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SCH4U. Unit 4. Thermo chemistry Gizmo

Gizmo: Reaction Energy, Student Exploration Sheets: Note: To record observations and answers create space as much as you need, move the lines down. Vocabulary: calorimeter, chemical bond, endothermic, enthalpy, exothermic, Hess’s law Prior Knowledge Questions (Do these BEFORE using the Gizmo.) 1. Two magnets are stuck together. What might you have to do to get them to separate? In order to separate two magnets which are stuck together, you must pull them apart. 2. Suppose you held two magnets a short distance apart, then let go. What would happen? The magnets will most probably snap together/attract towards each other. 3. Think about the magnets in terms of energy. In which case do you increase the potential energy of the magnets? In which case do you increase the kinetic energy of the magnets? Pulling the magnets away from each other/keeping them away from each other increases the potential energy in them. Whereas, the kinetic energy increase when they are together or moving towards each other. Gizmo Warm-up Just like magnets, atoms of different elements are attracted together to form chemical bonds. Breaking these bonds requires energy. When a new bond forms, energy is released and temperatures rise. In the Reaction Energy Gizmo, you will explore how the energy of chemical bonding relates to temperature changes that occur during chemical reactions. To begin, check that Reaction 1 and Forward are selected. In this reaction, hydrogen (H2) and oxygen (O2) react to form water (H2O). The reaction takes place inside a device called a calorimeter. Inside the calorimeter, a small chamber holds the reactants. The rest of the calorimeter is filled with water. 1. Click Play ( ). What happens? Hydrogen atoms pair up with oxygen atoms to form H2O. 2. How does the temperature change? The temperature increases from 21 degree C to 27.4 degree C.2019

Get Activity A: Energy of Reactions

SCH4U. Unit 4. Thermo chemistry Gizmo • Check that Reaction 1 and Forward are selected. Chemical bonds • Select the INVESTIGATION tab. Introduction: The heat energy stored in a chemical system is called the enthalpy (H) of the system. When atoms are joined by a chemical bond, energy must be added to pull them apart. This increases the enthalpy of the system. When a chemical bond forms, energy is released as shared electrons move into lower-energy orbitals. This causes the enthalpy to decrease. Question: How can you predict how much energy is released in a chemical reaction? You can predict how much energy is released or absorbed by reading the temperature and using the q=mc delta T formula. 1. Predict: In the warm-up activity, you observed how the reaction inside the chamber affected the temperature of the surrounding water. Based on what happens to the surrounding water, do you think heat energy (enthalpy) is absorbed in the reaction or released? Explain. Based on the warm up activity I predict that the energy was released since H hydrogen created bonds with O oxygen which released energy and caused temperature to warm up. 2. Observe: In the Gizmo, the energy required to break a chemical bond is modeled by placing a molecule into a set of mechanical claws. Place one of the hydrogen (H2) molecules between the claws, and press Break bond. A. What happens? The energy absorbed changed to 436 kJ/mol B. Look under the Energy absorbed column of the table. How much energy was required to break this bond? The amount of energy required to break this bond was 436 kJ. Note: The energy is given here in units of kilojoules per mole (kJ/mol). This is the energy, in kilojoules, required to break all of the H–H bonds in one mole of H2 gas. C. Remove the hydrogen atoms from the claws and then break apart the other H–H molecule. What is the total energy absorbed so far? The energy absorbed so far is 872KJ/mol. 3. Measure: Notice that the oxygen atoms are connected by a double covalent bond. This is because the oxygen atoms share two pairs of electrons. Place the oxygen molecule in the claws and press Break bond. A. How much energy is required to break the first O–O bond? When the first 02 bond’s broken, energy 349 kJ/mol is required. B. Press Break bond. How much energy is needed to break both bonds? 495 kJ/mol energy required to break both bonds. C. What is the total energy required to break up two moles of H2 molecules and one mole of O2 molecules? 1367 kJ/mol energy required. (Activity A continued on next page) 2019 Activity A (continued from previous page)

SCH4U. Unit 4. Thermo chemistry Gizmo 4. Create: Remove the two oxygen atoms from the claws. Now the claws disappear and you see a template for creating a water molecule. Drag an oxygen and a hydrogen atom into the template. (If necessary, use the Key on the right-hand side as a reference.) A. Click Create bond. What happens? Energy is released when bonds are created.

B. The “jiggling” animation you see represents the release of kinetic energy that occurs when a bond is formed. How much energy was released? 436 /mol energy was released. C. Drag another hydrogen molecule into the template and click Create bond to make a water molecule. What is the total energy released so far? 926 /mol D. Drag the first water molecule away from the template, then use the Gizmo to create a second water molecule. What is the total energy released now? 1852 mol 5. Calculate: Compare the energy absorbed in breaking up the molecules to the energy released when new bonds are formed. A. In this reaction, was more energy absorbed or released? In this reaction, energy was more released than absorbed. B. How does this relate to the change in temperature observed for this reaction? This relates to change in temperature since when energy is released temperature increases. C. The change in enthalpy (∆H) of the system is equal to the total energy absorbed minus the total energy released. What is the ∆H value for this reaction? The ∆H value for this reaction is -484 kJ/mol. Compare this value to the Theoretical ∆H listed on the right side. Theoretical is -485 kJ/mol which means the experimental is just off by 1. 6. Draw conclusions: The experimental ∆H value was determined by measuring how much heat the reaction produced inside the calorimeter. This is calculated based on the temperature change of the reaction, the amount of water inside the calorimeter, and the specific heat of the calorimeter. Compare the theoretical change in enthalpy to the experimental value. Are these values close? The value of the theoretical is -485 kJ/mol and experimental is -484 kJ/mol, which means the experimental is just off by1.

019 Get Activity B: Reaction Direction  Select the Reaction tab.

SCH4U. Unit 4. Thermo chemistry Gizmo 

With Reaction 1 selected, choose Reverse.

Introduction: Reactions that emit heat energy are called exothermic reactions. In an exothermic reaction, more heat energy is released by the formation of bonds than is absorbed in the breaking of bonds. In an exothermic reaction, the temperature of the surroundings will increase and the enthalpy of the system will decrease as energy is emitted from the system. In an endothermic reaction, more heat energy is absorbed in the breaking of bonds than is released in the formation of bonds. The temperature decreases and the enthalpy increases. Question: How can you predict the direction of a chemical reaction? 1. Observe: Notice that in the reverse reaction, the container is filled with water molecules. A. Click Play. What happens? The temperature remains the same and the speed of the molecules is constant. B. Did the temperature change? No, the temperature does not change. C. Is there any evidence that a reaction took place? Explain There is no evidence that a reaction occurred. This is because the reaction chamber still contains 0.2 moles of water. 2. Investigate: Switch to the INVESTIGATION tab. Use the claws to break up the two water molecules, and then form two hydrogen molecules and one oxygen molecule. A. How much energy was absorbed in breaking up the water molecules? The energy absorbed in breaking up the water molecules was 1852 kJ/mol. B. How much energy was released in forming the product molecules? 1,367 kJ/mol C. What is the total enthalpy change in the system? 485 kJ/mol E. Based on this result, is this reaction exothermic or endothermic? Explain. This reaction was endothermic since more energy was absorbed then released. Most endothermic reactions do not occur unless there is a continuous input of energy. For example, water molecules can decompose into hydrogen and oxygen gas by adding a salt and passing an electrical current through the water. (Activity B continued on next page)

19 Activity B (continued from previous page)

SCH4U. Unit 4. Thermo chemistry Gizmo 3. Investigate: Select Reaction 2. On the INVESTIGATION tab, go through the reaction in both the forward and reverse directions. State the energy absorbed, energy released, and theoretical ∆H value in each direction: Direction Energy absorbed Energy released Theoretical ∆H Direction

Energy absorbed

Energy released

Theoretical ∆H

Forward

2347 kJ/mol

2144kJ/mol

203 kJ/mol

Reverse

2144 kJ/mol

2347 kJ/mol

-203 kJ/mol

What do you notice about the theoretical ∆H values in the forward and reverse directions? Both values are different since in the forward reaction we have a +ve theoretical value whereas in the reverse we have a negative theoretical value 4. Predict: Based on your findings, do you think Reaction 2 is more likely to occur in the forward or reverse direction? Explain. Reverse since most reactions are exothermic. 5. Test: Select the REACTION tab and test your prediction by running the reaction in the forward and reverse directions. Describe your findings. When the reaction occurred, how did the theoretical ∆H value compare to the experimental value? The reaction occurred in the reverse order since most reactions are exothermic. The theoretical values were different from experimental value by 8 meaning theoretical value was -203 but the experimental value was -211. 6. Investigate: On the INVESTIGATION tab, go through the remaining reactions, in either direction. (You will only need to look at each reaction once.) Fill in the table, then predict if the reaction will proceed in the forward or reverse directions. Reaction Direction Theoretical ∆H Forward or reverse? Reaction 3 was Reverse (Exothermic) Reaction 4 was Forward (Exothermic)

7. Test: Test your predictions on the REACTION tab. What did you find? Reaction 3 was reverse and Reaction 4 was forward.

9 Get Activity C: the Gizmo ready: Bond enthalpy

SCH4U. Unit 4. Thermo chemistry Gizmo • Select Reaction 3 and Reverse. • Select the INVESTIGATION tab. Introduction: Each chemical bond has a “bond enthalpy” that describes how much energy is absorbed to break a bond and how much energy is released when the bond is formed. (These values are the same.) A chart of bond enthalpies for some common bonds is shown below. Bond Enthalpy (kJ/mol) Bond Enthalpy (kJ/mol) Bond Enthalpy (kJ/mol) C–H 413 O–H 463 H–H 436 C–C 348 O=O 495 N–H 391 C=C 614 O–S 265 N≡N 941 C=O 799 O=S 523 S–S 266 Question: How can you use bond enthalpy to predict the total enthalpy change of a chemical reaction? 1. Calculate: Consider the reaction CH4 + 2O2 → CO2 + 2H2O. A. In the reactants, how many C–H bonds are there? 1 bond B. Using the chart above, what is the total bond enthalpy of these bonds? 413 for one C and H bond, so 4 would be 1652 C. How many O=O bonds are there? 2 for 1 O and O double bond so it would be 696 D. What is the total enthalpy of these bonds? 4872 E. What is the total bond enthalpy of the reactants? 2348 F. Do the same calculation for the products of the reaction, CO2 + 2H2O. Carefully count how many of each bond there is, and consider whether bonds are single or double bonds. Show your work and list the total bond enthalpy of the products below. Total bond enthalpy of products: 2524 G. Based on the enthalpy of the reactants and products, what is the ∆H value for this reaction? (Recall that ∆H = energy absorbed – energy released.) H. Use the Gizmo to check your results, and correct any errors if necessary. (Activity C continued on next page)

2019 Activity C (continued from previous page) Bond Enthalpy (kJ/mol) Bond Enthalpy (kJ/mol) Bond Enthalpy (kJ/mol) C–H 413 O–H 463 H–H 436 C–C 348 O=O 495 N–H 391 C=C 614 O–S 265 N≡N 941 C=O 799 O=S 523 S–S 266

SCH4U. Unit 4. Thermo chemistry Gizmo 2. On your own: Using the chart above, predict the enthalpy change for the following reactions. Show your work. A key of all relevant molecule structures is shown on the right. A. N2 + 3H2 → 2NH3 N2: N≡N H2: H–H NH3: -97KJ Total ∆H: Exothermic or endothermic? Exothermic B. 8SO2 → S8 + 8O2 SO2: O=S–O S8: 8 S–S bonds O2: O=O -44KJ Total ∆H: Exothermic or endothermic? exothermic C. C2H4 + 3O2 → 2CO2 + 2H2O O2: O=O CO2: O=C=O H2O: H–O–H C2H4: -18KJ Total ∆H: Exothermic or endothermic? exothermic 3. Analyze: In all of the reactions you investigated today, did it make any difference in which order you broke bonds or formed bonds? Explain your answer. It does, because reactants break first in order to make new bonds. According the Hess’s law, the total enthalpy change for a reaction is simply the sum of all of the individual enthalpy changes in the reaction. The order in which you add these enthalpy changes does not matter. 2019...