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 KNO_{3}.
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 KNO_{3 }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 H_{2}O 
Final temperature of 50.0 ml of H_{2}O 
LiCl 


KNO_{3} 


Unknown mixture of LiCl and KNO_{3} 


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 KNO_{3} absorbed it.
b. Divide this amount of heat by the mass of KNO_{3} 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 KNO_{3} 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.