Question

are number of mole conserved in a chemical reaction?

Answer 1

Law of Conservation of Mass: Mass can neither be created nor destroyed (except in nuclear reactions).

Because of this, we can write equations called "mass balances" or "material balances". Any process being studied must satisfy balances on the total amount of material, on each chemical component, and on individual atomic species. Later in the course, we'll use the Law of Conservation of Energy (1st Law of Thermodynamics) to write similar balance equations for energy.

General Balance Equation

(Applies to mass, components, energy, etc.)



The system is any process or portion of a process chosen by the engineer for analysis. A system is said to be "open" if material flows across the system boundary during the interval of time being studied; "closed" if there are no flows in or out.

Accumulation is usually the rate of change of holdup within the system -- the change of material within the system. It may be positive (material is increasing), negative (material decreasing), or zero (steady state).

If the system does not change with time, it is said to be at steady state, and the net accumulation will be zero.

For our purposes, the generation and consumption terms are the consequence of chemical reaction. Note that while the total mass of a system and elements (or "atoms") are conserved, individual species are not.

If there is no chemical reaction, the production and consumption terms are typically zero.

Your bank statement can be thought of as a "dollar balance". Specifically, an "integral form balance" on dollars.

Forms of Balance Equations

Differential Form

All the terms are rates, so the balance describes an instant in time. Usually the best choice for a continuous process. When formulated for an instant in time, the result is an ordinary differential equation. This is what we used on steady flow systems in Thermo I.

Integral Form

(Also called cumulative form) Written using total amounts as terms, so it describes the overall effect. Often a good choice for batch processes. In Thermo I, we used these on both closed and uniform state problems.

In this class, most problems will be differential balances on steady state systems. Consequently, accumulation will usually be zero.

The flow terms can usually be easily identified from the problem statement. If the process is batch, these may be zero.

Production and consumption are almost never present when balancing total mass, and are only present in component balances when reaction occurs.

WARNING: Mass and atoms areconserved. Moles are conserved only when there is no reaction. Volume is NOT conserved. You may write balances on total mass, total moles, mass of a compound, moles of an atomic species, moles of a compound, mass of a species, etc.
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