Question #9439b

1 Answer
Mar 6, 2016


You are indeed dealing with a first-order reaction.


The idea here is that you can determine the order of the reaction by using the units of the rate constant, #k#.

As you know, the rate of a reaction tells you how the concentrations of the reactants or of the products that are taking part in a chemical reaction change as the reaction proceeds.

For all intended purposes, the decomposition of sulfuryl chloride, #"SO"_2"Cl"_2#, can be written like this

#color(blue)("SO"_2"Cl"_2 -> "products")#

Now, the rate of this reaction will tell you how the concentration of sulfuryl chloride changes per unit of time. This means that you can write

#color(blue)("rate" = -(Delta["SO"_2"Cl"_2])/(Deltat))#

The minus sign is used here because the concentration of a product decreases during a chemical reaction.

Since the rate of a reaction expresses change in concentration per unit of time, its units will usually be given as #"mol L"^(-1)color(red)("s"^(-1))#. Keep this in mind.

For a given chemical reaction, the differential rate law tells you how the concentration of a reactant affects the rate of the reaction.

In simple terms, the differential rate law establishes a relationship between the rate of the reaction and the concentration of the reactant by using a proportionality factor, i.e. rate constant, #k#.

Let's assume that this reaction is #n# order. The differential rate law will take the form

#color(blue)("rate" = k * ["SO"_2"Cl"_2]^n)#

Rearrange the above equation to get

#k = "rate"/["SO"_2"Cl"_2]^n#

Now focus on the units of the rate constant and of the rate of the reaction. The rate constant is said to be equal to

#k = 4.68 * 10^(-5)color(red)("s"^(-1))#

Since concentration is expressed in #"mol L"^(-1)#, you can say, using only units, that

#color(red)("s"^(-1)) = ("mol L"^(-1))/("mol L"^(-1))^n * color(red)("s"^(-1))#

In order to have a valid equality, you need to have

#("mol L"^(-1))/("mol L"^(-1))^n = 1#

This can only happen if #n=1#, since

#("mol L"^(-1))/("mol L"^(-1))^1 = color(red)(cancel(color(black)("mol L"^(-1))))/color(red)(cancel(color(black)("mol L"^(-1)))) = 1#

Since #n# represents the order of the reaction, the decomposition of sulfuryl chloride will indeed be a first-order reaction, which means that the rate of the reaction depends linearly on the concentration of the product.

#"rate" = k * ["SO"_2"Cl"_2]#