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Constant Pressure Calorimetry
Objective
To become acquainted with the use of a coffee cup calorimeter and determine the heat of reaction of a neutralization reaction, the enthalpy of solution of salts and the specific heat of a metal.
Equipment
- Coffee cup calorimeter • 1 cardboard lid • Vernier LabQuest w/ Thermometer probe
- 50.0-mL graduate cylinder - 400 - mL beaker • Hotplate
Chemicals
- 1.00 M NaOH solution • 1.00 M HCl • DI Water
- MgSO 4
Introduction
Chemical reactions are accompanied by heat change. When heat is released, the reaction is called exothermic (exo-out, thermic-heat). When heat is absorbed, the reaction is called endothermic (endo-in, thermic-heat). If substances mixed in a flask undergo an exothermic reaction, the contents of the flask become warmer. If the substances undergo an endothermic reaction, the flask contents become colder. The heat change of a reaction is generally called the heat of reaction. For a reaction performed at constant pressure, the
heat of reaction is equal to the enthalpy change, D H , for the reaction. The D symbol means
“change in”. Every substance has an enthalpy, H. Enthalpy is a thermodynamic quantity equivalent to the total heat content in the system and is equal to the internal energy plus the product of the pressure and volume (eq. 7.1). 𝐻 = 𝐸 + PV (𝟕. 𝐱) Since we will be conducting this experiment at a constant pressure and volume for our purposes H = E. Be careful about confusing heat ( H ) with temperature ( T ). The temperature of a match flame and a bonfire flame may be equal however the heat within the match is vastly different from the heat found in the bonfire. Generally, the sum of the enthalpies of the products differs from the sum of the enthalpies
of the reactants. The enthalpy change, D H , is equal to the sum of the enthalpies of the
products minus the sum of the enthalpies of the reactants.
When D H is negative, heat is released by the reaction and thus, the reaction is exothermic.
This is key to understanding why the heat of reaction is equal to the negative of the heat change in the calorimeter (eq 7.3). The SI unit of heat is the joule (J). It takes 4.184 joules to raise the temperature of one gram of water one degree Celsius, also known as the specific heat of water ( Cs ). Heat is also commonly measured in units of calories. One calorie (cal) is the amount of heat needed to raise the temperature of one gram of water one degree Celsius. One calorie is equal to 4.184 joules. One kilocalorie (kcal) equals 1000 calories. You will be using the terms specific heat ( Cs ) and heat capacity (C). Specific heat has units of J • g-^1 ˚C-^1 whereas heat capacity has units of J • ˚C-^1. Calorimetry is the study of heat transferred in a chemical reaction, and a calorimeter is the tool used to measure this heat. Calorimetry can be used to find heats of reaction. In a calorimeter, a chemical reaction is generally performed in a water bath. The heat of reaction will change the temperature inside the calorimeter. For an exothermic reaction, the temperature inside the calorimeter will increase. For an endothermic reaction, the temperature inside the calorimeter will decrease. The heat change associated with the temperature change inside the calorimeter is equal to the heat capacity of the calorimeter
contents ( C contents.) times the temperature change ( D T = T final – T initial):
𝑞&#+')+'( = 𝐶&#+')+'( × ∆𝑇 (𝟕. 𝟐)
Now what is this “ q ”? It is a term to describe the change in heat, i.e. q = D H , in a system
at constant pressure. To understand this more fully refer to Enthalpy in your textbook. The heat of reaction is equal to the negative of the heat change of the calorimeter because heat flows out of the reaction into the calorimeter (notice the change of direction): 𝑞",+ = −𝑞&#+')+'( (𝟕. 𝟑) In today’s experiment, you will determine a heat of reaction in a coffee cup calorimeter. A coffee cup calorimeter is surprisingly efficient and consists of two Styrofoam cups, a cardboard lid, and a thermometer. Two solutions are mixed in the calorimeter and the temperature change of the mixed solution is measured. We will assume that the heat capacity of the solution is equal to the heat capacity of water. The heat capacity of water is equal to: 𝐶-')" = 𝑚-')" × 𝐶(,-*')" (𝟕. 𝟒) Where m is mass in grams, g, and C s,water is the specific heat of water 4. 184 0 1 ∙ ℃ We will also assume that the density of the solution is about 1. 00 1 45
. For example:
- 0 mL H 6 O ×
- 00 g mL = 50. 0 g H 6 O
∆ Hwarm water = 4. 184 0 1 ∙ ℃
- 50 g • 9.8 ˚C = 2.050 x 10^3 J or 2.050 kJ ∆ Hcold water = 4. 184 0 1 ∙ ℃
- 50 g • 8.5 ˚C = 1.778 x 10^3 J or 1.778 kJ Hgained by calorimeter = 2.050 kJ – 1.778 kJ = 0.2720 kJ or 272 J This would be the amount of energy that is taken up by the calorimeter and is the amount of energy that needs to be added (exothermic reaction) or subtracted (endothermic reaction) to each reaction to give the total amount of energy released or absorbed by the system. B. Heat of Neutralization The transfer of heat that results from an acid/base neutralization reaction carried out at constant pressure is called the enthalpy of neutralization , Δ H neutralization , and is expressed in units of kcal/mol or kJ/mol. The reaction to be studied is: HCl(𝑎𝑞) + NaOH(𝑎𝑞) → NaCl(𝑎𝑞) + H 6 O(𝑙) (𝟕. 𝟗) Since HCl and NaOH are strong electrolytes, this net ionic equation associated with this molecular equation is: HH(𝑎𝑞) + OHI(𝑎𝑞) → H 6 O(𝑙) (𝟕. 𝟏𝟎) As with any chemical reaction, the extent of the reaction is dependent on the amount of limiting reactant present. Given the moles of limiting reactant undergoing reaction and the measured heat of the reaction, Δ H neutralization can be determined as shown below, keeping in mind that: q rxn = − q calorimeter
∆𝐻+)%'"JKL'K#+ =
moles reacted
C. Enthalpy of Solution of Salts When a salt dissolves in water at constant pressure, there is a transfer of heat associated with the reaction called the enthalpy of solution, Δ H solution. It is expressed in units of kcal/mol or kJ/mol of salt. ∆𝑯𝒔𝒐𝒍𝒖𝒕𝒊𝒐𝒏 =
The solution process can be written as follows: NHVNOW(𝑠) → NHV^ H(𝑎𝑞) + NOW^ I(𝑎𝑞) (𝟕. 𝟏𝟑) Heat may be given off or absorbed by the salt as it dissolves as ions in water.
Procedure – Create data tables similar to the Report Sheet below in the Data section of your lab notebook. A. Heat Capacity of a Calorimeter
- Obtain 2 Styrofoam cups and a piece of cardboard from the supply table. Nest the cups and insert the LabQuest2 thermometer probe through the small hole in the center of the cardboard. Rest the entire apparatus in a 400 mL beaker for stability. On the LabQuest2 module select “Sensors” then “Data Collection”. Interval should be set to 1 s/sample. Duration should be set to 180 s.
- Using a graduated cylinder, place exactly 50.0 mL of distilled water in the inner cup of the calorimeter and replace the cardboard lid. Allow 5-10 minutes for the system to reach thermal equilibrium and record the temperature to the nearest 0. ˚C.
- While waiting for the system to equilibrate, put exactly 50.0 mL of distilled water in a clean, dry 250 mL beaker. Heat the water on a hot plate until the temperature of the water is approximately 40 – 50 ˚C. Allow the water to stand for a minute or two on the bench and then record the temperature to the nearest 0.1 ˚C.
- Once both water temperatures are recorded and immediately after recording the warm water temperature, quickly pour all of the warm water in the calorimeter, replace the lid with the temperature probe and start measuring the temperature by selecting the start icon on the LabQuest2. Observe the temperature curve until the temperature reaches a stable maximum, then stop.
- Export your data to a USB stick. Results section: Create an Excel chart (graph) for inclusion with your results, plotting the temperature as a function of time. Determine ∆T from your plot and perform the calculations as described above to determine the heat capacity of your calorimeter. Use this value in the following experiments for determining the heat of reactions. B. Heat of Neutralization
- Using the calorimeter you prepared, carefully measure 50.0 mL of 1.00 M HCl solution with a graduate cylinder and transfer it to the inner cup. Put the probe into the HCl solution and use LabQuest2 to measure and record the temperature of the HCl to the nearest 0.1oC.
- Rinse the graduated cylinder with DI water, then rinse it with small amount of NaOH solution, then carefully measure 50.0 mL of 1.00 M NaOH solution. The temperature of the NaOH is assumed to be the same as the HCl solution.
- On the LabQuest2 module select “Sensors” then “Data Collection”. Interval should be set to 1 s/sample. Duration should be set to 180 s. Put the probe into the HCl solution. Start running and measuring the temperature by selecting the start icon. Observe the curve on LabQuest2. When you get 5 points (~5s) with the same temperature, add the 50.0 mL of the NaOH solution to the HCl in the cup, swirl gently, observe the temperature curve until the temperature reaches a maximum,
Report Sheet for EXP
Report Sheet EXP7: Constant Pressure Calorimeter
A. Heat Capacity of Calorimeter
Temperature of calorimeter and water before mixing.
Temperature of warm water. __________ Maximum temperature of mixture after reaction __________ Temperature difference __________ Heat gained by calorimeter ( J ) __________
B. Heat of Neutralization
Trial 1 Trial 2 Trial 3 Volume of HCl __________ __________ __________ Temperature of HCl __________ __________ __________ Volume of NaOH __________ __________ __________ Temperature of mixture after reaction __________ __________ __________ Temperature difference __________ __________ __________ Number of joules evolved __________ __________ __________ Moles of H+^ that were neutralized __________ __________ __________
Report Sheet for EXP Joules evolved per mole of H+^ __________ __________ __________ Average of the three trials of joules evolved per mole of H+^ __________ Standard Deviation and RSD __________ __________ C. Enthalpy of Solution of Salts MgSO^4 Mass of salt __________ Volume of DI water __________ Mass of DI water __________ Temperature of DI water __________ Temperature of mixture after dissolution
Temperature difference __________ Total mass in reaction __________ Enthalpy of solution D Hsolution __________
