Calorimetry is the measurement of heat transfer. Devices that measure the amount
ID: 495180 • Letter: C
Question
Calorimetry is the measurement of heat transfer. Devices that measure the amount of heat released or absorbed during a process are called calorimeters. Coffee cup calorimeter is the simplest calorimeter and is often used to measure the heats of chemical reactions at constant pressure (also called enthalpies). This calorimeter consists of a Styrofoam cup with a plastic lid and a thermometer. In today’s workshop we will use calorimetry to calculate the standard molar enthalpy of reaction or the enthalpy change that occurs in a system when 1 mole of matter is transformed by a chemical reaction at 1 atm: H = qrxn (in J/mol, at constant pressure and pV-work only), where q = cmT Let’s consider the following chemical experiment: A student wants to measure the heat of neutralization reaction. To do this she mixed in a coffer cup calorimeter 35.0 mL of 0.70M HCl solution and 65.0 mL of 0.50M NaOH solution both at 22.0oC initially. She then continued to measure the temperature of the mixture for 10 minutes. 1. Write a chemical equation, a complete ionic and a net ionic equations for this reaction. Chemical equation:_____________________________________________________________________________________ Complete ionic equation:_______________________________________________________________________________ Net ionic equation:_____________________________________________________________________________________ 2. Now discuss with your group the following questions: (a) What is(are) the product(s) of this reaction? (b) What is the system in this experiment and what are the surroundings? (c) Is this system an open one, a closed one or an isolated one? Before you write down your answers make sure that EVERYONE in your group can justify each them. Now write your answers below and don’t forget to explain them. (a) ___________ is(are) the product(s) in this reaction. (b) The system: The surroundings: (c) We can consider this system to be an ________________ system, because: General Chemistry II 3. Now calculate how many moles of water were produced in this experiment. 4. Below is the graph that the student attached to her report. Let’s think how we can use it to calculate the heat of neutralization reaction. Notice: In this experiment, the student measured the change in temperature not for the system itself but for the surroundings (solution). The solution is diluted and we can safely consider its specific heat and its density to be the same as for pure water: c = 4.18 J/goC and d = 1.00 g/cm3. (a) What is the change in temperature recorded in this experiment? T = ________________=________ oC Did the system loose or gain energy in this experiment? Explain. General Chemistry II (b) Calculate how much heat (in joules) transferred between the system and the surroundings in this experiment. (c) Calculate the molar heat of the neutralization reaction (in kJ/mol) (d)Is the neutralization reaction endo or exothermic? Can you suggest one reason why this can be true? 5. In the second experiment, the student mixed in a coffer cup calorimeter 35.0 mL of 0.70M H2SO4 solution and 65.0 mL of 0.50M NaOH solution. Do you think that the change in temperature in the second experiment was similar to the one recorded in the first experiment? Why or why not? T2 ________ T1
Explanation / Answer
Calorimetry
1. HCl + NaOH
Ionic equation : H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) --> Na+(aq) + Cl-(aq) + H2O(l)
net-ionic equation : H+(aq) + OH-(aq) --> H2O(l)
2. (a) Products for the reaction : NaCl, H2O
(b) System is HCl + NaOH
surrounding : calorimeter
(c) This can be considered as an isolated sytem because the heat lost or gained remained inside the calorimeter itself.
3. moles of water formed = 0.7 M x 35 ml = 24.5 mmol
4. From the graph,
final temperature = 25.3 oC
(a) change in temperature dT = 25.3 - 22 = 3.3 oC
(b) heat of neutralization = q = mCpdT
= 100 x 4.18 x 3.3
= 1379.4 J
(c) molar heat of neutralization dH = 1379.4/24.5 mol
= 56.30 kJ/mol
(d) This is an exothermic reaction as the final temperature is greater than the initial temperature
5. Experiment with H2SO4 + NaOH
The change in temperature would be higher than HCl+NaOH experiment as the number of moles of H2O formed as product is greater.
Here NaOH is the limiting reactant.
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