4.1: Lab 4 Instructor Notes

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Jessica Garber-Morales
Tidewater Community College

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Lab 4: Reactions of Calcium

Remind Students:

Goggles MUST be worn whenever ANY group has chemical at their station.
Go through the safety in a good amount of detail
- Acids are caustic.
- Bases are caustic. (They do not know to be careful about this!)
- Hydrogen gas is explosive.
- Metals react with water (don’t touch).
- Glassware should be dry when they add metal.
- The reaction between Ca and HCl is violent and hot.
Make sure they secure all equipment in their bin when moving the bin between the cart and their stations.
All glassware must be cleaned and dry (in their bin) before leaving lab.

When reviewing the protocol explain:

Part A: This is the exact reaction and set-up they did last week except on a smaller scale and in a qualitative (not quantitative manner).
- Demonstrate how to use the beaker and test tube.
- Make sure they have the test tube AND water in the beaker ready BEFORE they get the calcium.
- Have them put the test tube over the Ca to collect gas.
- If it produces too much gas for the test tube to hold, it is okay (students will be upset by this, but remind them this is a qualitiative analysis).
- The point here is to see the reaction happening. They will be doing signs of a chemical reaction in class soon and this is a GREAT example.
- They will not have seen litmus paper so you will need to show them how to use it. If they do not get the litmus paper to change color, have them add another small piece of calcium and try again.
- Part B: They need to calibrate the pipettes the way they did the graduated cylinder in Lab 2 (Density). Hopefully they will see the connection here.
  - Suggest the same student handle the same pipette throughout the procedure to ensure all drops are the same.
  - Make sure they understand the pipette need to be held vertically to get identical drops and best accuracy.
  - Tell them A is for acid and B is for base. The same pipette should ONLY touch that chemical for the entire lab to avoid contamination and errors.
  - You will probably need to explain how to convert to mol/drop. They usually want to stop at mol/mL instead, but Part C is easier with the unit mol/drop in case they add the wrong number of drops.
- Part C: Here they will react Ca and HCl. Explain that they are intentionally adding too much HCl so that we can react the extra acid with base. This is the only way to find the experimental mol Ca: mol HCl reacting with Ca ratio.
  - Use DRY test tubes. Or Ca will react with excess moisture in test tube.
  - The reaction will be violent. The test tube may get hot (depending on their definition of hot). You may want to consider: either having students put the test tube in a beaker while they add acid OR use a paper towel or mitt to hold the test tube.
  - Explain how the indicator works.
  - Tell them to swirl (to the best of their ability) between each drop of NaOH.
- You will need to walk them through how to get each column.
  - Mass Ca: scale
  - Mol Ca: molar mass conversion.
  - Drops HCl added.
  - Mol HCl added: use the conversion from Part B to find this.
  - Mol NaOH added: use the conversion from Part B to find this.
  - Mol EXCESS HCl is the SAME as mol NaOH
  - Mol HCl reacting with Ca will be the Mol HCl added – Mol Excess HCl.
- They should complete the chart for their two trials. Check their numbers.
- Have them put columns mass Ca, mol HCl added, and mol NaOH added on the board. DO NOT let them put any other information on the board. Each student should fill out their own chart to give them adequate practice with stoichiometry.
- Graph – They will need to graph mol HCl reacting with Ca vs Mol Ca.
  - Hopefully they can predict the 2:1 ratio based on compound formula and see from their data that there is a 2:1 ratio, but if not, the graph will confirm this relationship.
  - When they attach the graph, it should have a trendline.
  - The SLOPE gives them the ratio of mol HCl : mol Ca.
  - If the trendline gives them a weird number (ie 0.4), then they have EITHER graphed Ca vs HCl instead of HCl vs Ca OR they need to set their intercept at 0.

Contributors and Attributions

College Chemistry 1. Authored by: Jessica Garber-Morales. Provided by: Tidewater Community College. Located at: http://www.tcc.edu/. License: CC BY: Attribution