Lecture 2 : Mass and energy balances

Question 1

What are the equations for law of conservation of mass?

Answer 1

Mass in - mass out = mass accumulation in system

thus mass flow in - mass flow out = rate of mass accumulation in system

Question 2

What are the equations for energy balance?

Answer 2

Energy in - Energy out = Energy accumulation in system

thus Energy flow in - Energy flow out = rate of Energy accumulation in system

Question 3

The energy accumulated in the system can be a positive or negative value. True or False?

Answer 3

True.

Energy accumulated > 0 : means the energy of products is less than energy of reactants, and that reactants lose energy to the surroundings, and is accumulated in the system.

Energy accumulated < 0 : means the energy of products is more than energy of reactants, and that reactanats gains energy from the system (surroundings)

Question 4

What is the equation for calculating the amount of heat gained / lost (q) by a sample?

Answer 4

q = m.c.ΔT

Question 5

What is latent heat?

Answer 5

It is the amount of heat added or removed to a substance, which induces a phase change, with temperature remaining constant.

Question 6

What is the equation to calculate the amount of heat needed to cause a phase change?

Answer 6

Q = m.Li

m = mass (kg) ; Li: latent heat required for a phase change per kg of substance (kJ/kg)