Question

1. If we use symbol U_sys to represent the energy (also referred as internal energy) contained in the system, what is a reasonable symbol to represent the energy of the surroundings? Provide an expression relating the total energy of the Universe, U_univ to the energy of the system and energy of the surroundings? Explain your answer

2. Now consider the system undergoes a process, let DU_sys represent the change in system’s energy, DU_univ represents the change in energy of the universe. Provide an expression for DU_univ in terms of energy change of the system and the surrounding. Explain your answer.

3. Consider your answer in (2), what would be the reasonable sign for DU_univ? Explain.

4. Consider your answer in (2), what would be the reasonable sign for DU_sys? Explain

5. Let q and w represent two ways to exchange energy between system and surrounding. What is a reasonable expression that relates the internal energy change DU_sys with q and w?

6. Consider burning of propane gas (propane and oxygen gases are our interested system), is this an exothermic reaction? What is the sign of q? what is the sign of DU_sys ? Describe in your own words what happened to the internal energy of the system?

7. Identify each energy exchange as heat or work and determine the sign of heat or work is positive or negative

(a) An ice cube melts and cools the surrounding beverages (ice cube is the system here)

(b) A metal cylinder is rolled up a ramp (the metal cylinder is the system)

(c) Steam condenses on skin causing a burn (the condensing steam is the system)

8. Consider a closed container that contains some hydrogen gas (H₂) and oxygen gas (O₂). These gases reacted (burned) and formed water vapor.

(a) Write a balanced chemical equation to represent this process (reaction)

(b) Do you expect this is an exothermic or endothermic reaction? What is the sign of q?

(c)What is the sign of internal energy change DU for this reaction?

(d) List the types of energy stored in the system before and after reactions. How do you explain the sign of internal energy DU observed in part (c)?

Answer 1

A reasonable symbol to represent the energy of the surrounding is U_surr

Expression relating the total energy of the Universe (U_univ) ot the energy of system and energy of the surroundings :-

U_univ =U_surr + U_sys

Explaination :- according to law of conservation of mass and energy , mass or energy cannot be destroyed or created, it can only be transformed from one form to another. thus energy cannot be destroyed, which thereby means that energy is contant for an universe. if we treat ourselves as an observer standing outside the Earth, the energy content of Earth won't change. but once we get into the Earth, we would be treated either as a part of system or surrounding. thus we can either lose or gain some amount of energy. but the overall change in energy will be constant. in other words U_univ will remain zero in ideal condition ( no other source for leak of energy).

U_univ= U_sys + U_surr

thus if U_univ is zero, that will eventually mean that

U_sys = - U_surr

which means that if in any reaction or mechanism, system loses some amount of energy, the lost energy will be gained by the surrounding, and vice versa. but the overall energy will be constant for the universe.