Mrs. Farrales Nikita Pandya October 23, 2012
December 3, 2012
In the method of continuous variations the total number of moles of reactants is kept constant for the series of measurements. Each measurement is made with a different mole ratio of reactants. A mole ratio is ratio between the amounts in moles of any two compounds involved in a chemical reaction. Mole ratios are used as conversion factors between products and reactants in many chemistry problems. The optimum ratio, which is the stoichiometric ratio in the equation, form the greatest amount of product, and, if the reaction is exothermic, generate the most heat and maximum temperature change, Double replacement reactions are generally considered to be irreversible. The formation of an insoluble precipitate provides a driving force that makes the reaction proceed in one direction only. In a double reaction, the two reactants which are aqueous solutions (which can be broken down), can form two products one also an aqueous solution, and another which can be a precipitate, water, or a gas, which cannot be broken down, therefore making the reaction irreversible.
The objective/goal of this laboratory is to find the optimum mole ratio for the formation of a precipitate in a double replacement reaction and use this information to predict the chemical formula of the precipitate.
How can the products of a double reaction be predicted?
How will it be determined if a product is aqueous or a precipitate? How will the method of continuous variations help determine the mole ratio of the two reactants?
1. Copper (II) chloride solution, CuCl2, 0.05 M, 210 mL
2. Iron (III) nitrate solution, Fe(NO3) 3, 0.1 M. 110 mL
3. Sodium hydroxide solution, NaOH, 0.1 M, 320 mL
4. Sodium phosphate, tribasic, solution, Na3PO4, 0.05 M, 210 ml 5. (14+) Test tubes (some graduated, some without graduations) 6. Black Marker
7. Marking tape
8. (2) Stirring rods, large
10. (2) Test tube racks
11. (2) Pairs of gloves
12. Lab goggles
13. Lab apron
15. Para film
The lab was set up, as it is seen in figure 1, with seven test tubes in a test tube rack. Using a clean 10mL graduated cylinder, the appropriate volume of iron(III) nitrate solution was taken from its container and transferred/added to each test tube using a pipet. Using a another clean 10mL graduated cylinder, the appropriate volume of sodium hydroxide solution was taken from its container and transferred/added to each test tube, which already contained iron(III) nitrate, using a pipet.
Before the timer was started, each of the solutions in the test tube was stirred/mixed with a large stirring rod. This ensured that both the reactants mixed properly. After stirring the solutions, observations were noted for any signs of chemical changes.
The mixtures were to be left for 10 minutes (a timer was used) to sit undisturbed, because any movement of the test tube could cause a hindrance in the settlement of the precipitate. Though each test tube was left undisturbed for 10 minutes, final observations were made after the solutions were left to sit undisturbed for 24 hours.
After the 24 hours of settling, the volume of the precipitate in each test tube was measured and recorded. For test tube with graduations, seeing the numbers at eye level made the calculations, but for test tubes with no graduations a different method was used to measure the volume of the precipitate. First another test tube of the same size was found, then using a pipet, 1ml of water was measured in a 10mL graduated cylinder, and then poured into the similar size test tube. Using a black marker graduations were...