Reaction Rates and Stoichiometry

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Reaction Rates and Stoichiometry - PowerPoint PPT Presentation

-  [C 4 H 9 Cl]  t. Rate =. =.  [C 4 H 9 OH]  t. Reaction Rates and Stoichiometry. C 4 H 9 Cl( aq ) + H 2 O( l )  C 4 H 9 OH ( aq ) + HCl( aq ). In this reaction, the ratio of C 4 H 9 Cl to C 4 H 9 OH is 1:1 .

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-[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.

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)

aA + bB

cC + dD

= −

=

=

Rate = −

1

b

1

a

1

c

1

d

[D]

t

[C]

t

[A]

t

[B]

t

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

Sample Exercise 14.3 p. 563

Concentration and Rate

One can gain information about the rate of a reaction by seeing how the rate changes with changes in concentration.

N2(g) + 2 H2O(l)

NH4+(aq) + NO2−(aq)

Concentration and Rate

If we compare Experiments 1 and 2, we see that when [NH4+] doubles, the initial rate doubles.

N2(g) + 2 H2O(l)

NH4+(aq) + NO2−(aq)

Concentration and Rate

Likewise, when we compare Experiments 5 and 6, we see that when [NO2−] doubles, the initial rate doubles.

Concentration and Rate
• This means the Rate  [NH4+] & Rate  [NO2−]

Rate  [NH4+] [NO2−]

which, when written as an equation, becomes

Rate Law Eqn= k [NH4+]m[NO2−]n

• This equation is called the rate law,

andkis the rate constant.

mandnare calledreaction orders.

Therefore,

What are m and n exponents?
• Exponents indicate how the rate is affected by each reactant concentration – termed reaction orders.
• Rate Law must be determined experimentally.
Overall Reaction Order

Rate = k [NH4+]m[NO2−]n

• The overall reaction order can be found by adding the exponents on the reactants in the rate law … so m + n = ? 1 + 1 = 2
• This reaction is second-order overall.
Rate Constant (k)
• Reveals how fast or slow rxn proceeds.
• LARGE k (~ 109 ) = FAST
• small k ( < or = 10 ) = SLOW
Sample Exercise 14.6
• Homework Chemical Kinetics: Rate Laws WS