6: Acid-Base Equilibria Part 2

Overview

In this lab you will run 4 titrations using the ChemCollective Virtual Lab, report your results in a Google sheet that your instructor will give your team and you need to register the Google Sheet with your instructor's grading sheet through a Google Form. This part of the experiment introduces you to the Google Form and how to use the Google Sheet.  Each titration has its own Virtual Lab page in LibreText and its own tab in the Google Sheet (in parenthesis of list below)

1. Weak Base by Strong Acid (WB/SA)
2. Diprotic Acid by Strong Base (diprotic_A/SB)
3. Weak Acid and Strong Base (WA/SB), note this is not really a titration
4. Solid Weak Acid by Strong Base (Solid_A/SB)

Students should work in pairs of 2 with one student running the virtual lab and the other writing the data in the appropriate tab (each experiment has its own tab) and they should switch roles between experiments.  It is important that all students know how to navigate the virtual lab as you will need to use it during your final exam.

ChemCollective Virtual Lab

In this experiment we will be running the ChemCollective Virtual Lab developed by David Yaron at Carnegie Mellon University. This lab is a computational simulation where the equilibrium concentrations of products and reactants are calculated by applying the thermodynamic equations we will be studying in Chapter 18.  That is by knowing the enthalpy and entropy of a reaction the free energy can be calculated, which in turn gives the equilibrium constants, and these can then be used to determine equilibrium concentrations from initial conditions.  From the enthalpy of reaction the amount of heat can be generated and temperature changes can also be calculated with calculations involving the specific heat capacity. The virtual lab considers at 25^oC to be the ambient temperature and so any exothermic reaction instantly shows an increase in temperature and then cools to 25^oC  while any endothermic reaction has an instant drop in temperature and then warms to 25^oC. As it is cooling or heating the equilibrium constant is recalculated and the concentrations change accordingly.

Note:

The Virtual Lab uses the following relationships that are covered in section 18.5

\[\begin{align}\Delta G^o & = \Delta H^o - T\Delta S^o \\ \Delta G^o & = -RT\ln K \\  & \therefore  \\ K & =e^{\frac{-\Delta G^o}{RT}} \\  & and \\ K & =e^{\frac{-(\Delta H^o-T \Delta S^o)}{RT}} \end{align} \]

and from section 5.2 

\[ c=\frac{q(J)}{m(g)\Delta T(K)}\]

where \[\Delta H^o=q_p\]

The Virtual Lab Does not account for kinetics, and the equilibrium values are instantly displayed. The change in concentration as it warms or cools to 25^oC may look like kinetics but this is because of the way the Virtual Lab models heat transfer, where \(\frac{\Delta T}{\Delta t} =k \Delta{T}\). That is, the rate of heat transfer (temperature change) is proportional to the temperature change and so a hot object cools from 99^oC to 98^oC  faster than a cooler object cools from 26^oC to 25^o (the math is the same as first order kinetics. If you think about it, this is the same relationship of first order kinetics where the rate of change of the concentration is proportional to the concentration (R=-k[A]), it is just that here the independent variable is the temperature difference and not the concentration.

 

Virtual Lab Interface

The virtual lab has a "Windows intuitive" graphical user interface where you can perform actions like mix solutions by dragging containers with your mouse and right clicking on objects.  The left pane is a stockroom where you can obtain supplies and chemicals and the right (main) region is the workbench where you can mix material.  Be sure you have loaded the correct lab as sometimes it loads the "Default Lab", which will not have your unknowns.  When you click on a container the stockroom viewer converts to an information viewer that provide information on the substance in the container you mixed.  If you have problems you can refresh your page, or do a hard refresh, by holding the <ctrl> key while also hitting <F5>.

 

 

Figure \(\PageIndex{1}\): Overview of the virtual lab

 

Transferring Fluids

To transfer a fluid you use the mouse to move the container it is in to the top of the container you want to transfer to, and when you release the mouse click when you see some a dotted square line appear around the container you want to transfer.

Figure \(\PageIndex{2}\): Copy and Paste Caption here. (Copyright; author via source)

Controlling Temperature

There are essentially two ways to control temperature, using the Bunsen burner to heat a container or by right clicking on the container (figure \(\PageIndex{3}\).  We will use the right click option as it also gives you the ability to isolate the container from the surroundings.  This does not mean it maintains a constant temperature and if you mix two solutions that undergo an exothermic reaction the temperature will rise, but it will not cool back to room temperate (25^oC) and will stay at the elevated temperature. In effect it converts a container into an "ideal calorimeter" that looses no heat.

Figure \(\PageIndex{3}\): Left image shows the mouse "right click" options and the right image shows the "thermal properties" options, which is an easy way to change the temperature and isolate a solution from the surrounds.

 

In a normal semester we use the Virtual Lab for postlabs and prelabs, often with the goal of comparing the results in the real lab to those of the virtual lab. Due to COVID19 this has partially pivoted to replacing the real lab, but we are not going to use it to emulate the real lab.  That is, you could run your experiments in "realistic mode" (figure \(\PageIndex{2}\)) where and use devices like burets to transfer fluids (where it is transferred as long as you hold down the mouse button), but we will use in in "precise mode", where you type in a value and precisely that amount of material is transferred.  The former technique emulates uncertainty in measurements and your results would have significant digits, but we are not going to worry about that in these activities.

View Menu Options

The view menu allows you to toggle different options in the Information Display.  Figure \(\PageIndex{4}\) shows some of these options.  For example, you can see how much solid is undissolved or the current concenctration of the different species present.  It should be noted that if species are changing values it is not because equilibrium is not achieved, but because the system has not reached room temperature and you will often want to right click and instantly change the temperature to 25^oC (room temperature in the Virtual Lab).

Figure \(\PageIndex{4}\): Through the view menu option you can display different options in the Information display.  Here we are clicked on the Erlenmeyer flask that clearly has a solid acid added to it, and not all the acid is dissolved.

 To remove the View menu pop-up window you click on the view menu tab again.

 

 

Google Sheet

In this lab two students will share a Google Form that will be sent to you by your instructor through Google Classroom.  The form will have 5 tabs, a "Dashboard" (cover page) on which you will submit your answers and copies of your titration curves and a data tab for each experiment where you will place your data and work it up (make the titration curve, or whatever it is that the assignment has you do)

 

Figure \(\PageIndex{5}\): Dashboard tab of Google Sheet

 

 

Google Form

You need to fill out this form, Submit only one form per group and include all partners (if there is a group of three include all three names)

Figure \(\PageIndex{6}\): Your instructor will send you a form to register your Google Sheet in so that it can be graded.