15=5

    • x=1, y=1, z=2

Experience Chemistry | Lesson 12.1

​Collision Theory and Reaction Rates

• Collision Theory states that a reaction only takes place if reactants combine with enough energy at the right orientation

  • As ​reaction takes place, less reactants are available to collide

Experience Chemistry | Lesson 12.1

​Concentration, Temperature, Particle and Reaction Rates

• The higher the concentration, the more molecules to collide and react

  • High Concentration= faster rate​

• The higher the temperature, the more faster molecules move, and the more likely they are to collide​

  • High temperature= faster rate​​

• A larger particle size, the longer the reaction will take place​

  • Smaller Particles have more surface area​

    • More Surface area = Faster rate​

Experience Chemistry | Lesson 12.1

Multiple Choice

How could we make this reaction happen more quickly?

• Decrease the concentration of the acid

• crush the chalk to increase the surface area

• Put the test tube in an ice bath

Multiple Choice

Which factors increase the rate of a reaction.

• increasing temperature
• increasing concentration
• increasing surface area
• all of these

Multiple Choice

Grinding a seltzer tablet into powder increases the rate of reaction due to increased

• concentration
• surface area
• temperature
• reactants

Multiple Choice

Increasing the concentration of the reactants will slow down the reaction.

• false 
• true