Single Step Rate Laws

Rate Law

• A B where,

• Rate = -1 x ∆A and Rate = 1 x ∆B 1 ∆t 1 ∆t

• Or: Rate = -1 x ∆A = 1 x ∆B 1 ∆t 1 ∆t

Reaction Rate• Proportional to the [concentration] of

reactants raise to a power = to their coefficients in a balanced equation

• Rate ∝ [Reactants]coefficients

• Rate = k[reactants]coefficients • K: – unique based on reaction– Depends on temperature

Order of Reactant

• = to the coefficient of a reactant in a balanced equation

• =the exponent of the reactant in the rate law

• Use these reactant coefficients to determine overall order of reaction = sum (+) of all the exponents in a rate law

Example

• aA + bB cC + dD

• Rate ∝ [A]a[B]b• Rate = k[A]a[B]b

• Order of A = a• Order of B = b• Order of Reaction = a+b

Graphs

Order can be used to predict the shape of:

Concentration vs. Time Graph

Rate vs. Concentration Graph

Zero Order

• Rxn rate independent of reactant concentration

• (insert graphs from single step handout)

First Order

• Reaction rate ∝ concentration• (ie. Rate decreases, concentration

decreases)

• (insert graph)

Second Order

• Rate increases exponentially as concentration increases

2NO N2O2

a) Write a rate lawb) Determine the order of the reactionc) Determine k if R = mol/L s when [NO] = 2.1 ∙

mol/Ld) Find the new R value if temperature is

unchanged and [NO] = 1.2 mol/Le) How does the value of R change if the [NO] is

tripled?

e) How does the value of R change if the [NO] is tripled?

• Assume [NO] = 1• Then, rate = k[1]2 = 1 x k

• If [NO] is tripled, then the new [NO] = 3 where

• Rate = k[3]2 = 9 x k

• When the [NO] is tripled, rate increases by a factor of 9!

a) Write a rate law

• 2NO N2O2

• Rate = k[NO]2

• (reactants only!)

b) Determine the order of the reaction

• Rate = k[NO]2

• Order = 2

c) Determine k if R = mol/L s ∙when [NO] = 2.1 mol/L

• K = rate = 4.3 mol/L s∙ = 0.98L/mol s∙[NO]2 (2.1 mol/L)2

Rate = k[NO]2

Rate = k[NO]2

Rate = k[NO]2

d) Find the new R value if temperature is unchanged and [NO] = 1.2 mol/L

• Rate = k[NO]2

= (0.98 L/mol s)(1.2mol/L)∙ 2

= 1.4 mol/L s∙

Don’t forget BEDMAS and order of operations!