Calculations vs observations

Seite 1

Why Calculations Do Not Always Match Observations

Rememer: The principle of conservation of mass states that mass in a closed system is neither created nor destroyed by chemical reactions or physical transformations.

Even though no atoms are gained or lost in a chemical reaction, it is not always possible to obtain the calculated amount of a product because:

The reaction may not go to completion because it is reversible

Some of the product may be lost when it is separated from the reaction mixture

Some of the reactants may react in different ways to those which we expected

Seite 2

Non-enclosed systems

Gases

One of the important properties of gases is that they diffuse into all available space. This is because their molecules have a lot of kinetic energy, so they have a lot of translational freedom.

Non-enclosed system: A system where the gas is free to escape

Example: An open beaker or flask

Reaction that produces gas

There may be an apparent decrease in mass between reactants and products if this reaction is carried out in a non-enclosed system.

This is because some of the reactants have been turned into gas and have escaped the reaction vessel.

If we could weigh the gas produced, we would find that no mass is lost.

Reactions that involve heating of solids

There may be an apparent increase in mass between reactants and products if this reaction is carried out in a non-enclosed system.

This apparent gain is because some of the solid reactants have reacted with gases to form solid products.

Seite 3

Yield

The yield of a reaction is the actual mass of product obtained.

The theoretical yield is the mass of product we would obtain according to the balanced equation.

\( \text {Percentage yield = } \frac{\text{Actual mass of product obtained}}{\text{Maximum theoretical mass of product obtained}}\)