The rate law for the decomposition of phosphine \(\left(\mathrm{PH}_{3}\right)\) is $$ \text { Rate }=-\frac{\Delta\left[\mathrm{PH}_{3}\right]}{\Delta t}=k\left[\mathrm{PH}_{3}\right] $$ It takes 120. s for \(1.00 M \mathrm{PH}_{3}\) to decrease to \(0.250 \mathrm{M}\). How much time is required for \(2.00 M \mathrm{PH}_{3}\) to decrease to a concentration of \(0.350 M ?\)

Understanding how quickly a chemical reaction occurs is essential for many fields of science and engineering. This is where the concept of the 'reaction rate' comes into play. In simple terms, the reaction rate indicates how fast the concentration of a reactant decreases or the concentration of a product increases over time. The formula
\[\text{Reaction rate} = -\frac{\Delta[\mathrm{A}]}{\Delta t}\]
represents the rate at which a reactant A is consumed in a chemical reaction. The negative sign indicates the concentration of the reactant is decreasing. In the exercise, this was showcased by finding how fast phosphine (\(\mathrm{PH}_3\)) decomposes.
To ensure students grasp this concept fully, incorporating visual aids like graphs showing concentration versus time can be beneficial. It's important to note that reaction rate can be affected by various factors such as temperature, catalysts, and concentration of reactants, which could form part of a more advanced lesson.

Diving deeper into kinetics, the 'rate law' expresses the relationship between the reaction rate and the concentration of reactants. This mathematical equation reflects how the rate depends on the concentration of one or more reactants. In the given exercise, the rate law for the decomposition of phosphine is written as
\[\text{Rate} = k[\mathrm{PH}_3]\]
where \(k\) is the rate constant, and [\(\mathrm{PH}_3\)] represents the concentration of phosphine. Students should learn that the rate law is specific to each reaction and must be determined experimentally; it cannot be deduced from the chemical equation alone. To clarify the concept, providing examples of different types of rate laws, like zero-order, first-order, or second-order can illustrate how the rate varies with concentration.

At the heart of the rate law lies the 'rate constant', symbolized by \(k\). This coefficient is a measure of the intrinsic reactivity of the reaction, and it remains constant at a given temperature. The rate constant essentially provides a link between the concentration of the reactants and the speed at which they react. In quantitative terms, it's a proportionality factor in the rate law equation, as seen in the problem's solution where
\[k = 0.00625 \, \frac{\text{M}}{\text{s}}\]
was calculated. Despite its name, the rate constant can be influenced by environmental conditions like temperature, with its magnitude providing insight into the reaction's speed. A larger \(k\) means a faster reaction at the same concentration levels. Simple animations or simulations showing how altering \(k\) influences the rate can be very instructive at driving this point home.