Question

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"`?

Answer 1

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"`