H2 Chemistry 8 - Reaction Kinetics: Rate Equation from Experimental Data

Step 1 - State the problem

The reaction between nitrogen dioxide and carbon monoxide follows the rate equation: rate equals k times the concentration of NO two raised to some power m, times the concentration of CO raised to some power n.

Step 1 - State the problem

Use the experimental data to determine the orders m and n, write the rate equation, and calculate the rate constant k.

Step 1 - State the problem

We have three experiments with different initial concentrations and measured initial rates.

Step 2 - Find the order with respect to nitrogen dioxide

Compare experiments one and two, where the concentration of CO is held constant at zero point one zero molar.

Step 2 - Find the order with respect to nitrogen dioxide

The concentration of NO two doubles from zero point one zero to zero point two zero.

Step 2 - Find the order with respect to nitrogen dioxide

The rate quadruples from zero point zero zero five zero to zero point zero two zero.

Step 2 - Find the order with respect to nitrogen dioxide

Since doubling the concentration causes the rate to increase by a factor of four, the order with respect to NO two is two.

Step 3 - Find the order with respect to CO

Compare experiments two and three, where the concentration of NO two is held constant at zero point two zero molar.

Step 3 - Find the order with respect to CO

The concentration of CO doubles from zero point one zero to zero point two zero.

Step 3 - Find the order with respect to CO

The rate stays the same at zero point zero two zero.

Step 3 - Find the order with respect to CO

Since doubling the concentration of CO has no effect on the rate, the order with respect to CO is zero.

Step 4 - Write the rate equation and calculate k

The rate equation is: rate equals k times the concentration of NO two squared.

Step 4 - Write the rate equation and calculate k

CO does not appear because it is zero order.

Step 4 - Write the rate equation and calculate k

Now calculate k using data from experiment one: k equals rate divided by the concentration of NO two squared.

Step 4 - Write the rate equation and calculate k

k equals zero point zero zero five zero divided by zero point one zero squared, which equals zero point five zero mol to the minus one dm cubed per second.

Step 5 - Discuss the mechanism implication

Since CO is zero order, it does not appear in the rate-determining step.

Step 5 - Discuss the mechanism implication

The rate-determining step likely involves two molecules of NO two reacting together.

Step 5 - Discuss the mechanism implication

A possible mechanism is: step one, two NO two molecules react to form NO three and NO, which is slow. Step two, NO three reacts with CO to form NO two and CO two, which is fast.

Step 5 - Discuss the mechanism implication

Common mistake: assuming the overall equation gives the rate equation. The rate equation must be determined experimentally.