An energy balance on a steady flow process with no shaft work reduces to
Answer: A
Flow work is folded into enthalpy, and with no shaft work and negligible velocity terms nothing else remains. This is the form used for nearly every heater, cooler and exchanger calculation. Kinetic and potential terms are dropped as negligible.
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State whether the following is true or false.
Enthalpy is an absolute quantity that can be measured directly.
Answer: B
Correct answer: False
Only enthalpy *changes* are measurable; the absolute value depends on an arbitrary reference state, so tables quote enthalpy relative to a chosen datum such as liquid water at the triple point. That is not a limitation in practice, because every energy balance involves a difference and the datum cancels - provided the same datum is used throughout.
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Assertion (A): A reference state must be chosen before enthalpies can be tabulated.
Reason (R): Only enthalpy differences are physically meaningful.
Answer: C
Both are true and the reason gives the necessity: steam tables take liquid water at the triple point as zero, and any other choice shifts every value by a constant that cancels in every difference. The choice is a convention rather than a physical fact.
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Match each kind of enthalpy change with what causes it.
List I
A. Sensible heat
B. Latent heat
C. Heat of reaction
D. Heat of mixing
List II
1. a chemical change at fixed temperature
2. a change of temperature within one phase
3. combining two substances into a solution
4. a change of phase at constant temperature
Answer: D
Any real enthalpy change is a sum of these contributions along a chosen path. Splitting a problem into such steps is exactly how a hypothetical path is constructed for calculation. Each step draws on a different kind of tabulated data.
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A hypothetical path is used in energy balance calculations because
Answer: A
Correct answer: enthalpy is a state function of the stream
Because enthalpy depends only on the end states and not on how the change occurred, any convenient path between them gives the same answer. That licenses the standard construction: bring the reactants to 25 degrees, react them there where the tabulated data apply, and heat the products to the outlet temperature - a path nothing in the real process follows.
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How much heat is needed to raise 10 kilograms of a liquid of heat capacity 2.5 kilojoules per kilogram kelvin from 20 to 60 degrees Celsius?
Answer: B
Correct answer: 1000 kilojoules
\[ Q = mc_p\Delta T = 10 \times 2.5 \times (60-20) = 10 \times 2.5 \times 40 = 1000\ \mathrm{kJ}. \] The temperature difference may be taken in Celsius because a kelvin and a Celsius degree are the same size - only a *difference* permits that, and an absolute temperature must be used wherever the value itself enters.
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Heat capacity data are usually correlated as
Answer: C
Correct answer: a polynomial in temperature
Heat capacity varies with temperature, and the standard correlations express it as \( c_p = a + bT + cT^2 + dT^3 \) with tabulated coefficients for each substance. Integrating that polynomial between two temperatures gives the enthalpy change; using a single constant value is an approximation acceptable only over a narrow range.
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Consider the following statements about heat capacities.
1. The value at constant pressure exceeds that at constant volume for a gas.
2. For liquids and solids the two are nearly equal.
3. The difference is negligible for gases as well.
Answer: D
The first two are correct. The third is wrong, since for an ideal gas the difference is the gas constant, which is far from negligible against typical molar heat capacities. For a diatomic gas it is about a quarter of the value.
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Steam condenses at 100 degrees Celsius with a latent heat of 2257 kilojoules per kilogram. How much heat is released by condensing 2 kilograms?
Answer: A
Correct answer: 4514 kilojoules
\[ 2 \times 2257 = 4514\ \mathrm{kJ}. \] The latent heat is per kilogram, so it multiplies by the mass. Note the magnitude against the previous item: condensing two kilograms of steam releases four and a half times the heat needed to warm ten kilograms of liquid through forty degrees, which is why latent heat dominates any balance in which a phase change occurs.
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Assertion (A): Latent heat is much larger than the sensible heat over a modest temperature range for water.
Reason (R): Breaking the intermolecular bonds of the liquid takes far more energy than raising its temperature by a few tens of degrees.
Answer: B
Both are true and the reason gives the comparison: the latent heat of water is about five hundred and forty times its heat capacity per degree, which is why steam is such an effective heating medium. Condensing steam delivers heat at constant temperature.
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