Calorimeter: A device in which the heat associated with a specific process is measured.

Transcription

1 1 CALORIMETRY p (simple), (bomb) Calorimeter: A device in which the heat associated with a specific process is measured. There are two basic types of calorimeters: 1. Constant-pressure calorimeter: also called simple calorimeters 2. Constant-volume calorimeter: also known as bomb calorimeters Constant-Pressure Calorimeter Used to measure the heat absorbed or released by a process or reaction taking place at constant pressure. Examples: The heat of acid-base neutralizations or the heat associated with dissolving a salt in water can be measured. A constant-pressure calorimeter can be constructed using styrofoam cups. This is called a coffee-cup calorimeter. A coffee-cup calorimeter is considered to be an isolated system. If an exothermic process occurs, the heat released will be absorbed by the water in which the reaction took place and by the calorimeter. If an endothermic process takes place, heat will be absorbed from the water and from the calorimeter.

2 2 Recall from the First Law of Thermodynamics that: q system = -q surroundings For a process in a simple calorimeter, the system is the chemical reaction, and the surroundings are the water and the calorimeter, so: q reaction = - (q calorimeter + q H2O ) For simple calorimeters, q calorimeter is often so small that it can be omitted leaving us with: q reaction = - q H2O Heat absorbed or released from the H 2 O is calculated from q = mc T. Examples: g of metal X at o C is dropped into 200.0mL of H 2 O at 20.0 o C in a coffee cup calorimeter. Metal X and H 2 O reach thermal equilibrium at 25.0 o C. Calculate the specific heat capacity of X. It may help to visualize the equilibrium temperature as follows: Temperature ( o C) Temperature vs. Time (mcδt) X (mcδt) H2O Time (arbitrary)

3 g of CaCl 2 (s) is dissolved in 100 g of water in a simple calorimeter. Before CaCl 2 was added, the water s temperature was 22.0 o C. When all the CaCl 2 dissolved, the water was at a final temperature of 24.2 o C. What is the heat of solution for this process? Note: a) For a process where the reactants dissolve, the mass of the surroundings is the total mass of the solution. That is: b) We will assume that the solution is dilute enough so that we can use the specific heat capacity of H 2 O. Note: Since q CaCl2 is negative, we know that dissolving this salt in water (the heat of solution) is an exothermic process! 3. In the previous example, dissolving 4.00 g of CaCl 2(s) released 0.96 kj of energy. What is the molar enthalpy ( H) of solution for CaCl 2(s)?

4 4 4. Consider the following neutralization reaction. HCl (aq) + NaOH (aq) NaCl (aq) + H 2 O (l) When 50.0 ml of 1.00 mol/l HCl (aq) and 50.0 ml of 1.00 mol/l NaOH (aq) are mixed in a coffee cup calorimeter the temperature of the resulting solution increased from 21.0 o C to 27.5 o C. What is the heat of this reaction, measured in kj per mole of HCl (aq)? Note: a) Assume that density, d solution = d H2O (1g = 1mL) b) Assume that specific heat capacity, c solution = c H2O c) Total mass, m surroundings = m NaOH + m HCl d) Assume that all heat released is absorbed by H 2 O.

5 5 Constant-Volume Calorimeter Constant-volume calorimeters are also called bomb calorimeters. Used to measure the heat released by a combustion reaction taking place at constant volume. A bomb calorimeter consists of several basic components: A substance is placed in a steel container (the bomb!) The bomb is then filled with high pressure oxygen. The bomb is surrounded by water, and the entire system is isolated from the external environment. The substance in the bomb is ignited electrically. The resulting change in temperature of the water and the bomb is measured at the end of the reaction. A bomb calorimeter is considered to be an isolated system.

6 6 Applying the First Law of Thermodynamics gives us: q system = -q surroundings For a process in a bomb calorimeter, the system is the chemical reaction, and the surroundings are the water and the calorimeter, so: q reaction = - (q calorimeter + q H2O ) Bomb calorimeters are normally calibrated for a constant mass of water, with the heat capacity supplied by the manufacturer. This leaves us with the calculation: q reaction = -(q calorimeter ) The heat absorbed by the calorimeter is calculated from q= C T. Example A 1.00 g sample of sucrose (C 12 H 22 O 11 ) is burned in a bomb calorimeter which has a heat capacity of 7.00kJ/ o C. The temperature increases from 25.0 o C to 27.3 o C. How much heat is released from this combustion reaction? What is the molar enthalpy of combustion ( H comb )?

7 7 Calorimetry THERMOCHEMISTRY WORKSHEET #6 1. A student mixed ml of 1.0 mol/l sulfuric acid with ml of 1.0 mol/l potassium hydroxide in a simple calorimeter. Both solutions were initially at o C and the highest temperature reached by the reaction mixture was o C. a) Find the heat change for this reaction. (Ans: kj) b) Find the molar heat of neutralization for the acid. (Ans: kj/mol) 2. A 5.00 g sample of acetylene undergoes complete combustion in an oxygen bomb calorimeter. The calorimeter has a heat capacity of 6.49 kj/ o C. The maximum temperature increase is recorded as 9.50 o C. Find the molar heat of combustion for acetylene, C 2 H 2. (Ans: kj/mol) 3. A very cold piece of silver with a mass of g is added to a simple calorimeter that contains ml of water. The temperature of the calorimeter water changes from o C to o C. Note: Do not add the mass of silver to total mass since it does not dissolve in the water. a) Find the heat change for the piece of silver. (Ans: 2.03 kj) b) How cold was the piece of silver, originally (find T i )? (c Ag = J/g o C) (Ans: -97 o C)

8 8 4. A 2.24 g sample of ammonium nitrate is added to ml of water in a simple calorimeter. The sample completely dissolves and causes the water temperature to change from o C to o C. a) Calculate the heat change associated with dissolving the ammonium nitrate. (Ans: kj) b) Is the process exothermic or endothermic? Explain. c) What assumptions are being made in your solution to the question? d) Find the molar heat of solution for ammonium nitrate. (Ans: 23.9 kj/mol) 5. A 5.00 g sample of solid benzene (C 6 H 6 ) is added to a simple calorimeter that contains kg of ethanol. Given that the specific heat capacity of ethanol is 2.4 kj/kg. o C, and that the temperature changed from o C to o C, what is the molar heat of fusion for the benzene? Note: Ethanol is the surroundings, not water and benzene dissolves in the ethanol. (Ans: 8.1 kj/mol)