Eye protection is mandatory in this lab, and you should not wear shorts or open toed shoes.
Any spills should be cleaned up immediately
Minimize Risk
Check the cord on the hotplate, inform the instructor if it is frayed.
Make sure the electrical cord never touches the surface of the hotplate
Recognize Hazards
You will be pourning a hot solution of calcium hydroxide through a filter paper and must take care not to spill the solution.
All waste is placed in the labeled container in the hood and will be recycled when the lab is over. Contact your instructor if the waste container is full, or about full.
Equipment and materials needed
Table \(\PageIndex{1}\): Supplies for Experiment 6
saturated Ca(OH)2(aq)
hot plate
ring stand
Vernier pH probe
Vernier temp probe
LabQuest
long stem funnel
filter paper
Background
In this lab students will calculate the solubility product (Ksp) for an slightly soluble hydroxide salt (Ca(OH)2) at two temperatures and use that information to calculate the standard state Gibbs Free Energy, entropy and enthalpy for the dissolution reaction. The ion concentrations will be determined by measuring the pH.
Solubility Product
For the slightly soluble salt calcium hydroxide the equilibrium equation can be written as:
Eq. \ref{eqtag3} calculates \( \Delta G^o\) as a function of T by measuring K at a given T. In \ref{eqtag4} we can calculate the temperature dependence of \(\Delta G^o\) on \(\Delta H^o\) and \(\Delta S^o\), but it needs to be noted that in this treatment we are considering both \(\Delta H^o\) or \(\Delta S^o\) to be constant with respect to temperature. This is normally true for the enthalpy but is often not true for the entropy, especially in the gas phase.
We can rearrange Gibbs Free Energy Expression so it is equal the constant Enthalpy value
If we measure Ksp at two temperature we can use eq. \ref{eqtag3} to calculate the free energy at those two temperatures, which when substituted into eq \ref{eqtag5} gives:
Now that we know one \(\Delta G^o\) and \(\Delta G^o\) we can solver for ∆Ho at any temperature.
\[\Delta H^o = \Delta G^o +T \Delta S^o\]
Theoretical Values
In this experiment you are going to calculate the percent error for ∆Gorxn, ∆Horxn and ∆Sorxn. The theoretical value will be determined from the standard thermodynamic formation values data at 298K. The reaction we are studying is:
\[Ca(OH)_2(s) \leftrightharpoons Ca(OH)_2(aq)\]
Standard thermodynamic formation energies at 298 K
Species
∆Hof (kJ/mol)
Sof (J/mol-K)
∆Gof (kJ/mol)
Ca(OH)2(s)
-986.09
83.39
-898.49
Ca(OH)2(aq)
-1002.82
-74.5
-868.07
We will consider ∆Horxn and ∆Sorxn to be independent of temperature and so you will use the above values of ∆Hof and Sof at 25oC to calculate ∆Horxn and ∆Sorxn at both temperatures. On the other hand ∆Gorxn is dependent on temperature and so you will need to calculate it at both of the temperature values you measured the pH at using Gibb's Free energy equation:
At first glance you would think that the entropy of dissolving solid calcium hydroxide would be positive because the ions start in the structured form of a crystal and end up dispersed by the solvent, but in fact the entropy of dissolution is negative and the product (dissolved) state is more structured than the reactant state. The reason is that you need to properly identify the system, which is the solute (Ca(OH)2) and the solvent (H2O), and it is the water that is driving the decrease in entropy. Before dissolution you have a very structured solute and a fairly unstructured solvent. As the solute dissolves both the cation and the anion interact with water. The cation (Ca+2) functions as a Lewis base and forms a complex ion (last lab) with the water which functions as a Lewis base and donates electrons to the vacant metal valence orbitals (forming a coordinate covalent bond) and thus the water becomes more structured. Due to ion-dipole interactions the complex ion also orients water molecules around it in a manner that maximizes attractions and minimizes repulsions (review section 11.2) . The anion (OH-) is part of the dissociation of water and according to Le Chatlier's principle (review section 15.6) the added hydroxide will push the water equilibria to the undissociated side and thus further reduce entropy (OH- + H+ --> H2O). So the solute is undergoing an increase in entropy while the solvent is undergoing a decrease, and the latter effect is the greater, resulting in an overall decrease in entropy for the system.
Experimental Procedures
In this lab we are going to use Vernier's pH and temperature probes to measure the pH and temperature of saturated solutions of calcium hydroxide at two different temperatures.
Hook up pH probe and temperature probe to LabQuest and view in meter mode
be sure to rinse tips with water everytime you make a measurement
Take some saturated calcium hydroxide solution and cool in an ice bath until the temperature is between 10 and 15oC.
Pour enough into a 50 mL beaker so the pH probe tip will be fully submerged and measure the temperature and pH, recording the values into your data sheet.
Be sure to read both measurements at the same time.
Take some saturated calcium hydroxide solution and heat on a hot plate until the temperature is between 45 and 60oC.
Pour enough into a 50 mL beaker so the pH probe tip will be fully submerged and measure the temperature and pH, recording the values into your data sheet.
be sure to read both measurements at the same time
Properly dispose of reagents
Data Analysis
This workbook only has two sheets, a cover sheet and a datasheet. Each student works on their own Google Sheet which they need to register with the instructor through the Google Form an the report page of this lab.
Cover Sheet
As always, the first sheet is the coversheet. Place numerical values in the results column and units in the units column (do not put both a number and the unit in the same data cell).
Data Sheet
The orange cells of the data sheet are where you paste the raw data and the blue cells are the results of calculations. You can do the calculations by hand or use a function within Google Sheets. Note, the theoretical values of enthalpy and entropy are determined from the standard state values, which are provided in the background section of this lab. The percent error equation is also in the background section of this lab.