Be sure to label initial and final pressures and volumes. Compare the work for e

Question

Be sure to label initial and final pressures and volumes. Compare the work for each of these processes (the sign and magnitude, i.e., absolute value). Predict whether the values of q, w, delta U, and delta H are positive, zero, or negative for each of the following processes: (a) reversible isothermal expansion of an ideal gas, (b) reversible adiabatic expansion of an ideal gas, (c) an ideal gas expands adiabatically against a vacuum, (d) for the reaction: 2K(s) + 2H_2O(l) rightarrow 2KOH(ag) + H_2(g). One mole of an ideal monatomic gas (C_v = 3R/2) initially at 300 K is expanded from an initial pressure

Explanation / Answer

2. a. Reversible isothermal expansion of ideal gas

w ( work done by gas) = +ve (as volume increases)

dU = 0 ( as isothermal process, no temperature change)

q (heat supplied to gas) = +ve ( as q = w + dU)

dH = 0 ( as dU is 0 and it is an ideal gas)

b. reversible adiabatic expansion of ideal gas

w ( work done by gas) = +ve ( as gas expannds )

q ( heat supplied to gas) = 0 ( as adiabatic process)

dU = -ve ( as dU = q - w)

dH = -ve ( as dU is -ve )

c. ideal gas expands adiabatically against vaccum

w ( work done by gas) = 0 ( as gas expannds against vaccum )

q ( heat supplied to gas) = 0 ( as adiabatic process)

dU = 0 ( as dU = q - w)

dH = 0 ( as dU is 0 )

d. for 2K(s) + 2H2O(l) -> 2KOH(aq) + H2(g)

w ( work done by gas) = +ve ( as gas is created in products, and there is no gas in reactants, so volume increases )

q ( heat supplied to gas) = -ve ( as this is an exothermic process)

dU = +ve ( as temperature of the mixture increases in exothermic reaction)

dH = +ve ( as dU is +ve )

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