Heat of Fusion of Ice

The objective of this lab is to measure the latent heat of fusion of ice by using a calorimeter. Students will utilize various tools and equipment, demonstrate safe practices in the chemical laboratory, and apply their understanding of chemical reactions, quantities in chemical reactions, molecules, compounds, and chemical composition. They will also demonstrate the proper use of exponential notation and significant figures, record their results, and employ scientific reasoning to evaluate physical and natural phenomena.

The discussion begins by explaining the concept of energy changes when a substance undergoes a change in its physical state. In the case of solid ice melting, a specific amount of energy is required to overcome the attractive forces between the ice particles and convert them into liquid water. This energy is known as the latent heat of fusion and represents the number of calories of heat energy needed to melt one gram of a solid at its melting point. The experiment aims to measure this quantity of heat energy by using a calorimeter.

Calorimetry is based on the Law of Conservation of Energy, which states that energy can be transferred from one object to another without any loss or gain in the total energy involved. In an isolated system like a well-insulated calorimeter, the energy lost by one part of the system is equal to the energy gained by the other part(s) of the system. The calorimeter acts as a container that absorbs or releases heat as the temperature of its contents changes.

The lab procedure consists of two parts:

Part A involves determining the heat capacity of the calorimeter. A known mass of water is placed in the calorimeter, and its initial temperature (t1) is recorded. Boiling water at a known temperature (t2) is added to the calorimeter, and the mixture is stirred to reach a final temperature (t3). The heat gained by the cold water, heat gained by the calorimeter, and heat lost by the hot water are calculated using specific heat equations. By applying the Law of Conservation of Energy, the heat capacity of the calorimeter can be determined.

Part B focuses on determining the heat of fusion of ice. Water is added to the calorimeter, and its initial temperature (T1) is recorded. Ice, which is assumed to be at 0°C, is added to the water, and the lowest temperature reached (T3) is recorded. The heat gained by the ice in melting, heat gained by the water from the melted ice, heat lost by the initial water, and heat lost by the calorimeter are calculated using specific heat equations. The heat of fusion of ice can then be determined by rearranging the equation and solving for it.

In both parts of the experiment, the calculated values are obtained by applying specific heat equations and the Law of Conservation of Energy. Students will perform the necessary calculations using the given formulas and values measured during the lab. They are also encouraged to consider the significance of their results and how they relate to the energy changes that occur during phase transitions.

This lab allows students to practice using calorimetry as a tool to measure energy changes and reinforce their understanding of specific heat, heat capacity, and the Law of Conservation of Energy. It also emphasizes the importance of accurate measurements, proper use of significant figures, and recording observations and calculations in a structured manner.

Objective

· Use the tools and equipment necessary for basic scientific analysis and research

· Demonstrate safe practices in the Chemical Laboratory

· Demonstrate an understanding of Chemical Reaction and Quantities in Chemical Reactions

· Demonstrate an understanding of Molecules, Compounds and Chemical Composition

· Demonstrate the proper use of Exponential Notation and Significant Figures

· Demonstrate an understanding of the composition of matter and energy

· Record the results of investigation through writing

· Use scientific reasoning to evaluate physical and natural phenomena

· Identify the unifying themes of the scientific field of study

Materials

· Styrofoam calorimeter cup

· 250 mL Beaker

· Thermometer

· Ice

Discussion

When a substance undergoes a change in its physical state, there is a corresponding change in its energy. For example, when a solid melts, a certain amount of energy must be absorbed to overcome the attractive forces between the particles that hold them in rigid positions. The energy required depends on the strength of these attractive forces. The stronger the forces, the more energy is needed to bring about the change in state.

The objective of this experiment is to measure the amount of heat energy required to overcome the crystal forces of attraction in ice. This quantity of heat energy is known as the latent heat of fusion and is expressed in terms of the number of calories needed to melt one gram of a solid at its melting point. To determine this constant, a calorimeter will be utilized.

The principle of calorimetry is based on the Law of Conservation of Energy, which states that energy can be transferred between objects without any loss or gain in the total energy involved. In an isolated system, the energy lost by one part of the system is equal to the energy gained by the other part(s). The contents of a well-insulated calorimeter constitute such an isolated system. While achieving perfect insulation where no heat is lost is rare, it is especially challenging when dealing with liquids, as they must be contained in a vessel that can absorb or release heat as the liquid changes temperature. The amount of heat released or absorbed by the container is determined by its heat capacity and the change in temperature. The heat capacity of the container represents the amount of heat that it would release or absorb for each degree of temperature change in its contents.

To work with heat transfers, it is crucial to understand the fundamental terms involved:

a. Specific heat: It is the ratio of the heat required to raise the temperature of one gram of a substance by 1°C to that needed to raise the temperature of 1 gram of water by 1°C.

b. Calorie: It is the quantity of heat required to raise the temperature of 1 gram of water by 1°C.

c. Heat capacity: It is the amount of heat required to raise the temperature of a body or object by 1°C.

d. Density: It is the mass per unit volume.

According to the Law of Conservation of Energy, when ice is added to a sample of water, the heat lost by one part of the system is equal to the heat gained by another part of the system.

heat gained = heat lost

During this process, the resulting temperature will be an intermediate value between that of the ice and the original water. This means that heat is gained by the ice as it melts, and also by the water formed from the melted ice as it warms up to the final temperature. Simultaneously, the original water and the calorimeter lose heat as they cool down to the final temperature.

Incorporating these ideas into Equation (1), we obtain the following:

heat gained by ice being converted to water at 0°C

+

heat gained by water (from ice) being warmed to final temperature

=

heat lost by initial water in calorimeter

+

heat lost by the calorimeter

Therefore, it is necessary to determine the amount of heat lost by the calorimeter, which can be calculated by multiplying the heat capacity of the calorimeter by the change in temperature. Once the heat lost by the calorimeter is known, it becomes possible to calculate the heat of fusion.

Determination of Calorimeter Heat Capacity

1. Weigh 100 g of water using a triple beam balance and carefully place it in a Styrofoam calorimeter cup. Note the initial temperature as t1.

2. Take approximately 110 mL of water in a beaker or flask and heat it until it reaches boiling point. Assume the water temperature is 100.0°C and record this value as t2.

3. Combine the boiling water with the water in the calorimeter. Gently stir the mixture with a thermometer and record the highest temperature reached as t3. Measure the total weight of water in the calorimeter. Calculate the weight of boiling water added by subtracting the original weight of water from the total weight. Perform two separate calculations to determine the heat capacity and average your results. Express the results in calories per degree gained or lost by the calorimeter.

Calculations:

Heat gained by cold water = (Mass of cold water) (specific heat of water) (t3-t1)

The specific heat of water is 1.00 cal/g ∙ °C, so this expression simplifies to:

(1) Heat gained by cold water = (MCW) (1.00 cal/g ∙ °C) (t3-t1)

(2) Heat gained by calorimeter = (heat capacity of calorimeter) (t3-t1) = (HCcal) (t3-t1)

Heat lost by hot water = (Mass of hot water) (specific heat of water) (t3-t2)

This equation simplifies to:

(3) Heat lost by hot water = (Mhw) (1.00 cal/g ∙ °C) (t3-t2)

The Law of Conservation of energy says:

HEAT GAINED = HEAT LOST

(4) (Mcw) (1.00 cal/g ∙ °C) (t3-t2) + (HCcal)(t3-t1) = -(Mhw) (1.00 cal/g ∙ °C) (t3-t2)

(5) Rearranging this equation gives:
(HCcal) (t3-t1) = -(Mhw) (1.00 cal/g ∙ °C) (t3-t2) - (Mcw) (1.00 cal/g ∙ °C) (t3-t1)

Solving Equation (5) for the heat capacity of the calorimeter, we get:

(6) (HCcal) = - (Mhw) (1.00 cal/g ∙ °C) (t3-t2) - (Mcw) (1.00 cal/g ∙ °C) (t3-t1)
(t3-t1)

Equation (6) should be used for the calculations of the heat capacity of the calorimeter cup.

Determination of the Heat of Fusion of Ice

1. Place 200 grams of water into the calorimeter and note down the initial temperature (T1) of the water.

2. Take one or more ice lumps weighing between 15 and 25 grams in total. It is not necessary to be precise in measuring the ice. Remove any excess liquid water from each lump by blotting them, and add these blotted lumps to the water in the calorimeter. Stir the mixture of ice and water until the ice has completely melted, and record the lowest temperature reached (T3). Assume the original temperature of the ice to be 0°C (T2).

3. Determine the total weight of the water in the calorimeter, following the same procedure as described in Part A. The difference between the total weight of water and the initial weight represents the weight of water resulting from the melted ice.

4. Conduct two separate measurements of the heat of fusion of ice and calculate the average of the results. Incorporate the previously determined heat capacity of your calorimeter into this calculation.

Calculations:

Heat gained by ice in melting = (Mass of ice) (Heat of fusion of ice)

(7) = (Mice) (∆Hfus)

Heat gained by water from ice = (Mass of ice) (specific heat of water) (T3-T2)

The specific heat of water is 1.00 cal/g ∙ °C, and T2 is 0°c, so this becomes:

(8) Heat gained by water from ice = (Mice) (1.00 cal/g ∙ °C) (T3)

(9) Heat lost by initial water = (Mass of water) (specific heat of water) (T3-T1) = - (Mwater) (1.00 cal/g ∙ °C) (T3-T1)

Heat lost by calorimeter = (heat capacity of calorimeter) (T3 -T1)

(10) = (HCcal) (T3-T1)

From the Law of Conservation of Energy HEAT GAINED = - HEAT LOST

(11) (Mice) (∆Hfus) + (Mice) (1.00) (T3) = - (Mwater) (1.00) (T3-T1) - (HCcal) (T3-T1)

Rearranging gives:

(12) (Mice) (∆Hfus) = - (Mwater) (1.00) (T3-T1) - (HCcal) (T3-T1) - (Mice) (1.00) (T3)

When Equation 12 is solved for the heat of fusion of ice the following results:

(13) ∆Hfus = - (Mwater) (1.00) (T3-T1) - (HCcal) (T3-T1) - (Mice) (1.00) (T3)
Mice

Equation 13 should be used for the calculations of the Heat of Fusion of Ice.

Report Sheet for Heat of Fusion of Ice

Name_______________________________________ Date___________________________

Lab / Section_________________________________

Review Questions

1. What is the purpose of measuring the latent heat of fusion in this experiment?

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2. Define the term "latent heat of fusion."

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3. How does the Law of Conservation of Energy apply to calorimetry?

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4. Explain why energy is required to melt a solid substance.

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5. How does the heat capacity of the calorimeter affect the measurements in this experiment?

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6. Describe the process of determining the heat capacity of the calorimeter.

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7. What is the significance of the specific heat of water in the calculations performed in this lab?

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8. How does the concept of energy transfer relate to the Law of Conservation of Energy?

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9. Discuss the role of a well-insulated calorimeter in maintaining an isolated system.

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10. Why is it important to blot the ice before adding it to the calorimeter?

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11. Describe the steps involved in determining the heat of fusion of ice.

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12. Explain how the heat gained by the ice in melting and the heat gained by the water from the melted ice are calculated.

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13. How does the change in temperature affect the calculations for heat gained and lost in this experiment?

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14. Discuss the relationship between the heat capacity of the calorimeter and the heat lost by the calorimeter.

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15. Why is it necessary to perform multiple trials and average the results in Part A of the experiment?

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16. How can the results of this experiment be applied to understand the energy changes during phase transitions in other substances?

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17. Explain the concept of efflorescence and deliquescence as discussed in the background information.

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18. What factors determine the stability of hydrates?

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19. How do the equations provided in the discussion section illustrate the decomposition of hydrated aluminum chloride?

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20. Discuss the importance of accurate measurements and significant figures in conducting this lab and obtaining reliable results.

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