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

## $2.$ The reaction $\text{A"_2 + "2B + C"_2 →"2ABC}$ is zero-order in ${\text{A}}_{2}$ and $\text{B}$ 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"?

Aug 14, 2017

#### Answer:

1. endothermic; 2. $\text{0.36 mol·L"^"-1""s"^"-1}$

#### 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

$\text{A"_2 + "2B + C"_2 → "2ABC}$

The rate law is

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

The reaction is zero order in $\left[{\text{A}}_{2}\right]$ and $\left[\text{B}\right]$, so $x = y = 0$

Thus, the rate law becomes

$\text{rate} = k {\left[{C}_{2}\right]}^{z}$

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

The rate law is then

$\text{rate} = k {\left[{C}_{2}\right]}^{2}$

Inserting values, we get

$\text{rate" = 9 × 10^"-2"color(white)(l) "L·mol"^"-1""s"^"-1" × ("2 mol·L"^"-1")^2 = "0.36 mol·L"^"-1""s"^"-1}$