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Experiment 13 - Heat of Reactions.docx

Experiment 13. Heat of Reactions

Student Name (Print):__________________________

Pre-lab Study Questions (Submit upon arrival for experiment)

1. Calculate the enthalpy of reaction DH_rxn^o for each of the following reactions using the tabulated standard enthalpy of formation DH_f^o. (Show calculation)

a. H₂O(g) H₂O(l)

b. CaCO₃(s) CaO(s) + CO₂(g)

c. CH₄(g) + 2 O₂(g) CO₂(g) + 2 H₂O(g)

2. Consider the following reaction and the given standard enthalpy of formation DH_f^o.

NH4NO3(s) NH4+(aq) + NO3-(aq)
DHfo (kJ/mol)	-365.6	-132.0	-205.0

a. Calculate the enthalpy of reaction DH_rxn^o for the above reaction using the given standard enthalpy of formation DH_f^o. (Show calculation)

b. The above reaction equation represents the dissolving process of 1 mol of solid ammonium nitrate NH₄NO₃ in water. This process is endothermic or exothermic? This process will cause the temperature of the surroundings to rise or drop? Explain.

c. If 2.00 g of solid NH₄NO₃ is dissolved in 50.0 mL of water in a coffee-cup calorimeter, how much heat is released or absorbed? Show calculation. (Hint: the calculation in a gives the heat transferred when 1 mol of NH₄NO₃ is dissolved in water. If there are 2.00 g of NH₄NO₃, how many moles of NH₄NO₃ are there?)

d. Calculate the temperature change Dt of the 50.0 mL of water in the calorimeter if 2.00 g of solid NH₄NO₃ is dissolved in 50.0 mL of water. Show calculation.

Experiment 13. Heat of Reactions

Student Name (Print):__________________________

I. Objectives

(1) Understand reaction enthalpy: endothermic and exothermic and relate reaction enthalpy to the sign of the temperature change of the surroundings

(2) Determine enthalpy change DH of dissolving solid NH₄NO₃ in water

(3) Determine enthalpy change DH of acid-base neutralization reactions

II. Materials

(1) Coffee-cup calorimeter

(2) Ammonium nitrate NH₄NO₃

(3) 1 M hydrochloric acid, HCl

(4) 1 M sodium hydroxide, NaOH

III. Safety Information and Requirements

(1) Wear goggles or safety glasses

(2) Dump waste according to instruction

IV. Procedure and Data Collection

IV.1 Information

The First Law of Thermodynamics (aka Law of Energy Conservation) states that the total energy of the universe must remain constant. Therefore, the heat (q) lost by a system is equal to the heat gained by the surroundings. The only difference between these two values is the sign:

q_system = -q_surroundings ..Equation 1

The standard S.I. unit for heat is the joule (J). The specific heat C_s of a substance is the heat it takes to raise the temperature of one gram of the substance by 1 ^oC. The specific heat of water is 4.18 J/g ^oC.

In this experiment, a coffee-cup calorimeter is used to study the heat transfer involved in chemical reactions. The calorimeter contains two-nested Styrofoam coffee cups. Water is often used in the calorimeter to make thermal contact with the process under investigation (i.e. water is the surroundings). If the process releases heat, the heat transferred to the surroundings causes the temperature of the water to rise and vice versa. The temperature change (Dt) can be measured as the difference between the final and initial temperatures of the water in the calorimeter. The heat transfer in a calorimeter may be expressed with the following equation:

q_water = m C_s Dt .. Equation 2

where m is the mass of water in the calorimeter, C_s of water is 4.18 J/g ^oC and Dt is the temperature difference of water before and after the process occurs. Note the water is the surroundings to the process and one has to reverse the sign of q_water to find the heat transfer of the system (Equation 1).

Coffee-cup calorimeter

To determine the temperature difference Dt, the temperatures over a certain time period will be recorded and plotted on y-axis versus time on x-axis. The temperature difference Dt will be calculated as

Dt = t_{after,average} t_{before,average} Equation 3

In this experiment, two reactions will be studied. The first reaction is the dissolving process of solid ammonium nitrate NH₄NO_3. Dissolving NH₄NO₃ is an endothermic reaction. This is the reaction often used in cold packs sold at drug stores. When NH₄NO₃ is dissolved in water in a coffee-cup calorimeter, the temperature of water will drop.

The second type of reaction that will be studied in this experiment is acid-base netrulization reactions. Two acid-base reactions can be studied. The first reaction is between a sodium hydroxide NaOH solution and a hydrochloric HCl solution:

NaOH(aq) + HCl(aq) NaCl(aq) + H₂O(l)

The second reaction is a NaOH solution reacting with an acetic acid CH₃COOH solution:

NaOH(aq) + CH₃COOH(aq) CH₃COONa(aq) + H₂O(l)

Neutralization reactions are exothermic. Mixing of a base solution and an acid solution in a coffee-cup calorimeter will make the temperature of the mixed solutions higher than those of the original base and acid solutions.

IV. 2 Procedure and Data Collection

Heat of Solution

(1) Add 50 mL of deionized water into the calorimeter and allow the temperature of the water to equilibrate with the calorimeter. Record the temperature.

(2) Weigh approximately 2 grams of ammonium nitrate NH₄NO₃, Record the exact mass and covert the grams to moles.

(3) Add the ammonium nitrate to the calorimeter and quickly swirl to dissolve. Record the temperature every 15 seconds for 5 minutes.

Heat of Neutralization Reaction

NaOH HCl neutralization reaction

(1) Add 25 mL of 1.0 M NaOH to the calorimeter (Caution: NaOH is a strong base!). Cover it with the lid and let the solution temperature equilibrate for 5 minutes. Record the temperature after 5 minutes.

(2) Measure 25 mL of 1.0 M HCl in a clean graduated cylinder (Caution: HCl is a strong acid!). Allow to sit for 5 minutes and then record the temperature. The temperature of the HCl solution should be approximately the same as the NaOH solution.

(3) Quickly but carefully add the HCl solution to the calorimeter. Put the lid on and swirl the solution. Record the temperature every 15 seconds for 5 minutes.

Record

Temperature of 50 mL water in the calorimeter

___________________ ^oC

Mass of NH₄NO₃ = ________________ grams

= ________________ moles

Time	t (oC)	Time (sec)	t (oC)
0 min		45 sec
15 sec		3 min
30		15 sec
45		30
1 min		45
15 sec		4 min
30		15 sec
45		30
2 min		45
15 sec		5 min
30

Record

Temperature of 1.0 M NaOH solution

____________ ^oC

Temperature of 1.0 M HCl solution

____________ ^oC

Time	t (oC)	Time (sec)	t (oC)
0 min		45 sec
15 sec		3 min
30		15 sec
45		30
1 min		45
15 sec		4 min
30		15 sec
45		30
2 min		45
15 sec		5 min
30

V. Calculations

Heat of Solution

(1) Plot temperature (C) on the y-axis vs time (min) on the x-axis and find the temperature difference Dt of water.

Dt = _____________ ^oC

This reaction is ________ (endothermic or exothermic).

(2) Calculate heat absorbed from or released to the water using Equation 2.

q_water = _______________ J

(3) Calculate heat of solution (heat absorbed or released by the dissolving process) using Equation 1.

q_solution = _______________ J

(4) Calculate molar heat of solution using the following equation:

DH_solution = ________________ kJ/mol

(5) Compare the value calculated above to the theoretical value (Pre-Lab Study Question 2a.). Calculate the percent difference.

Heat of Neutralization

NaOH HCl neutralization

(1) Plot temperature (C) on the y-axis vs time (min) on the x-axis and find the temperature difference Dt of water.

Dt = _____________ ^oC

(2) Approximately 50 g of water H₂O is present for this neutralization reaction. Calculate heat absorbed from or released to the water using Equation 2.

q_water = _______________ J

(3) Calculate heat of neutralization (heat absorbed or released by the neutralization reaction between NaOH, a base and HCl, an acid) using Equation 1.

q_{neutralization} = _______________ J

(4) Approximately 0.025 mol (25 mL 1 M H⁺ with 25 mL 1 M OH^-) of water H₂O is produced from this reaction. Calculate DH_{neutralization} using the following equation:

DH_{neutralization} = ________________ kJ/mol

VI. Post-Laboratory Questions

1. In this experiment, reaction enthalpies DH_rxn are studied. In the chapter of Thermodynamics, another property called entropy is also studied. In your own words, describe entropy of a substance. Which phase, solid, liquid or gas, has the greatest entropy?

2. Predict the entropy change DS of the following reactions and briefly explain.

a. NH₄NO₃(s) NH₄⁺(aq) + NO₃^-(aq)

b. H⁺(aq) + OH^-(aq) H₂O(l)

3. In the chapter of Thermodynamics, you have also learned the Gibbs free energy change DG, which is a property that combines DH and DS:

DG = DH - TDS

where T is Kelvin temperature.

For a spontaneous process, DG < 0.

Based on your experimental results on DH and your predications on DS of the two reactions, predict if the following reactions are always spontaneous, spontaneous only at high temperatures, spontaneous only at low temperatures, or always nonspontaneous. Briefly explain.

a. NH₄NO₃(s) NH₄⁺(aq) + NO₃^-(aq)

b. H⁺(aq) + OH^-(aq) H₂O(l)