1. When we calculate how much energy is needed to break a chemical bond, it makes...

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1. When we calculate how much energy is needed to break a chemical bond, it makes a difference if we are talking about a single molecule in a vacuum, or lots of molecules contained in a fluid. The latter situation is more realistic and less abstract, but also requires that we consider not just the energy to break the bond, but the energy required to "deal with" the environment. If, for example, as is common in chemistry (or biology), a reaction takes place at a constant pressure and temperature, then the energy we put into the reaction is used not only to break apart bonds but also to do work-the volume must be increased to make room for the additional particles that are created so the pressure remains constant. This is why we might use enthalpy as our energy needed to cause a reaction rather than just the energy needed to break the bond itself: enthalpy also includes the amount of energy required to change the volume, i.e., the work. The energy needed to simply pull the bond apart is called the dissociation energy (it's the negative of the binding energy). In more complicated reactions, of course, we can both break and form bonds, and so there could be a net input or output of energy. In either case, enthalpy would still be the relevant quantity for a reaction taking place at constant pressure, and the change in the enthalpy is simply the heat that is added or removed from a system at constant pressure. Let's consider the following example: To break apart a single H₂ molecule, H₂ → 2H, the dissociation energy is 4.52 eV. a. How much energy would be required (in kJ) to dissociate 1 mol of hydrogen molecules? (1 eV 1.602 x 10 19 J and there are 6.02 x 1023 molecules in 1 mol) Suppose we are putting in energy to dissociate a bubble of 1 mol of H, molecules at STP (P= 10³ Pa and T = 300 K). We'll consider the bubble as our system and assume the pressure and temperature remain at STP. b. How many moles of H atoms are in the bubble after we have dissociated all the hydrogen molecules? c. What are the initial and final volumes of the bubble? Has the bubble expanded or contracted? d. Does the thermal energy of the system increase, decrease, or stay the same in this process? Explain your reasoning. If the thermal energy changes, calculate AE e. Does the chemical energy of the system increase, decrease, or stay the same in this process? Explain your reasoning. If the chemical energy changes, calculate Achem f. What is the change in internal energy AUint of the system for this process? g. Calculate the work for this process. Is the work transferring energy in or out of the system? How can you tell? h. Using the first law of thermodynamics, AUnt=W+Q, calculate the heat for this process. Is the heat transferring energy in or out of the system? How can you tell? i. Using the definition of enthalpy, AH =AUnt+ PAV, calculate the change in enthalpy for this process. Is the enthalpy increasing or decreasing? How does the enthalpy change compare to the heat you. calculated in part h? 1. When we calculate how much energy is needed to break a chemical bond, it makes a difference if we are talking about a single molecule in a vacuum, or lots of molecules contained in a fluid. The latter situation is more realistic and less abstract, but also requires that we consider not just the energy to break the bond, but the energy required to "deal with" the environment. If, for example, as is common in chemistry (or biology), a reaction takes place at a constant pressure and temperature, then the energy we put into the reaction is used not only to break apart bonds but also to do work-the volume must be increased to make room for the additional particles that are created so the pressure remains constant. This is why we might use enthalpy as our energy needed to cause a reaction rather than just the energy needed to break the bond itself: enthalpy also includes the amount of energy required to change the volume, i.e., the work. The energy needed to simply pull the bond apart is called the dissociation energy (it's the negative of the binding energy). In more complicated reactions, of course, we can both break and form bonds, and so there could be a net input or output of energy. In either case, enthalpy would still be the relevant quantity for a reaction taking place at constant pressure, and the change in the enthalpy is simply the heat that is added or removed from a system at constant pressure. Let's consider the following example: To break apart a single H₂ molecule, H₂ → 2H, the dissociation energy is 4.52 eV. a. How much energy would be required (in kJ) to dissociate 1 mol of hydrogen molecules? (1 eV 1.602 x 10 19 J and there are 6.02 x 1023 molecules in 1 mol) Suppose we are putting in energy to dissociate a bubble of 1 mol of H, molecules at STP (P= 10³ Pa and T = 300 K). We'll consider the bubble as our system and assume the pressure and temperature remain at STP. b. How many moles of H atoms are in the bubble after we have dissociated all the hydrogen molecules? c. What are the initial and final volumes of the bubble? Has the bubble expanded or contracted? d. Does the thermal energy of the system increase, decrease, or stay the same in this process? Explain your reasoning. If the thermal energy changes, calculate AE e. Does the chemical energy of the system increase, decrease, or stay the same in this process? Explain your reasoning. If the chemical energy changes, calculate Achem f. What is the change in internal energy AUint of the system for this process? g. Calculate the work for this process. Is the work transferring energy in or out of the system? How can you tell? h. Using the first law of thermodynamics, AUnt=W+Q, calculate the heat for this process. Is the heat transferring energy in or out of the system? How can you tell? i. Using the definition of enthalpy, AH =AUnt+ PAV, calculate the change in enthalpy for this process. Is the enthalpy increasing or decreasing? How does the enthalpy change compare to the heat you. calculated in part h?