5 Steps to a 5 AP Chemistry 2019

296 ❯ STEP 4. Review the Knowledge You Need to Score High

Comments

When plotting the data, the time is commonly the horizontal axis, while the mass lost or

mL of gas produced is the vertical axis.

Be especially careful when handling the hydrochloric acid.

Experiment 11: Rate Laws

Synopsis

In this experiment, you will determine the rate law for a specific chemical reaction.

Commonly the reaction involved is the reaction of crystal violet (CV) with sodium

hydroxide (NaOH). The progress of the reaction is followed with a spectrophotometer or

colorimeter. You will initially create a Beer’s law calibration curve by measuring the absor-

bance of solutions of crystal violet of varying concentrations. Then you will use the same

spectrophotometer to follow the change in concentration of crystal violet as it reacts with

NaOH as a function of time:

CV+(aq) + OH-(aq) → CVOH(aq)

The rate expression for this reaction would be:

rate = k [CV+]x[OH-]y

If we use a large stoichiometric excess of NaOH, then the rate equation becomes

rate = k* [CV+]x

since there is so much hydroxide ion present that its concentration essentially becomes

constant.

Equipment

Spectrophotometer (commonly SPEC 20)

Cuvettes (sample tubes for the spectrophotometer)

Pipettes and bulbs

Assorted glassware, including volumetric glassware

Measurements

You will be making measurements of absorbance and time. Be sure to use a blank containing

only water and NaOH but no crystal violet.

Calculations

You will be making different concentrations of the stock crystal violet solution by dilution,

so that you will use the dilution equation: M 1 V 1 = M 2 V 2. You will be making three graphs:

(1) concentration versus time (straight line indicates zero order with respect to CV [x = 0

in rate expression]); (2) ln(concentration) versus time (straight line indicates first order with

respect to CV [x = 1 in rate expression]) and (3) 1/concentration versus time (straight line

indicates second order with respect to CV [x = 2 in rate expression]).

Comments

Be especially careful when handling the sodium hydroxide solution.