First Law of Thermodynamics Contributions by: John L. Falconer & Will Medlin Department of Chemical and Biological Engineering University of Colorado Boulder, CO 80309-0424 Supported by the National Science Foundation

When a hydrogen molecule dissociates into two H atoms, energy is ______________. added removed neither added nor removed H H ANSWER: A. added. H H

Consider a completely insulated, closed system of fixed volume Consider a completely insulated, closed system of fixed volume. The exothermic thermite reaction takes place in the system. The system energy __________. Insulation System increases decreases stays the same need more information ANSWER: C. stays the same. No energy was added to or removed from the system

The internal energy of the container _____________. An endothermic reaction takes place in a completely insulated, closed container of fixed volume. The internal energy of the container _____________. increases decreases stays the same Need more information Insulation Container ANSWER: C. stays the same. No energy is added to or removed from the container.

The internal energy of the container ____________. An endothermic reaction takes place in an isothermal, closed container of fixed volume. The internal energy of the container ____________. increases decreases stays the same ANSWER: A. increases. Energy must be added to the container in order to keep the system isothermal (constant temperature).

An exothermic reaction takes place in an isothermal, closed container of fixed volume. The internal energy of the container _____________. increases decreases stays the same Need more information ANSWER: B. decreases – energy must be removed from container to remain isothermal (constant temperature).

An endothermic reaction takes place in a continuous-flow, isothermal reactor at constant pressure. The enthalpy of the effluent is _________________ the enthalpy of the inlet. higher than lower than the same as ANSWER: A. higher than the enthalpy of the inlet. The enthalpy of the effluent is higher because energy is added in order to keep the reactor isothermal. For a flow reactor, enthalpy appears in the first law.

An exothermic reaction takes place in a continuous-flow, isothermal reactor at constant pressure. The enthalpy of the effluent is ____________ the enthalpy of the inlet. higher than lower than the same as ANSWER: B. lower than the enthalpy of the inlet. The enthalpy of the effluent is lower because energy is removed in order to keep the reactor isothermal. For a flow reactor, enthalpy appears in the first law.

An endothermic reaction takes place in a continuous-flow, adiabatic reactor. The enthalpy of the effluent is ____________ the enthalpy of the inlet. higher than lower than the same as ANSWER: C. the same as the enthalpy of the inlet. The enthalpy is the same because no energy is added or removed in an adiabatic reactor. For a flow reactor, enthalpy appears in the first law.

An exothermic reaction takes place in a continuous-flow, adiabatic reactor. The enthalpy of the effluent is _____________ the enthalpy of the inlet. higher than lower than the same as ANSWER: C. the same as the enthalpy of the inlet. The enthalpy is the same because no energy is added or removed in an adiabatic reactor. For a flow reactor, enthalpy appears in the first law.

The energy of the box ___________. A weight hangs by a thin thread from a black box that is completely isolated from the surroundings. The weight slowly rises. The energy of the box ___________. BLACK BOX Weight slowly drawn up to box Very thin thread TABLE Completely isolated (except for thread) increases decreases does not change ANSWER: B. decreases. The potential energy of the weight increases.

Two adiabatic reactor setups contain the same reaction with 70% conversion at steady state. The preheater heats the feed stream and causes a 200°C decrease in the reactor effluent. Assume perfect heat exchangers. The exit temperature of the system with the preheater is ______________. Reactor 100oC 500oC X=0.7 T = ? 500°C 600°C 700°C Need more information ANSWER: A. 500°C. Assuming the pre-heater is a perfect heat exchanger, the energy removed from the reactor effluent (to lower it by 200°C) enters the feed stream (to raise it by 200°C). No energy is removed or added from the system as a whole (comprising of the pre-heater AND the reactor). Pre-Heater

A steady state, adiabatic reactor is fed a mixture of A and an inert; the flow rate of each is 10 mol/hr. The exothermic reaction A  B goes to completion. The outlet temperature is 400oC. When the flow rate of A is increased to 20 mol/hr while the inert remains at 10 mol/hr, conversion is still complete and the effluent temperature _____________. A  B A 10 mol/hr X = 1.0 T = 400oC 20 mol/hr T = ? Inert increases decreases remains the same Need more information ANSWER: B. decreases. Some of the heat of reaction is need to raise the temperature of the additional inert.

An endothermic reaction (A  C) takes place in an adiabatic flow reactor. The feed is a 50/50 mixture of reactant A and inert B. At a flow rate of 10 mol/hr, conversion of A is 100% and the outlet temperature is 60oC. The feed flow rate is doubled, and only 75% of A is converted to C. The outlet temperature is _____________ 60oC. A  C A B 10 mol/hr X = 1.0 T = 60oC 20 mol/hr X = 0.75 T = ? higher than lower than equal to ANSWER: A. higher than 60°C. Less energy is absorbed in the endothermic reaction when the conversion is only 75%.

gas flow What is the equation for work for steady-state flow? -PDV -D(PV) -VDP None of the above ANSWER: B. -D(PV) This is why delta H is used for flow systems, to account for this work.

An ideal gas goes from state 1 to 2 by two processes An ideal gas goes from state 1 to 2 by two processes. Process A is reversible and has 50 kJ of work done on the gas. Process B is irreversible and has 200 kJ of work done on the gas. If one process requires heat to be added and the other removed, which process has to have heat added? 50 kJ Process A Process B 1 2 ANSWER: A. Process A. Heat would have be added to satisfy first law since Q1 + W1 = Q2 + W2 200 kJ 1 2

Which process creates more |work|? A gas undergoes reversible expansion from 40 bar and 500 K to 5 bar and 500 K by two pathways. Which process creates more |work|? A constant pressure process to the final volume and then constant volume to 5 bar. A constant volume process to 5 bar and then constant pressure to the final volume. Same work for both ANSWER: A. A constant pressure process to the final volume and then constant volume to 5 bar. The P-deltaV term is much greater when calculated at the constant higher pressure, as in the pathway for Answer A.

Methane at 15 bar and 120°C flows into an evacuated container when the valve is open. When the pressure in the container reaches 15 bar, the temperature of the methane is _______ 120°C. Container CH4 120°C 15 bar greater than less than equal to Need more information ANSWER: A. greater than 120°C. The flow of the incoming gas is doing work on the container contents.

Methane at 15 bar and 120°C flows into a container that has methane at 120°C and 2 bar when the valve is opened. When the pressure in the container reaches 15 bar, the temperature of the methane in the container is ___________ 120°C. CH4 120°C & 2 bar 120°C 15 bar greater than less than equal to Need more information ANSWER: A. greater than 120°C. The flow of the incoming gas is doing work on the container content, and the pressure is increasing at constant volume.

Two reversible pathways are shown. Which one has the larger value of Q? 1  2  3  1 1  4  5  1 Both have the same Q Q is zero for both pathways ANSWER: B. The larger area means more work done by the system (for both paths the work is negative). Because the pathway is a cycle, the system energy does not change. Consequently, first law states that if more work is removed (Path B), more heat must be added.

An electric fan is turned on in a room with all the doors closed An electric fan is turned on in a room with all the doors closed. The room temperature ___________. increases decreases remains the same Need more information ANSWER: A. increases. Energy is added to the room in the form of electricity and that added energy is converted to heat.

A battery-operated fan is turned on in a room with all the doors closed. The room temperature ______________. increases decreases remains the same Need more information. ANSWER: A. increases. No energy is added or removed from the room, but the chemical energy in the battery decreases and the room temperature increases.

A battery-operated fan is turned on in a room with all the doors closed. The energy in the room ______________. increases decreases remains the same Need more information ANSWER: C. remains the same. No energy is added or removed from the room. The chemical energy in the battery decreases and the room temperature increases.

A refrigerator door is opened in a kitchen and all the doors to the kitchen are closed. The kitchen temperature ______________. increases decreases remains the same ANSWER: A. increases. Energy is added to the room in the form of electricity and that added energy is converted to heat. No energy is being removed from the room.

An ideal gas is compressed to 10 times its initial pressure An ideal gas is compressed to 10 times its initial pressure. The final volume is smaller for _________. an isothermal compression an adiabatic reversible compression neither. It is the same in both. ANSWER: A. an isothermal compression. For adiabatic compression, work done on the system increases its internal energy and thus its temperature. Higher temperature at the same pressure must have a larger volume.

Which substance at 25°C requires the most work per kg to compress from 1 bar to 25 bar? Water Hexane Mercury Air Same work for all ANSWER: D. Air. The largest volume change out of all the choices is possible with air.

Which of the following, when fed with an organic to an oxidation reaction, has the higher adiabatic temperature? A stoichiometric amount of pure O2 50% excess oxygen (pure) A stoichiometric amount of air ANSWER: A. A stoichiometric amount of pure O2. Choices B and C require some of the heat of reaction to raise the temperature of the excess O2 or N2, and the final temperature will be lower.

For which system is the value of the internal energy the highest? One kg of steam One kg of liquid water Both systems have the same internal energy ANSWER: A. One kg of steam.

A pot of water is boiling open to the atmosphere when the pot is sealed by a tight lid, and continued heating causes the pressure to rise. Compared to before the lid was added, the temperature _____________. increases   remains the same   decreases   H2O ANSWER: A. increases. The pressure increases and the volume now remains constant, so the temperature will increase.

Air is contained in a sealed tank of fixed volume Air is contained in a sealed tank of fixed volume. The air in the tank is initially at 20°C and 1 atm. It is then heated to 250°C. The energy of the air in the container ___________. Gas 20°C 1 atm Gas 250°C increases  decreases does not change   ANSWER: A. increases. Heating the air adds energy to the air in the container. Start After heating