Calorimetry Lab

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Imperial Valley College *

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101

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Chemistry

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Apr 3, 2024

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Relevant Course Level Learning Objectives: 1a, 4a, 6a Name: Calorimetry: Enthalpy of Neutralization Reactions Activity Learning Objectives: 1. To determine the calorimeter constant and use that information to determine the heat of neutralization of a strong acid with a strong base. Background Heat flow is studied using a calorimeter, which is essentially a well-insulated container with an accurate thermometer. Before we study any reaction, we need to look at the environment where the reaction will take place. Let us define the system to be what we are studying and the surroundings to be everything else in the universe outside our system. The law of conservation of energy states that energy is neither created nor destroyed, but simply converted from one form to another. Therefore, whatever heat is released by the surroundings is absorbed by the system, and whatever heat is absorbed by the surroundings is released by the system. Using a calorimeter, you can measure different properties of a substance, such as specific heat. Specific heat (c), or specific heat capacity, is defined as the amount of heat necessary to raise the temperature of 1 g of a substance by 1 °C, so specific heat generally has units of J/g ∙ °C or J/g ∙ K. The specific heat is represented in the equation below where q is the amount of heat transferred, m is the mass in grams, and Δ T is the change in temperature. Eqn 1 (definition of specific heat): c = 𝑞 𝑚∙∆𝑇 By assuming the calorimeter loses no heat to the surroundings, we can first calculate how much heat the calorimeter is expected to absorb by determining the calorimeter’s general heat capacity. The heat capacity (C) is the amount of heat necessary to raise the temperature by 1 kelvin (which is equivalent to change in temperature of 1 °C), so heat capacity generally has units of J/K. To calculate the calorimeter constant (sort of like the calorimeter’s heat capacity), you need to know two pieces of information: the heat absorbed or released by the calorimeter, and the temperature change of the calorimeter. To calculate the heat absorbed or released by the calorimeter we will use the law of conservation of energy. If a known amount of hot water at a measured temperature is added to a known amount of cold water in a calorimeter at a measured temperature, there will be heat transferred from the hot water to the cold water and to the calorimeter. Therefore, knowing that energy must be conserved and assuming that no heat is transferred to the surroundings (besides the calorimeter), we get the equation below where q hw is heat lost by the hot water, q cw is heat absorbed by the cold water, and q cal is heat absorbed by the calorimeter. Eqn 2 (heat transferred to calorimeter): q cal = - q hw – q cw or - q hw = q cw + q cal How do we calculate the heat lost by the hot water or the heat gained by the cold water? By rearranging Eqn 1 you can calculate the heat using the specific heat capacity of water as shown in Eqn 3 . Because this equation requires the difference in temperature it does not matter if the temperature is measured in °C or K, since their numerical difference is the same. Eqn 3 (heat in terms of specific heat capacity): q = m × c × Δ T

Relevant Course Level Learning Objectives: 1a, 4a, 6a Name: After calculating the heat transferred to the calorimeter, the last piece of information needed to calculate the heat capacity of the calorimeter would be the change in temperature of the calorimeter. We cannot directly measure the temperature change of the calorimeter. Instead, we can assume the temperature of the calorimeter follows the same temperature measurements as that of the cold water as long as the cold water has been kept in the calorimeter for a reasonable amount of time to achieve equilibrium. Keeping this in mind, we can obtain and use Eqn 4 : Eqn 4 (heat capacity of calorimeter): 𝐶 cal = 𝑞 cal ∆𝑇 cw Safety Eye protection and proper Personal Protective Equipment must be worn throughout the experiment. Waste Disposal All of the acids and bases have been neutralized, therefore all waste may go down the drain. It’s just salt water after all! Procedure: Part A: Determine the calorimeter constant. Construct your calorimeter from an insulated coffee cup with a cover and a temperature probe. Use a graduated cylinder to measure 50.0 mL of deionized water. Add this water to your calorimeter, replace the lid, and let the water come to room temperature ( about 10 minutes). This water will be the equivalent of your “cold” water. While the cold water is equilibrating to room temperature, obtain another 50.0 mL of deionized water and pour it into a dry 250 mL beaker. Heat this beaker of water on a hot plate until it reaches 60-70 °C and record the temperature. This water will be the equivalent of your “hot” water. Record the temperature of the cold water (your initial temperature at 0 sec). In the Vernier Graphical Analysis program, in the Data Collection Settings, set the mode to “Time Based”, change the interval to 0.5 s/sample, and set the “End Collection” setting to “Manually”. When you are ready to collect data, press the collect button. Quickly pour your hot water into the calorimeter and snap on the lid -making sure the temperature probe is immersed in the water. Stop data collection after three minutes. 1. Draw a picture of your temperature plot below. Make sure to label your plot on the x- and y-axes, including labeling the starting and final temperatures.

Relevant Course Level Learning Objectives: 1a, 4a, 6a Name: Data (calorimeter constant): Temperature of cold water and calorimeter ______________ Temperature of hot water ______________ Final temperature of water mixture ______________ Analysis (calorimeter constant): 2. What is the heat lost by hot water, q hw ? (show calculations) 3. What is the heat gained by cold water, q cw ? (show calculations) 4. What is the heat gained by calorimeter, q cal ? (show calculations) 5. What is the calorimeter constant, C cal ? (show calculations)

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Relevant Course Level Learning Objectives: 1a, 4a, 6a Name: Part B: Determine the enthalpy of reaction of a strong acid + strong base. Start with a clean, dry calorimeter. Use your graduated cylinder to measure out 50.0 mL of 1.0 M sodium hydroxide, NaOH( aq ), and transfer it to the calorimeter. Wash and dry your graduated cylinder, and use it to measure out 50.0 mL of 1.0 M hydrochloric acid, HCl( aq ). Check to make sure the acid solution and the base solution are at the same temperature, making sure to rinse and dry your temperature probe when switching between solutions. When your acid and base are at the same temperature, stir your NaOH solution and record the temperature of the solution and calorimeter (your initial temperature at 0 sec). Immediately pour the acid into the sodium hydroxide solution, hit collect on the graphical analysis program, and stir with the temperature probe. Stop data collection after three minutes. 6. Draw a picture of your temperature plot below. Make sure to label your plot on the x- and y-axes, and show the starting and final temperatures. Data (acid base reaction): 7. Record your temperature data for the strong acid and strong base Temperature of NaOH solution and calorimeter ______________ Δ T from the plot ______________ 8. What is the balanced molecular equation for the mixing of the strong acid and the strong base? 9. What is the heat gained by solution, q soln ? (show calculations)

Relevant Course Level Learning Objectives: 1a, 4a, 6a Name: 10. What is the heat gained by calorimeter, q cal ? (show calculations) 11. What is the heat of reaction, q rxn ? (show calculations) 12. How many moles of acid reacted? (show calculations) 13. How many moles of water were formed? (show calculations) 14. What is the e nthalpy of reaction, Δ H rxn ? (show calculations) 15. If instead of a strong acid we used a weak acid of the same concentration and volume, how do you think the enthalpy of reaction, Δ H rxn , would change?

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