Two slices of bread + 1 piece of cheese + 1 piece of ham = 1 ham and cheese sandwich

Transcription

1 Lab #7 Limiting Reactants Background /24 Name: When you are making a ham and cheese sandwich, you need two slices of bread and one piece of ham and cheese. How many complete sandwiches could you make if you had eight slices of ham, seven slices of cheese and eighteen slices of bread? Obviously, there is not enough cheese to make use of all of the ham available. There is some cheese to make some sandwiches but not enough cheese to use up all of the ham. If there was more cheese, you would get more sandwiches. Since you would run out of cheese for the sandwiches, the cheese becomes your limiting reagent and the ham and bread becomes your excess reagents. Two slices of bread + 1 piece of cheese + 1 piece of ham = 1 ham and cheese sandwich Similarly, in many reactions, one reactant becomes entirely consumed and the other reactant is left over. The reactant that becomes consumed is called the limiting reagent. When the limiting reagent is all consumed, no more product can be formed (reaction complete). The reactant limits the amount of product that can be formed. In the sandwich analogy above the bread, cheese and ham are not just analogies for the reactants; they also represent the number of moles or particles available for reaction. As soon as one reactant is consumed, the reaction stops because there are no longer particles of that reactant available for reaction. The reaction is limited by the availability of this reagent. In this experiment, you will predict and observe a limiting reactant during the copper (II) chloride oxidation. You will use the single displacement reaction of aluminum with aqueous copper (II) chloride. 2Al(s) + 3CuCl 2 (aq) 3Cu(s) + 2AlCl 3 (aq) Copper (II) chloride, CuCl 2, turns a light blue in aqueous solution. This is due to the Cu 2+ (aq) ion. Aluminum chloride is colourless in aqueous solution. Copper use dates back more than 10,000 years. The copper (II) chloride oxidation reaction has been used in petroleum industries for sweetening (a refining process used to remove sulfurous gases from natural gas). This process has also been used for etch regeneration, in which CuCl 2 is used to remove unwanted copper from printed wiring boards and discrete areas are etched away.

2 Purpose In this experiment, you will be: Materials predicting and observing the limiting and excess reagents in a reaction, distinguishing between different substances (reactants and products) based on their changes in colouration distinguishing between limiting and excess reagents in a reaction. conducting limiting reagent calculations able to understand the terms: limiting and excess reagent - 2 x 250-mL beakers - glass stirring rod - Scale or Balance - CuCl 2, anhydrous ( 0.65g and 0.13g) g Aluminum - Water, distilled - 50-mL graduated cylinder Safety - All waste liquids must be placed in the liquid inorganic waste - All solid waste should be handed to your teacher for disposal Procedure 1. Using the balance place a piece of weighing paper or weigh boat, zeroing the scale. 2. Onto the weigh paper, measure 0.65g of CuCl 2 solid. Record this mass: g 3. With the CuCl 2 still on the paper, zero the scale and add 0.25g worth of Aluminum foil cut into small squares. The chemical reaction will not start at this point because the atoms of aluminum and CuCl 2 are not able to collide in the solid state. Record the mass of Al used: g 4. Empty this weigh paper or weigh boat into a 250-mL beaker. Label this beaker with masking tape as beaker 1. Place this beaker aside and prepare the second beaker. 5. Onto the weigh paper, measure 0.13g of CuCl 2 solid. Record this mass: g 6. With the CuCl 2 still on the paper, zero the scale and add 0.25g worth of Aluminum foil cut into small squares. Record the mass of Al used: g 7. Empty this weigh paper or weigh boat into a 250-mL beaker. Label this beaker with masking tape as beaker 2. Place this beaker with your first one. 8. Observe the reactants before the reaction starts. Describe how each look. _

3 9. Look at the amount of each reactant in beaker 1 and beaker 2. In beaker 1 which do you think is the limiting reactant? In beaker 2 which do you think is the limiting reactant? Beaker 1: Beaker 2: 10. Using a graduated cylinder, measure 50-mL of distilled water into beaker 1. Measure a second amount of 50-mL of distilled water into beaker Record any observations you notice at this point of the experiment. (bubbling, colour change, etc). Identify any differences between beaker 1 or 2 if necessary. 12. Stir the substances in the beakers with a glass rod occasionally with the stirring rod. 13. When the reaction appears to have stopped in each beaker record your observations, differentiating between the two beakers clearly. (Did the beakers look different? The Same?) 14. When the reaction is complete empty the contents into the waste beaker. Wash the beakers and place to dry. Analysis 1. In this experiment what does the colour change represent/ tell you is happening? Where was the aluminum metal going? What is the solid that remains at the end of the experiment? [2 pts] 2. According to your observations, which reactant was present for the longest period of time in the first reaction with 0.65g CuCl 2? Is this the limiting or excess reactant? Explain. [2 pts]

4 3. According to your observations, which reactant was present for the longest period of time in the second reaction with 0.13g CuCl 2? Is this the limiting or excess reactant? Explain. [2 pts] 4. Based on your observations from this experiment what was the limiting and excess reactants in each beaker? [2 pts] (a) Beaker 1: Limiting Reactant Excess Reactant (b) Beaker 2: Limiting Reactant Excess Reactant 5. In beaker 1, 0.65g of CuCl 2 was used and 0.25g of Al. What was the limiting reactant and what was the excess reactant? Show your calculations. [2 pts] 6. In beaker 2, 0.13g CuCl 2 was used and 0.25g of Al. What was the limiting reactant and what was the excess reactant? Show your calculations. [2 pts]

5 7. Octane, C 8 H 18, is the principle fuel in gasoline. If kg of octane is allowed to burn in a room with 160kg of oxygen, O 2 by the following reaction: 2C 8 H 18 (l) + 25O 2 (g) 16CO 2 (g) + 18H 2 O(g) (a) Which is the limiting reactant, octane or oxygen? Show how you know. [2 pts] (b) How much excess reactant remains when the reaction is complete? [2 pt] (c) A room is 10.0m x 20.0m x 2.50m in dimensions (V room = l x w x h) (i) Find the volume of this room [1 pt] (ii) Given the limiting reactant above, find the amount of grams of CO 2 that would be produced by the above reaction.[3 pts] (iii) If an engine was running, completing the above reaction, it would produce the amount of CO 2 (grams) in the room. Assuming the room was sealed this could be very dangerous. CO 2 can suffocate an animal, including humans, when present at a concentration of 122.5g/m 3. Would this room become lethal if the above reaction took place in the room described in (c)? Show your work and put the amount of CO 2 present in the room as grams per m 3 [4 pts]