5 Steps to a 5 AP Chemistry

Experimental  291

Equipment

analytical balance buret clamp

Erlenmeyer flask graduated cylinder pipet

stopwatch support stand thermometer

Measurements

1. Weigh solid samples.


2. Use a pipet or a buret to measure the volume of any solutions.


3. Use the buret to measure the volume of any gas formed (multiple measurements at
   different times may be required).


4. Measure the temperature of the solutions.


5. Measure time intervals or record the time after mixing when an observable change occurs.

NOTE: When using a buret, an initial and a final measurement are always needed.

Calculations

Using the molar mass, calculate the moles of all weighed samples. The moles of substances

are converted to molarities by dividing by the volume (in liters) of the solution. Molarities

may also be determined from pipet or buret readings using the dilution equation. (If a buret

is used, one of the volumes is calculated from the difference between the initial and final

readings.) The dilution equation may be needed to calculate the concentration of each reac-

tant immediately after all the solutions are mixed.

If a gas is being generated, plot the volume of gas formed versus time. The volume of

the gas formed is the difference between the initial buret reading and the buret reading at

a particular time. The slope of this graph is the rate.

The rate may also be determined by taking the amount of any reactant divided by the

time required.

Tabulate the concentrations of the reactant solutions and the rates for various trials. The

rate law may be determined by comparing values in this table. (See the Kinetics chapter.)

Comments

Many types of reaction may be used. The simple recording of the time required for a notice-

able change is particularly applicable to “clock” reactions.

Experiment 13: Enthalpy Changes

Synopsis

In this experiment the heat change associated with a process is calculated. Various sub-

stances are added to a calorimeter (usually a polystyrene cup), and the initial and the final

temperatures are measured. (See the Stoichiometry chapter.)

Equipment

analytical balance calorimeter graduated cylinder

thermometer

Refer to the figure of a calorimeter in Chapter 9.

Measurements

1. The masses of various substances are determined.


2. The volume of a solution or solvent is determined with a graduated cylinder.