A chemist investigated the reaction rate of a reactant, Substance Y, at various initial co… — ACT

A chemist investigated the reaction rate of a reactant, Substance Y, at various initial concentrations. The initial rate of reaction, $R$ , in millimoles per liter per second ( $\text{mmol}\cdot\text{L}^{-1}\cdot\text{s}^{-1}$ ), was recorded for each concentration, $[Y]$ , in millimoles per liter ( $\text{mmol/L}$ ), at a constant temperature of $298\text{ K}$ . The results are presented in the table below:

Initial Concentration $[Y]$ ( $\text{mmol/L}$ )	Initial Rate of Reaction $R$ ( $\text{mmol}\cdot\text{L}^{-1}\cdot\text{s}^{-1}$ )
$1.5$	$4.5$
$3.0$	$18.0$
$4.5$	$40.5$
$6.0$	$72.0$
$7.5$	$112.5$

Based on the trend shown in the table, what would be the expected initial rate of reaction, in $\text{mmol}\cdot\text{L}^{-1}\cdot\text{s}^{-1}$ , if the initial concentration of Substance Y is increased to $12.0\text{ mmol/L}$ ?

Answer: 288 mmol*L^-1*s^-1

Answer

The expected initial rate of reaction at a concentration of 12.0 mmol/L is 288.0 mmol*L^-1*s^-1.

The rate of reaction scales quadratically with concentration. Calculating the ratio of the rate to the concentration for each data point reveals that the ratio is equal to 2.0 times the concentration, yielding the equation R = 2.0 * [Y]^2. Substituting the target concentration of 12.0 mmol/L gives R = 2.0 * (12.0)^2 = 288.0 mmol*L^-1*s^-1. Alternatively, using the method of finite differences, the second difference between successive values is constant at 9.0, and continuing this sequence to 12.0 mmol/L also results in 288.0.

Step-by-Step Solution

Calculate the ratio of the rate R to the concentration [Y] for each data point.

The ratios are 3.0, 6.0, 9.0, 12.0, and 15.0.

To determine whether a direct proportional or higher-order relationship exists.

Formulate the mathematical model that represents this trend.

The ratio R/[Y] increases by 3.0 for every 1.5 mmol/L increase in [Y], which corresponds to R/[Y] = 2.0 * [Y], or R = 2.0 * [Y]^2.

To establish the quadratic relationship governing the dataset.

Substitute the target concentration value of 12.0 mmol/L into the derived quadratic equation.

R = 2.0 * (12.0)^2 = 288.0.

To calculate the extrapolated reaction rate.

Key Concept

Extrapolation of Quadratic Trends

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