Mass And Energy Balances

Question 1

How do you calculate the mol fraction and how is it denoted?

Answer 1

Mol fraction = mol of particular substance/total moles present in system.
x - liquid phase mol fraction
y - vapour/gas phase mol fraction
z - mixed system

Question 2

How do you calculate mass fraction and how is it denoted?

Answer 2

Mass fraction = mass of component/total mass in system.

Mass fraction is denoted by w

Question 3

How do you calculate specific gravity/relative density?

Answer 3

Density of substance at stated T and P/density of reference substance at same stated conditions

Question 4

What is Dalton’s law of partial pressures?

Answer 4

The total pressure of a gas equals the sum of pressures exerted by the individual molecules of each component gas were they to occupy the volume of the system at the same temperature as mixture

Question 5

What is the partial pressure of a component in a gas mixture?

Answer 5

The pressure that component exerts were it isolated in the system volume at the system temperature.

Question 6

What is the ideal gas law?

Answer 6

PV = nRT

Question 7

What is the equation for calculating partial pressure from mol fraction and total pressure?

Answer 7

PA = yA x P

Question 8

What is Amagat’s law of additive volumes?

Answer 8

The total volume of an ideal gas mixture is equal to the sum of volumes of the individual components if they were measured at the same temperature and pressure as the whole mixture.

Question 9

What can be said about the volume fraction and mol fraction for an ideal gas?

Answer 9

They are equivalent.

Question 10

Define a gas.

Answer 10

A gas is a vapour phase component that cannot be condensed by isothermal compression.

Question 11

Define a vapour.

Answer 11

A vapour is a gas phase component that can be condensed by isothermal compression.

Question 12

What is the saturated vapour pressure?

Answer 12

The pressure of a system when the vapour and liquid phases of a component are in equilibrium.

Question 13

What is the triple point?

Answer 13

The point when liquid, solid and vapour are in equilibrium. This is a unique property of the material, it can’t be predicted, must be measured.

Question 14

What is the critical point?

Answer 14

The point on a vapour/liquid equilibrium line where the physical properties of the vapour and the liquid phases become the same.

Question 15

What does cooling a gas isobarically mean?

Answer 15

Cooling a gas at constant pressure.

Question 16

What is sensible heat?

Answer 16

A heat exchange that can be sensed by a temperature change.

Question 17

What is latent heat?

Answer 17

An energy change that doesn’t cause a change in temperature but causes a change in phase.

Question 18

How do you calculate the mol of a substance in mol and kmol?

Answer 18

In mol, it’s mass in grams/RMM

In kmol, it’s mass in kg/RMM

Question 19

When does a liquid boil?

Answer 19

When the system pressure equals SVP

Question 20

What is Raoult’s law?

Answer 20

At equilibrium for an ideal system the partial pressure of a vapour phase component equals the SVP of that component at system temperature multiplied by the mol fraction of that component in liquid phase.

Question 21

What is Cv?

Answer 21

The specific heat capacity at constant volume, and it’s the energy required to increase the temperature of a unit mass of material by 1 kelvin at constant volume.

Question 22

How can internal energy change be calculated using Cv?

Answer 22

du = CvdT so the integral of du equals the integral of CvdT so u2-u1 = integral from T2 to T1 CvdT
Then multiply the change in energy by the number of moles being heated.

Question 23

What is Cp?

Answer 23

The specific heat capacity at constant pressure, and it’s the energy required to increase the temperature of a unit mass of material by 1 kelvin at constant pressure.

Question 24

How can the change in enthalpy be calculated from Cp?

Answer 24

dh = CpdT so the integral of dh from h2 to h1 equals the integral from T2 to T1 of CpdT
so h2-h1 = the integral from T2 to T1 of CpdT.
Then multiply the change in enthalpy by the number of moles being heated.

Question 25

How do you find the Cp of a mixture?

Answer 25

The sum of each components Cp value multiplied by the components mol fraction, if units are on a molar basis (J/mol K) or if units are on a mass basis (J/mol K) then multiply each Cp value by the mass fraction of component.

Question 26

How would you calculate the enthalpy change of a reaction of the form A + B ---> C + D?

Answer 26

The sum of the enthalpy of formation of the products minus the sum of the enthalpy of formation of the reactants.

Question 27

What is the standard enthalpy change of formation?

Answer 27

Enthalpy change when a molecule is made from it's constituent elements at 298 K and 1 atm.

Question 28

What is Hess's law of heat summation?

Answer 28

The enthalpy change of any reaction may be expressed as the sum of the enthalpy changes of a series of reactions in to which the overall reaction may be formally divided.

Question 29

If a reaction is carried out at above 298 K, how can we determine the change in enthalpy of reaction?

Answer 29

Enthalpy received by cooling reactants + change in enthalpy of reaction at 298 K + enthalpy required to heat products from 298 back to reaction temperature. This can also be written as the enthalpy change of reaction at 298 K + the integral from reaction temperature to 298 of (Sum of each components Cp value multiplied by number of moles of component of the products minus the sum of each components Cp value multiplied by number of moles of component of the reactants)dT

Question 30

What's the equation for the equilibrium constant, Kp, of a reaction of the form aA + bB ---> cC + dD?

Answer 30

Kp = (PC^c x PD^d)/(PA^a x PB^b) where PC, PD, PA, and PB are the partial pressure of components and a, b, c, and d are stochiometric coefficients.

Question 31

What is the phase rule used for and what is the equation for it?

Answer 31

The phase rule helps you define how many variables the user needs to set to fully define the system, it refers only to a system at equilibrium. F = C - Ph + 2 where F is the number of degrees of freedom that the user can define to fully specify the system, C is the number of components present in the system, and Ph is the number of phases that can exist, i.e. gas, liquid or solution. All the properties used must be intensive - not depend on quantities of materials present.

Question 32

What is accumulation?

Answer 32

Input - output + generation - consumption It occurs when the inventory of a particular species within system increases or decreases during the time interval under consideration.

Question 33

What is stream flow?

Answer 33

Flow of material in the stream (mol/unit time or mass/unit time)

Question 34

How would you calculate the flow of a component into a system?

Answer 34

Multiply the total molar flow by the mol fraction of that component. Or multiply the total mass flow by mass fraction.

Question 35

If using molecular balances for reacting systems, how do you calculate the mass flow of an atomic species in a stream?

Answer 35

The sum of Wip x P x Fzi | Where Wip is the mass fraction of molecule i in stream P, and Fzi is the fraction of atoms z in molecule i

Question 36

What is recycle and why recycle?

Answer 36

It is return of part or whole of process stream to upstream process unit. It's to save energy or to maximise usage of reactants.

Question 37

What is a bypass?

Answer 37

The direction of part or whole of the process stream round rather than through a process unit and the recombination of the streams downstream of the unit.

Question 38

What is purge?

Answer 38

The continuous part or complete removal from either a unit or process stream of an undesirable component. The purge consists of the component itself or the component in association with other constituents.

Question 39

What is the gross calorific value?

Answer 39

The energy released per unit mass of fuel when any water released appears as liquid. gross = net + mhfg where m is the mass of water per unit mass of fuel and hfg is the enthalpy of phase change for water at datum conditions.

Question 40

What is the net calorific value?

Answer 40

The energy released per unit mass of fuel or per unit volume of gaseous fuel when any water produced appears as water vapour.

Question 41

For combustion, we use excess air. How do we calculate the fraction of excess air?

Answer 41

(Actual amount fed - amount for complete combustion)/(amount for complete combustion)

Question 42

What is work?

Answer 42

The energy transfer between the system and surroundings due to a vector force acting through a vector displacement. W = integral of FdL

Question 43

How do you calculate kinetic energy and potential energy?

Answer 43

``` KE = 0.5mv^2 PE = mgh ```

Question 44

What is internal energy?

Answer 44

Macroscopic measure of the molecular and sub-atomic energy in system - it is not measurable directly. There is no unified zero point.

Question 45

What is heat of solution?

Answer 45

Energy change when solvated is dissolved in solvent to form a solution.