Thermochemistry. Questions to ponder. Because 4/20/14. an ice-cube? an ice-cube? Part 2: Calorimetry. But I KNOW. Q=mc T, but T=0

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1 Thermochemistry Part 2: Calorimetry p p If you leave your keys and your chemistry book sitting in the sun on a hot summer day, which one is hotter? Why is there a difference in temperature between the two objects? Because p Different substances have different specific heats (amount of energy needed to raise the temperature of 1 g of a substance by 1 degree Celsius). Q=mc T, but T=0 But I KNOW 0 1

2 How can I solve this??? Calorimetry!!! Heat reuired to melt ice (a.k.a. latent heat of fusion) cannot be measured directly, but calorimetry provides an experimental method allowing this heat transfer to be measured indirectly. Calorimetry!!! Calorimetry: measurement of the amount of heat evolved or absorbed in a chemical reaction, change of state or formation of a solution. CALORIMETRY p The enthalpy change associated with a chemical reaction or process can be determined experimentally. n Measure the heat gained or lost during a reaction at CONSTANT pressure p A calorimeter is a device used to measure the heat absorbed or released during a chemical or physical process Coffee Cup Calorimeter The cup is filled with water, which absorbs the heat evolved by the reaction, so: Coffee Cup Calorimeter p A more high tech drawing Styrofoam cup ice = - water OR rxn = - cal 2

3 What happens in a calorimeter? p One object will LOSE heat, and the other will ABSORB the heat n System loses heat to surroundings = EXO = - n System absorbs heat from surroundings = ENDO = + When a hot chunk of metal is dropped in a cool glass of water, the metal cools off. Where did the heat from the metal go? Did the metal lose more heat then the water gained? Magnitude of HEAT GAINED = HEAT LOST (ALWAYS!) To do calorimetry problems p Make a chart: measurement Heat () Mass (m) Specific Heat (c) Final Temp ( ) Initial Temp( ) The numbers in these two boxes are always the same, but with different signs (+/-). What heat one lost, the other gained. Cal (often water) Object/ Rxn EXAMPLE 1: A small pebble is heated and placed in a foam cup calorimeter containing 25.0 g of water at 25.0 C. The water reaches a maximum temperature of 26.4 C. How many joules of heat were released by the pebble? The specific heat of water is J/g C. Heat Water (cal) Mass 25.0 g Specific Heat Final Temp Initial Temp 26.4 o C 25.0 o C Pebble (rxn) The numbers in these two boxes are always the same, but with different signs (+/-). What heat one lost, the other gained. Hints: p The pebble lost heat because the water heated up from 25.0 C to 26.4 C. p Pebble loses heat (-, exothermic) while water gains heat (+, endothermic) p Do you calculation based on water (since the problem gave all the water s information) Heat Water (cal) Mass 25.0 g Specific Heat Final Temp Initial Temp 26.4 o C 25.0 o C cal = m water c water ΔT water Pebble (rxn) cal = (25.0g)(4.184J/g o C)(26.4 o C-25.0 o C) cal = 146 J rxn = J If the water ABSORBED 146 J of heat, then the pebble RELEASED 146 J of heat. Example 2: When 1.00 g of ammonium nitrate, NH 4 NO 3, is added to 50.0 g of water in a coffee cup calorimeter, it dissolves, NH 4 NO 3 (s) NH 4+ (a) + NO 3 - (a), and the temperature of the water drops from C to C. Calculate for the reaction system. m cal c g o C o C rxn 1.00 g cal = m w c w Δt cal = (50.0g)(4.18 J/g C)(-1.68 C) cal = J rxn = - cal rxn = 351 J (endothermic) When one substance absorbs heat, the other substance releases heat (energy) 3

4 Example 3 (similar to what you will do in tomorrow s lab) p Suppose that g of water at 22.4 C is placed in a calorimeter. A g sample of Al is removed from boiling water at a temperature of 99.3 C and uickly placed in a calorimeter. The substances reach a final temperature of 32.9 C. Determine the SPECIFIC HEAT of the metal. The specific heat of water is J/g C. è MAKE YOUR CHART Example 3: Suppose that g of water at 22.4 C is placed in a calorimeter. A g sample of Al is removed from boiling water at a temperature of 99.3 C and uickly placed in a calorimeter. The substances reach a final temperature of 32.9 C. Determine the SPECIFIC HEAT of the metal. The specific heat of water is J/g C. m cal (H 2 O) c g 32.9 o C 22.4 o C rxn (Al) g 32.9 o C 99.3 o C 1. Make chart 2. Calculate for water 3. Q for Al is the same (but with different sign) as for metal. 4. Using metal, calculate c metal Example 3 cont d: specific heat of aluminum cal (H 2 O) rxn (Al) 4,393.2 J -4,393.2 J m g g c J/g C 32.9 C 32.9 C 22.4 C 99.3 C cal = m w c w ΔT cal = (100.00g)(4.184J/g C)(10.5 C) cal = 4,393.2 J rxn = - cal rxn = -4,393.2 J rxn = m Al c Al ΔT -4,393.2 J = (75.25 g)(c Al )(-66.4 C) c Al = J/g C p What if you wanted to measure the heat of a reaction or process that couldn t be measured in a simple coffee cup calorimeter (e.g., heat of combustion of Mg)? You would need something like this Bomb Calorimeter 4

5 Another type of calorimeter is a Bomb Calorimeter If Heat Capacity (C) is known It is possible to calculate the amount of heat absorbed or evolved by the reaction if you know the heat capacity, C cal, and the temp. change, ΔT, of the calorimeter: cal = C cal ΔT NOTE: In a bomb calorimeter, heat is transferred from the sample to the oxygen-enriched chamber, to the metal that makes up the chamber, to the water thus we cannot just use the specific heat of water; instead heat capacity of the calorimeter, C cal, can be used or calculated. Everything else is the same (remember, the heat lost from the reaction goes into the calorimeter) EXAMPLE 4: The reaction between hydrogen and chlorine, H 2 + Cl 2 2HCl, can be studied in a bomb calorimeter. It is found that when a 1.00 g sample of H 2 reacts completely, the temp. rises from C to C. Taking the heat capacity of the calorimeter to be 9.33 kj/ C, calculate the amount of heat evolved in the reaction. EXAMPLE 5: When 1.00 mol of caffeine (C 8 H 10 N 4 O 2 ) is burned in air, 4.96 x 10 3 kj of heat is evolved. Five grams of caffeine is burned in a bomb calorimeter. The temperature is observed to increase by C. What is the heat capacity of the calorimeter in J/ C? c Cal 9.33 kj/ o C o C o C Rxn cal = C cal ΔT cal = (9.33 kj/ C)(9.82 C) cal = 91.6 kj rxn = kj *** must have a negative sign if heat evolved in the reaction = released 4.96 x 10 3 kj is for 1.00 mol of caffeine. We are burning only 5.00 g of caffeine to increase the temp by o C. We FIRST need to figure out how much heat energy is given off by just 5 grams. c kj = 194.0gC H N O Cal kj? ΔT o C 4 2 Rxn x 5.00 g kj x = kj KJ l = (C cal )(11.37 C) kj J C cal = 11.2! or11, 200! C C EXAMPLE 6: When twenty milliliters of ethyl ether, C 4 H 10 O. (d=0.714 g/ml) is burned in a bomb calorimeter, the temperature rises from 24.7 C to 88.9 C. The calorimeter heat capacity is kj/ C. (a) What is for the calorimeter? (b) What is when 20.0 ml of ether is burned? (c) What is for the combustion of one mole of ethyl ether? c Cal kj/ o C 88.9 o C 24.7 o C Rxn cal = C cal Δt cal = (10.34 kj/ C)(64.2 C) cal = 664 kj EXAMPLE 6: When twenty milliliters of ethyl ether, C 4 H 10 O. (d=0.714 g/ml) is burned in a bomb calorimeter, the temperature rises from 24.7 C to 88.9 C. The calorimeter heat capacity is kj/ C. (a) What is for the calorimeter? (b) What is when 20.0 ml of ether is burned? (c) What is for the combustion of one mole of ethyl ether? rxn = - cal rxn = -664 kj 5

6 EXAMPLE 6: When twenty milliliters of ethyl ether, C 4 H 10 O. (d=0.714 g/ ml) is burned in a bomb calorimeter, the temperature rises from 24.7 C to 88.9 C. The calorimeter heat capacity is kj/ C. (a) What is for the calorimeter? (b) What is when 20.0 ml of ether is burned? (c) What is for the combustion of one mole of ethyl ether? 664kJ 20.0mL 1mL g 74.0 g = mol 3 kj mol 6