1. Mole-Mole Ratios Post-Lab Discussion

2. 1. Continuous Variations  In the continuous variations method the ratio of moles of reactants is gradually changed while a constant solution volume is maintained. The reaction is exothermic. The maximum amount of heat will be given off when the correct mole ratio is combined. Because the solution volume is constant, the change in temperature at the optimum mole ratio will be the greatest.

3. 2. Total volume kept constant…  The maximum amount of heat is given off when the optimum mole ratio of reactants is combined. If the solution volume is a constant, the change in temperature will be proportional to the amount of heat evolved. If the volumes were not constant, a calculation would have to be made relating temperature change to heat evolved for each measurement.

4. 3. Same concentrations?  If the concentrations were not the same, a calculation must be made for each measurement to relate the temperature change to the heat evolved and moles reactants present.  Having the same concentrations eliminates the need for this extra calculation.

5. 4. Limiting Reagent?  The limiting reagent is the reagent that is completely consumed in a chemical reaction.

6. 5. Limits the precision?  The volume reading limits the precision. This measurement relies on the human eye to measure EXACT amounts with a graduated cylinder. The temperature reading was given out with decimal places.

7. 6. Limiting Reagent upward?  On the upward sloping line of the graph, the limiting reagent is the Hypochlorite ion and the excess reagent is the thiosulfate ion, until the 40 mL mark.

8. 6. Downward sloping line?  Thiosulfate ion is the limiting reagent and the excess reagent is the hypochlorite.  The perfect ratio: Where there is no limiting or excess reagent?  Where the lines intersect: 40 mL hypochlorite to 10 mL thiosulfate.

9. 7. Physical Properties?  Some other properties which could be used are the intensity of the color of a reactant or product, the mass of a precipitate that forms, or the volume of a gas that forms.

10. 8. Why graph?  In may be that the exact mole ratio was not chosen as a data value. Also, the graph averages several values to find the optimum ratio rather than relying on only one value.

11. 9. If the two solutions are not at the same initial temperature, a correction must be made to find the correct change. How?  Graph the Delta Temp vs. Volume if temp is not the same of two solutions.  Graphing the Final Temp vs. Volume is sufficient if the two solutions are at the same temp.

12. 61. Aluminum sulfide reacts with water to form aluminum hydroxide and hydrogen sulfide.  A. balanced equation?  B. how many grams of aluminum hydroxide from 6.75 g of aluminum sulfide?

13. 67.Define the terms:  Limiting Reagent: - determines the amount of product formed. It runs out first.  Excess Reagent: -is left over when the reaction stops.  Why are the amounts of products formed in a reaction determined only by the amount of the limiting reactant?  The L.R. is completely used up. No more product can be made when one of the reactants is gone.

14. 68. Define the following terms:  A. Theoretical Yield: the quantity of product that is calculated to form when all of the limiting reactant reacts.  Actual Yield – the amount of product actually obtained in a reaction.  Percent Yield: relates the actual yield to the theor. Yield.  % Yield = actual/theor x 100%

15. 73. Given 1 gram of both reactants, Find the L.R. and grams of CO 2 formed, excess remain  3NaHCO 3 + H 3 C 6 H 5 O 7  3CO 2 + 3 H 2 O + Na 3 C 6 H 5 O 7