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Typical Graphs . Rate of Reaction = Chemical Kinetics. Δ [Concentration]. Δ Time. Rate of Rxn = = Slope. Reaction Rates. Rates of reactions can be determined by monitoring the change in concentration of either reactants or products as a function of time . Watch This!.

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rate of reaction chemical kinetics
Rate of Reaction = Chemical Kinetics

Δ [Concentration]

Δ Time

  • Rate of Rxn = = Slope
reaction rates
Reaction Rates

Rates of reactions can be determined by monitoring the change in concentration of either reactants or products as a function of time.

par example
Par Example

C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)

In this reaction, [the concentration]of butyl chloride, C4H9Cl, was measured at various times.

reaction rates1

Average rate =

Average rate =

[.10 - .0905]

[50 – 0]

[C4H9Cl]

t

Reaction Rates

The average rate of the reaction over each interval is the change in concentration divided by the change in time:

= 1.9 x 10 -4

average rate changes
AVERAGE RATE CHANGES!
  • It is not constant.
  • What’s happening to the average rate?
reaction rates2
Reaction Rates
  • Note that the average rate decreases as the reaction proceeds.
  • This is because as the reaction goes forward, there are fewer collisions between reactant molecules.
change of rate over time
Change of Rate over Time

Practice Example p. 598 #14.4

YES! Linear Function with positive slope.

b. Yes! The slope = 0 indicating that the reaction is over evidenced by no change in [M].

instantaneous rate of change
Instantaneous Rate of Change
  • Instantaneous Rate of Change = slope of tangentline to curve at a point “t”

@ t = 0, initial rate

think of it this way
Think of it this way!
  • Your instantaneous rate is
  • You drove 98 miles to Charlotte in 2 hours.
  • Your average rate is 49 mi/hr.
reaction rates p 561
Reaction Rates p. 561

C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)

  • A plot of [C4H9Cl] vs. time for this reaction yields a curve like this.
  • The slope of a line tangent to the curve at any point is the instantaneous rate at that time= RATE @ instant.
  • Examine the slope at t = 0 vs. slope at t = 600 s.
  • Which is greater?

Steeper Slope

what s happening over time
What’s happening over time?

Slope is decreasing.

Rate is decreasing.

Reaction is slowing.

reaction rates3
Reaction Rates

C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)

  • All reactions slow down over time.
  • Therefore, the best indicator of the rate of a reaction is the instantaneous rate near the beginning of the reaction.
how do calculate instantaneous rate
How do calculate instantaneous rate?
  • NON-Calculus Method
  • Find slope of line at point: HOW???
  • USE GRAPH!
  • Draw in tangent line
      • Calculate ~ slope
      • Approximation of actual slope of tangent line to curve @ t = seconds
  • Calculus Method
  • In order to find the ACTUAL slope of tangent line at t = X seconds
  • MUST know function
  • DON’T know function
  • IF we knew the function, THEN we could use the 1st derivative to find the actual instantaneous rate of change
calculus application
Calculus Application

First Derivative = slope of tangent line to curve at t = 2

First Derivative = Velocity

let s practice p 600 14 21
Let’s Practice p. 600 #14.21
  • (a) Calculate averages between intervals of time.
  • (b) Calculate average rate over entire time interval.
  • (c) Use LoggerPro to graph data. Select natural exponent function.
  • ANSWERS FOUND ON p. A-18 at back of book.

AVERAGE = OVER SPECIFIC TIME INTERVAL

INSTANTANEOUS = @ SPECIFIC TIME VALUE

reaction rates and stoichiometry

-[C4H9Cl]

t

Rate =

=

[C4H9OH]

t

Reaction Rates and Stoichiometry

C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)

  • In this reaction, the ratio of C4H9Cl to C4H9OH is 1:1.
  • Thus, the rate of disappearance of C4H9Cl is the same as the rate of appearance ofC4H9OH.
reaction rates and stoichiometry1

1

2

[HI]

t

Rate = −

=

[I2]

t

Reaction Rates and Stoichiometry

What if the ratio is not 1:1?

2 HI(g)  H2(g) + I2(g)

reaction rates and stoichiometry2

aA + bB

cC + dD

= −

=

=

Rate = −

1

a

1

b

1

c

1

d

[C]

t

[D]

t

[A]

t

[B]

t

Reaction Rates and Stoichiometry
  • To generalize, then, for the reaction