Chemistry 534 Name:_________________

Energy in Chemical Reactions Unknown Number:_________

Purpose: To see the difference between an exothermic and endothermic reaction and to use calorimetry to determine the mass % of LiCl in an unknown mixture containing LiCl_(s) and KNO_3(s) .

Materials: Chemicals: unknown mixture, LiCl_(s) and KNO_3(s) .

Apparatus : spatula, thermometer, 50 mL graduated cylinder, a styrofoam cup , weighing boat

Instrument: electronic balance

Procedure:

1. Tare a weighing boat. Weigh out exactly 1.0 g of LiCl.

2. Pour exactly 50.0 mL of water into a styrofoam cup.

3. Record the initial temperature of the water.

4. Add the 1.0 g of LiCl to the water. Stir to dissolve.

5. Record the maximum temperature.

6. Clean everything from the above 5 steps.

7. Tare a weighing boat. Weigh out exactly 1.0 g of KNO₃.

8. Pour exactly 50.0 mL of water into a styrofoam cup.

9. Record the initial temperature of the water.

10. Add the 1.0 g of KNO₃ to the water. Stir to dissolve.

11. Record the maximum temperature.

12. Clean everything from the above 5 steps.

13. Tare a weighing boat. Weigh out exactly 1.0 g of the unknown.

14.. Pour exactly 50.0 mL of water into a styrofoam cup.

15. Record the initial temperature of the water.

16. Add the 1.0 g of the unknown to the water. Stir to dissolve.

17. Record the maximum temperature.

18. Clean everything from the above 5 steps.

Data:

Solid added ( 1.0 g)	Initial temperature of 50.0 ml of H2O	Final temperature of 50.0 ml of H2O
LiCl
KNO3
Unknown mixture of LiCl and KNO3

Analysis:

1. Which of the above reactions was exothermic?

Write an equation to represent what happened. Remember you dissolved a solid which then turned into aqueous ions.

2. Which of the above reactions was endothermic?

Write an equation to represent what happened.

3. a. Use Q = mcDT to calculate the amount of heat that water absorbed when LiCl lost it.

Remember m = mass of water.

b. Divide this amount of heat by the mass of LiCl used. Make sure you include the unit. It's the only thing that changes! (In # 5 we'll refer to this amount as W. )

4. a. Use Q = mcDT to calculate the amount of heat that water lost when KNO₃ absorbed it.

b. Divide this amount of heat by the mass of KNO₃ used. Make sure you include the unit. It's the only thing that changes! (In # 5 we'll refer to this amount as Z. )

5. Find the % of LiCl in the unknown mixture.

Hint 1:

let x = grams of LiCl in unknown

y = grams of KNO₃ in unknown

xW + yZ = mcDT,

where W = amount calculated in 3b

Z = amount calculated in 4b

m = mass of water

DT = temperature change caused by unknown

Hint 2: You'll need a second equation to solve for x.

Hint 3: Once you know x, you can find the mass % of LiCl in the unknown mixture.