1. A reaction has #Δ_text(r)H = "+100 kJ·mol"^"-1"#. Is the reaction exothermic or endothermic?

#2.# The reaction #"A"_2 + "2B + C"_2 →"2ABC"# is zero-order in #"A"_2# and #"B"# and second-order overall. What is the reaction rate if the rate constant is #9 × 10^"-2" color(white)(l)"L·mol"^"-1""s"^"-1"# and #["A"_2] = ["B"] = ["C"_2] = "2 mol/L"#?

1 Answer
Aug 14, 2017

Answer:

  1. endothermic; 2. #"0.36 mol·L"^"-1""s"^"-1"#

Explanation:

1.

The common sign convention in chemistry is that anything going into a system is positive, while anything going out of a system is negative.

Since #ΔH = "+100 kJ/mol"#, energy is going into the system.

This indicates that the reaction is endothermic (endo = "in").

2.

The reaction is

#"A"_2 + "2B + C"_2 → "2ABC"#

The rate law is

#"rate" = k["A"_2]^x[B]^y["C"_2]^z#

The reaction is zero order in #["A"_2]# and #["B"]#, so #x = y = 0#

Thus, the rate law becomes

#"rate" = k[C_2]^z#

Since the reaction is second order overall, we must have #z = 2#.

The rate law is then

#"rate" = k[C_2]^2#

Inserting values, we get

#"rate" = 9 × 10^"-2"color(white)(l) "L·mol"^"-1""s"^"-1" × ("2 mol·L"^"-1")^2 = "0.36 mol·L"^"-1""s"^"-1"#