5 Steps to a 5 AP Chemistry 2019

Experimental Investigations ❮ 297

Experiment 12: Calorimetry

Synopsis

In this experiment, you will be measuring the heat produced during the dissolving of vari-

ous ionic substances in water with the goal of determining which of the salts is most effi-

cient (with respect to cost) in generating heat. Substances to test might include anhydrous

calcium chloride (CaCl 2 ), anhydrous sodium carbonate (Na 2 CO 3 ), anhydrous ammonium

nitrate (NH 4 NO 3 ), anhydrous sodium acetate (NaC 2 H 3 O 2 ), and similar salts. You will

calculate the change in enthalpy of dissolution in kJ/mol (DHsoln) by using a coffee-cup

calorimeter (see Figure 9.1 in Chapter 9, Thermodynamics). You may be using a magnetic

stirrer instead of the stirring wire shown in the figure.

Equipment

Thermometers or temperature probes

Polystyrene cups

Magnetic stirrers and stir bars

Assorted glassware

Measurements

You will be making measurements of the initial and final temperatures of the solutions

formed by adding a certain mass of solute to a measured amount of water. The DT is the

final temperature minus the initial temperature. The value of DT is a calculated number

and not a measured number.

Calculations

You may be given or may have to calculate the calorimeter constant, C, for your calo-

rimeter—the heat absorbed by the calorimeter per degree of temperature change. The

energy of solution formation (qrxn) is calculated by multiplying the mass times the

specific heat of the solution (given) times the change in temperature (qrxn = mcDT) and

the energy of solution (qsoln) is calculated by: qsoln = - (qrxn + CDT). The enthalpy of

dissolution (DHsoln) is calculated by dividing the qsoln (in kJ) by the number of moles

of salt used.

Comments

Be especially careful with the ammonium nitrate—it is a strong oxidizer.

Experiment 13: Chemical Equilibrium—Le Châtelier’s

Principle

Synopsis

Experiments that fall into this category examine systems that are at equilibrium and what

happens when that equilibrium is disturbed. Many times this involves having a small tray of

reagents and testing an equilibrium system by mixing selected reagents and making obser-

vations. You may change concentrations (adding more reagent) or change the temperature

of the solutions. This may involve an acid–base equilibrium or complex ion equilibriums.

Reactions will be given, and you should be able to describe the stress that you imposed and

how the system reacted to that stress.