9.2: Lab - Thermodynamics

 

Learning Objectives

Goals: 

• Calculate solubility product for an unknown salt using concentration data of a saturated solution
• From the solubility product
  • Calculate K_sp at two temperatures
  • Calculate \(\Delta\)G^o at two temperatures
  • Calculate S^o
  • Calculate \(\Delta\)H^o

By the end of this lab, students should be able to:

• Understand the difference between Q and K for solubility problems
• Be able to design an experiment where they can calculate ΔG^o,  ΔH^o, and S^o.

Prior knowledge:

• Solubility Product (section 17.4)

Concurrent Reading:

• Free Energy (section 18.5)

 

 

 

Safety

• Emergency Preparedness
  • 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 use the Virtual Lab to calculate the solubility product (K_sp) 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.

 

\[CaOH_{2}(s) \leftrightharpoons Ca^{+2}(aq) + 2OH^{-}(aq)\]
The equilibrium constant expression is:
\[K_{sp}=[Ca^{+2m}][OH^{-n}]^2\]

We can measure [OH^-] by measuring pH, noting pH + pOH = 14, so pOH = pH-14:

\[ [OH^-]=10^{-pOH}=10^{-(pH-14)}=10^{14-pH} \\  \; \\ [Ca^{+2}]=\frac{[OH^-]}{2} = \frac{10^{14-pH}}{2}  \]

Substituting into K_sp gives:

\[K_{sp}=[Ca^{+2}][OH^-]^2= \frac{10^{14-pH}}{2}\left ( 10^{14-pH}  \right )^2= \frac{\left ( 10^{14-pH}  \right )^3}{2}\]

 

Now that you know K you can calculate \(\Delta G^o\)

\[ \Delta G^o =-RTlnK \]

We also know that \[\Delta G^o =\Delta H^o - T\Delta S^o \]

In the above expression we calculate \( \Delta G^o\) as a function of T by measuring K at a given T, and are treating \(\Delta H^o\)  and \(\Delta S^o\) as not being a function of temperature.  This is normally true for the enthalpy, but is often not true for the entropy, which for example undergoes a huge change if over the temperature difference there is a phase transition.  But in this experiment the change in entropy for dissolving a salt in liquid water is negligible and so it can be treated as a constant

We can rearrange Gibbs Free Energy Expression so it is equal the constant Enthalpy value

\[\Delta H^o = \Delta G^o +T \Delta S^o\]

So for state (1) and (2) 

\[\Delta H^o = \Delta G_1^o +T_1 \Delta S^o = \Delta G_2^o +T_2 \Delta S^o\]

Now we can solve the right hand equalty for entropy and have it as a function of \(\Delta G^o\) and T at two temperatures

\[ \Delta G_1^o +T_1 \Delta S^o = \Delta G_2^o +T _2\Delta S^o \] 

Solving for ∆S^o gives:

\[\Delta S^o=\frac{\Delta G_1^o - \Delta G_2^o}{T_2-T_1}\]

Now one can use the data from either temperature to solve for ∆H^o . 

\[\Delta H^o = \Delta G^o +T \Delta S^o\]

 

Theoretical Values

In this experiment you are going to calculate the percent error for  ∆G^o_rxn,  ∆H^o_rxn and ∆S^o_rxn, which can be determined from the standard thermodynamic formation values.  The reaction we are studying is:

\[Ca(OH)_2(s) \leftrightharpoons Ca(OH)_2(aq)\] 

Standard thermodynamic formation energies at 298 K

Species	∆H^of (kJ/mol)	S^of (J/mol-K)	∆G^of (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  ∆H^o_rxn and ∆S^o_rxn to be independent of temperature and so you will use the above values of  ∆H^o_f and S^o_f at 25^oC to calculate  ∆H^o_rxn and ∆S^o_rxn.  On the other hand ∆G^o_rxn is dependent on temperature and so you will need to calculate it at both of the temperature values you measured the pH at.  We will use the Gibb's Free energy equation to calculate ∆G^o_rxn.

\[\Delta G_{rxn}^{o} =\Delta H_{rxn}^{o}-T\Delta S_{rxn}^{o}\]

 

Percent Error

In this lab you will be required to report the percent error, which is defined as:

\[\% Error=\left | \frac{theoretical-experimental}{theoretical} \right |100\]

Entropy of Solution

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 in a second sphere around it.  The anion (OH^-) is part of the dissociation of water and according to LeChatlier's principle 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.

1. Hook up pH probe and temperature probe to LabQuest and view in meter mode
   • Section 0.2.1 (Temperature Probes)
   • Section 0.2.2 (pH probes) note, the pH has a functional temperature range of 5 to 80^oC.  Do not use below 10^oC or above 60^oC.
2. Check pH probe using pH=10 buffer.
   • If necessary, calibrate (section 0.2.2.1)
   • be sure to rinse tips with water everytime you make a measurement
3. Take some saturated calcium hydroxide solution and cool in an ice bath until the temperature is between 10 and 15^o​​​​​​​C.
   • 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.
4. Take some saturated calcium hydroxide solution and heat on a hot plate until the temperature is between 45 and 60^o​​​​​​​C.
   • 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
5. 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).

Figure \(\PageIndex{1}\): Report datasheet. (Copyright; Bob Belford & Liliane Poirot, CC-0)

 

Data Sheet

The brown 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.

Figure \(\PageIndex{2}\): Copy and Paste Caption here. (Bob Belford/Liliane Poirot, CC0.0)

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