Purpose
The goal of this lab is to verify Hess’s Law: if the sum of two chemical reactions is a third reaction then the sum of the two reaction enthalpies is the enthalpy of the third.
Procedure Notes [LINK]
- make sure that the calorimeter is clean and dry before each use
- do not contaminate the solutions with each other as that will distory the results
- any acid spill – neutralize with baking soda
- any base spill – neutralize with vinegar
- the smell of ammonia is powerful – be very careful and do not directly smell
- be sure and ask how to be coached on using the GLX to save files
Data Processing
- be sure and determine the heat capacity of the calorimeter
- determine the change in enthalpy for each reaction (per mole)
- determine the change in enthalpy for the unknown reaction (per mole) using your data
- calculate the percent difference between the theoretical and experimental values
- be sure and complete pre-lab if you did not already do so
Posting Information
- make sure the data, calculations, graphs are in the LNB
- the final data, calcs, graphs, results should be posted to Tumblr in polished, final form
Friday, October 21 – Lab Notes
The lab explores two gas laws: Boyle’s Law and Gay-Lussac’s Law. The data collection devices will be used to collect pressure and temperature information which will then be used to verify each of the laws. In addition, the second experiment will be used to estimate absolute zero. [LAB INSTRUCTIONS]
The first lab is quite straightforward
- be sure and start with the volume in the tube at its largest for the lab. Each trial should be smaller.
- Be sure and get to the smallest volume possible as this is where you will see the greatest pressure differences
The second lab has a few more intricacies
- you should examine the widest possible temperature range you can during the lab – this maximizes the possibilities for obtaining a meaningful data
- you should start with the hottest temperature and get colder – this will prevent the stopper from popping out of the erlenmeyer flask
- at no time should the stopper break the seal, as this will invalidate all of your data moving ahead
Analyzing the data
- for the first lab plot two graphs: P vs. V and P vs. 1/V (put the line of best fit on each)
- for the second lab, plot a graph of P vs. T (add the line of best fit)
- did the results verify Boyle’s Law?
- what did you determine to be absolute zero?
Friday, October 7 – Lab Notes
The lab is an example of calcualting the volume of a gas produced by a reaction [Instructions]. The reaction is quite simple as magnesium is reacting with hydrochloric acid, producing hydrogen gas. What if this gas was captured? IS it possible to use this as a way to determine the molar volume of a gas?
Obviously the answer is “yes” as this is what we will do. There are a few aspects to keep in mind when performing this lab.
- make sure the magnesium is the limiting reactant in this process so that it can predict the moles of hydrogen gas produced
- be sure and level the tube to the same level as the water outside of the tube, guaranteeing that the pressure of the gas inside the tube is the same as that outside the tube (atmospheric pressure)
- the temperature of the water is assumed to be the temperature of the gas, which is a great assumption given that the gas was produced and then bubbled through the water.
- the partial vapor pressure of water depends only upon the temperature of the water [LINK] and is to be subtracted from the total pressure of the gas to find the pressure of just the hydrogen
- once corrected, the molar volume of the gas is a simple “combined gas law” problem.
Safety & Other Notes
- strong acids are neutralized with sodium bicarbonate
- be sure and keep bubbles from entering the tube when inverting the tube
Friday, September 30 – Lab Notes
The lab is an example of the stoichiometry of a reaction in solution [Instructions]. Two solutions will be mixed in varying ratios with and the temperature change recorded for each trial. From this data it is possible to determine the mole ratio of the participants in the reaction.
Notes to consider for the procedure:
- It is critical to record the temperature of the solutions before mixing. These starting points will serve as the baseline for temperature rises needed to determine the change.
- The starting temperature for each trial is a weighted average of the starting temperatures of each of the two solutions used.
- All of the data will be combined into one spread sheet shared through Google Docs [LINK]. You must enter your data into this for all of the class to use. Your lab report will depend on using everyone’s data.
- Be mindful fo the reaction solutions as the bleach and the thiosufate are irritants and can harm your skin and eyes if they come into contact with you. The thiosulfate is a basic solution and can be neutralized with weak acid (vinegar)
For Monday, please find the results for the lab and answer the discussion questions.
NOTE – SUNDAY EVENING
Solution using class data (See graph above)
y = 1.4778 x
y = -3.9663 x + 99
set together and solve
1.4778 x = -3.9663 x + 99
x = 18.2 mL
mole ratio = volume ratio
mole ratio = (18.2 mL / 6.8 mL) = 2.7 moles A / 1.0 moles B
Monday, September 19 – Lab Notes
The lab is an example of quantitative analysis, where we use a precipitation reaction to determine the composition of a solid. [Instructions] Besides reading the instructions for the lab, there are a few pointers that need to be heeded:
- pipette skills – when dispensing very string acids, it is necessary to use a pipette and bulb
- safety with acid – care should be taken to be safe when using acids, with a neutralizer at the ready in case of spillage
- weigh boats for everything – when using an analytical balance, it is necessary to use weigh boats to eliminate contamination
- drying oven – be sure and place the crucible in the drying oven after the initial washing and after forming the PPT.
- be sure and use only distllied water when washing glassware, and wash everything
The lab will be done in two days: the first part will be done during class on Monday. The second part will be done during office hours, before or after school (or lunch) on Tuesday. The final massing of the PPT will be done before class on Wednesday.The calculations will be done before class on Wednesday for discussion.
Lab Schedule
- Monday class – wash crucible leave in oven, mass alloy, react alloy, PPT with NaCl
- Before Wednesday – filter PPT, dry in oven, find mass of crucible with contents
- Wednesday – lab quiz, discuss calculations and results
