L q + w. CHM 1041 Thermochemistry Heats of Solution (Reaction) J. Bieber. Section: Date:

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1 CHM 1041 Thermochemistry Heats of Solution (Reaction) J. Bieber Name: Partner: Section: Date: To study quantitatively the heat of solution when (1) a salt dissolves in water and (2) to study the heats of reaction when (a) a strong acid reacts with a strong base and (b) a strong acid reacts with an active metal (a redox reaction). DISCUSSION Thermodynamics of the field of study which deals with the relationships between all forms of energy (for example, heat and work) that are involved in physical and chemical changes. We can study a particular reaction without interference from other, extraneous factors by dividing the universe (artificially) into the system, the part on which we wish to focus, and the surrounding, everything else. A system which cannot exchange energy with its surroundings is called an isolated system. In this experiment we will use an isolated system to be able to determine the amount of heat generated by the reactions that we will study. Both physical and chemical changes are accomplished by the gain or loss of heat and other forms of energy. If a system releases heat to the surroundings, the change in the energy of the system is negative (it decreases because the system gave off heat) and the change is exothermic. If a system absorbs heat from the surroundings, the change in the energy of the system is positive (it increases because the system took on heat) and the change is endothermic. Internal Energy and Enthalpy Although there is no way to determine it, a system in a given state possesses a definite amount of internal energy, E. What can be determined is any change in the internal energy of a system. Physical and/or chemical changes within the system result in changes in its internal energy. The internal energy change,, depends only on the initial an final states of the system. For a given initial and final state, is always the same regardless of whether the change occurred in a single step or in multiple steps. A property such as, which depends only on the initial and final states of the system, is called a state function; its value is independent of the pathway or mechanism involved to get from one state to the other. is defined as the combination of the heat absorbed by the system, q, and the work done on the system, w: L q + w

2 The system s internal energy is increased by the amount of heat it absorbs and also increased by the amount of work performed on it. Notice that q is negative if heat is evolved fro the system and w is negative if work is done by the system on the surroundings. (You may sometimes see different conventions for the meaning of w, so be careful how it is used.) When you use the car battery to start the engine, the chemical reaction is the battery performs electrical work. In most physical and chemical changes however, the work done is often the work of expansion against atmospheric pressure, known as pressure-volume work, or PÈV work. Most commonly, reactions are carried out it an open container at constant pressure, usually atmospheric pressure. The heat absorbed at constant pressure is defined as the enthalpy change, ÈH: 2 ÈH L q p (where the subscript P means at constant pressure) Therefore, if heat is evolved, this is a negative absorption and the value of q p will be negative. At constant pressure, the work done is >PÈV and ÈE = q p + w = q p > PÈV and ÈH = ÈE + PÈV The enthalpy is often called the heat content. Like the internal energy change, ÈE, the enthalpy change, ÈH, is a state function. A chemists is usually not concerned with the ability of a system to do work, unless it s to prevent unwanted work, i.e., to prevent a explosion from occurring. Therefore, we will deal only with ÈH, now that we have shown how it comes about. Heat Exchange In the calorimeter, when heat is released by a reaction, the water absorbs the energy and the temperature of the water increases. The temperature change produced in any substance when a given amount of heat is gained or lost depends on the mass of the substance and its specific heat: ÈT = heat/m`s where m is the mass (grams) of the substance, s its specific heat (per gram). The specific heat is the heat capacity of one gram, i.e., the amount of heat necessary to raise the temperature of 1 gram of a substance 1 C. The specific heat of liquid water is approximately 1 cal`g >1` C >1 and depends on the exact temperature. At a typical room temperature the value is cal`g >1` C >1 or 4.18 J`g >1` C >1. Calorimeters A calorimeter is an insulated container with a stirrer and sensitive thermometer, in which a reaction can be carried out. We will use a calorimeter made from two nested styrofoam cups and a styrofoam cover. The air spaces in and between the pieces of styrofoam provide excellent insulation so that heat loss to the air is minimal. (It will be assumed that any heat absorbed by the styrofoam cup is negligible and can be ignored.) the calorimeter contains the substance(s) undergoing a physical or chemical change and water to absorb (or provide) heat. The known relationship between the temperature change which occurs in the water and the heat absorbed or released by the water makes it possible to calculate ÈH for the change/reaction which occurred inside the calorimeter. In this

3 experiment, you will use such a calorimeter to measure the heat of solution of a salt, and the heats of two chemical reactions. 3

4 CHM 1041 Thermochemistry Heats of Solution (Reaction) J. Bieber Name: Partner: Section: Date: EXPERIMENTAL Check out from the stockroom an analog thermometer graduated in 0.2 C increments or a digital thermometer graduated in 0.1 C increments. This is a very expensive and delicate thermometer, so please handle it gently and use it with care. Obtain a calorimeter with a stirrer and weigh it. Mass of the calorimeter is g Heat of Solution Obtain your unknown salt. Record its code number and molar mass g/mole. Place a piece of weighing paper on an electronic balance and zero the balance. Weigh a sample of 2-3 grams of the unknown to the greatest accuracy possible. Mass of the salt is g Place ml of deionized water in the calorimeter and weight it as accurately as possible. Mass of the calorimeter + the water is g Mass of the water is g Record the temperature of the water in the calorimeter as accurately as possible. Temperature of the water is C Carefully transfer all of the salt to the water in the calorimeter. Cover the calorimeter and stir the contents continuously, swirling it occasionally, and observe the temperature. When the temperature has remained constant for 15 seconds, record it. Temperature of the salt solution is C The change in temperature is C Empty the calorimeter, rinse it with tap water, then with deionized water, and invert it on paper towels to drain.

5 As the solid dissolved in the water inside of the calorimeter, a dilute aqueous solution of the salt formed. Assuming that the specific heat of the dilute salt solution is the same as that of water, the amount of heat gained or lost by the water (solution) can be calculated. If heat as evolved y the dissolving salt, the solution temperature is now higher than the water temperature was and q is negative. If the heat was absorbed by the dissolving salt, the solution temperature is now lower than the water temperature was and q is positive. 2

6 3 Calculate the heat in joules gained/lost when the salt sample dissolved. Calculate the heat in kilojoules gained/lost if one mole of the salt were dissolved. Using AB, A 1+ and B 1- to represent the salt and its ions, write a thermochemical equation for the dissolution of the salt. Include the symbol for the physical state the each reactant and product; also write ìh in kilojoules with the correct algebraic sign to the right. OK Heat of Reaction: Acid/Base Reaction You will study the reaction of a strong acid and a strong base. Every reaction between a strong acid and a strong base releases the same amount of energy per mole because the same net reaction is occurring. Record the concentration to 3 significant figures of the approximately 1 M HCl solution. Its density is 1.02 g/ml. M HCl Record the concentration to 3 significant figures of the approximately 1 M NaOH solution. Its density is 1.04 g/ml. M NaOH Use a clean, dry graduated cylinder to measure as accurately as possible 50.0 ml of the HCl solution and transfer it all to the calorimeter. Measure and record its temperature as accurately as possible. Initial temperature is C Since the HCl solution and the NaOH solution were both at room temperature, assume the temperature of the NaOH solution is the same. Use a clean, dry graduated cylinder to measure as accurately as possible 40.0 ml of NaOH solution. Carefully pour the NaOH solution the calorimeter as quickly and completely as possible. Cover the calorimeter and swirl it while observing the temperature. Record the maximum temperature reached. Final temperature is C The change in temperature is C

7 Again empty the calorimeter, rinse it with tap water, then deionized water, and invert it on paper towels to drain. Write the net ionic equation for the principal reaction which occurred. 4 How many moles of HCl are in the 50.0 ml of HCl solution? How many moles of NaOH are in the 40.0 ml of NaOH solution? How many moles of H 2 O are formed in the reaction you carried out? Calculate the mass of the HCl solution? Calculate the mass of the NaOH solution? Assuming that all the solutions involved have the same specific heat as water, and that the mixed solutions absorbed all of the heat evolved in the reaction, i.e., assume that no heat escaped, calculate the heat generated in joules. Calculate the heat in kilojoules that would be generated if 1.00 mole of water were formed.

8 5 Write the net ionic thermochemical equation of the reaction, including the value of ìh on the appropriate side of the reaction so that it has a + algebraic sign. If the acid used had been acetic acid instead of hydrochloric acid, would the magnitude (absolute value) of ìh that you determined have been larger or smaller? State your reasoning in a complete, concise sentence. OK Heat of Reaction: Redox Reaction You will study the displacement of an acidic hydrogen my magnesium. The amount of energy released here will be unique to this red-ox reaction because the reactants and products are unique to this red-ox reaction. Use a clean, dry graduated cylinder to measure as accurately as possible 80.0 ml of the same HCl solution and transfer it all to the calorimeter. Measure the temperature as accurately as possible. Initial temperature is C Obtain a sample of magnesium ribbon about 6 inches long. (If necessary, use some steel wool to clean both surfaces of the magnesium ribbon.) Then zero the electronic balance with a sheet of weighing paper already on it and weight the magnesium ribbon to the greatest accuracy possible. The sample weight should be gram. Mass of Mg is g Loosely coil the magnesium ribbon and carefully drop it into the calorimeter. Cover the calorimeter and stir the contents while the reaction occurs. When the temperature stops rising, briefly uncover the calorimeter to be sure all of the magnesium has reacted. Record the highest temperature reached. Final temperature is C Write a balanced equation for the reaction which occurred. The change in temperature is C

9 6 What was the mass of the HCl solution? How many moles of acid are in 80.0 ml of the HCl solution? How many moles of Mg are in your sample? Therefore, which reactant is the limiting reagent? How many moles each of Mg and the acid reacted? moles Assuming that the solution has the same specific heat is water, and that all of the heat evolved by the reaction was absorbed by the solution, i.e., assume that no heat escaped. Calculate the amount of heat generated when the magnesium reacted. Calculate the amount of heat (in kilojoules) that would be evolved if 1.00 mole of magnesium reacted with an hydrocloric acid solution. Write the net ionic thermochemical equation of the reaction, including the change in the heat content (to indicate that the reaction was endothermic or exothermic) on the appropriate side of the equation with a + algebraic sign. final OK

10 7 QUESTIONS Using AB, A 1+ and B 1- to represent a salt and its ions 1. a) Write a thermochemical equation for the breakup of the crystal lattice to form isolated gaseous ions, an endothermic process. Use X to represent the value of ìh along with the appropriate algebraic sign. (ìh = -ìh xtal ) b) Write a thermochemical equation for the hydration of the isolated gaseous ions, an exothermic process. Use Y to represent the value of ìh along with the appropriate algebraic sign. (ìh = ìh hydr ) c) Add the equations in (1) and (2) to obtain the thermochemical equation for the dissolution of the salt, including the ìh value. (ìh = ìh soln ) 2. For your unknown salt, was the amount of heat absorbed in the breakup of the crystal lattice larger or smaller than the amount of heat released in hydrating the ions? Explain. 3. If some water had vaporized as the result of an exothermic reaction, would your calculated heat of reaction be too large or too small? State your reasoning in complete, concise sentences.