Gizmo Answers: Reaction Energy

Activity A: Get the Gizmo ready: Energy of chemical bonds Check that Reaction 1 and Forward are selected. 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 achemical reaction? 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. 2. I think that the heat energy is released because the chemical bonds that are formed between the oxygen and hydrogen atoms would release their energy as electrons moving into the lower orbitals. 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. 3. A. What happens? What ends up happening is that the bond breaks and the two hydrogens are now separated. B. Look under the Energy absorbed column of the table. How much energy was required to break this bond? 436 kJ/mol was required to break this bond. 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 H 2 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 total energy absorbed so far is 872 kj/mol. 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? B. Press Break bond. How much energy is needed to break both bonds? 349 kJ/mol 495 kJ/mol C. What is the total energy required to break up two moles of H2 molecules and one mole of O₂ molecules? _ 1367 kJ/mol 4. 5. 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? What ends up happening is that the hydrogen and oxygen atoms connect via a single covalent bond. B. The "jiggling" animation you see represents the release of kinetic energy that occurs when a bond is formed. C. How much energy was released? There was 463 kJ/mol of energy released. Drag another hydrogen molecule into the template and click Create bond to make a water molecule. What is the total energy released so far? The total energy released was 926 kJ/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? The total energy released now is 1852 kJ/mol. 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? B. How does this relate to the change in temperature observed for this reaction? Released It means that more energy released means higher temperature. 6. C. The change in enthalpy (AH) of the system is equal to the total energy absorbed minus the total energy released. What is the AH value for this reaction? The value for this reaction is -484 kJ/mol. Compare this value to the Theoretical AH listed on the right side. Draw conclusions: The experimental AH 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? Yes these values are close. Activity B: Get the Gizmo ready: . Select the Reaction tab. Reaction 1 2H,0 2H,+O Reaction direction With Reaction 1 selected, choose Reverse. Forward 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? B. Did the temperature change? C. Is there any evidence that a reaction took place? What ends up happening is nothing actually, they end up bouncing around. No, the temperature did not change. No, there is no evidence that a reaction took place. 2. Explain. This is because it remains at the same molar concentration and temperature. 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. B. How much energy was released in forming the product molecules?. A. How much energy was absorbed in breaking up the water molecules? 1852 kJ/mol 1367 kJ/mol 0 kJ/mol Endothermic C. What is the total enthalpy change in the system?_ D. Based on this result, is this reaction exothermic or endothermic? _ Explain. This is because there was more total energy being absorbed than being 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. 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 AH value in each direction: 4. Direction Energy absorbed Energy released Theoretical AH Forward 2347 kJ/mol 2144 kJ/mol 203 kJ/mol 02 Reverse 2144 kJ/mol 2347 kJ/mol -203 kJ/mol 02 They are opposite of each other. What do you notice about the theoretical AH values in the forward and reverse directions? Predict: Based on your findings, do you think Reaction 2 is more likely to occur in the forward or reverse direction? I predict that the reaction is more likely to occur in reverse. Explain This is because most chemical 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. I found that the reaction occurred in the reverse direction. When the reaction occurred, how did the theoretical AH value compare to the experimental value?_ The theoretical value was -203 kj/mol O2 but the experimental value was -211 kJ/mol 02. 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 AH Forward or reverse? 3 Reverse -808 kJ/mol CH4 Reverse 4 Forward -2328 kJ/mol CH4 Forward 7. Test: Test your predictions on the REACTION tab.