Chemical Reactions and Stoichiometry

Inquiry question: What happens in chemical reactions?

During laboratory investigations, we can use electronic balances (sometimes called "electronic scales") are used to measure the mass of solid substances. Typically, when we measure the mass of a sample, we press the button labelled "TARE" on the balance to exclude the mass of the glassware or "weight boat" before we add the substance.

To measure liquids, we can use a measuring cylinder, pipette, burette or a volumetric flask.

Mearing gases given off during an experiment requires the use of an inverted measuring cylinder in water, as shown below.

The law of conservation of mass states that during a chemical reaction, the total mass of the products must be equal to the total mass of the reactants, since mass cannot be created or destroyed during a chemical reaction, but is always conserved.

Mole Concept

Inquiry question: How are measurements made in chemistry?

The atomic masses of all atoms are based on a standard, the carbon-12 isotope. However, working with the small masses of atoms is difficult and a more convenient scale was developed. Instead we use Avogadro’s constant defines the exact number of atoms that are present in 12 g of carbon-12.

This is called a mole (n), and is a unit that allows us to reference the relationship of elements to their atomic mass.

For example, a mole of copper contains 6.022 x 10^23 atoms of copper, with a mass of 63.55 g.

Avogadro’s number is defined as: 

Concentration and Molarity

Inquiry question: How are chemicals in solutions measured?

The concentration of a solutions is a measure of the amount of solute that is dissolved in a fixed volume of solvent. We can describe solutions as concentrated or dilute. A more useful measure to us in Chemistry is molarity. This is a measure of the amount of moles of solute dissolved in solution and is given by the following formula. 

Molarity is calculated by the formula:

Gas Laws

Inquiry question: How does the Ideal Gas Law relate to all other Gas Laws?

The French chemist Joseph Gay-Lussac (1778-1850) experimented with gases and determined the volume ratio in which they could be combined. Gay-Lussac soon discovered that gases always combined in simple whole number ratios. 

Hydrogen gas + chlorine gas → hydrogen chloride gas 

This led to the formation of his gas law, stating that all gases involved in a chemical reaction can be expressed in a simple whole number ratio (at constant temperature and pressure). 

Boyle’s law states that the pressure of a gas at a constant temperature is inversely proportional to its volume.

Charles’s law states that the temperature of a gas at a constant pressure is directly proportional to its volume.

The kinetic energy of gas molecules depends on the temperature of the gas. This was called the ideal gas model. The features of this ideal gas are: 

• The gas molecules behave as point particles with zero atomic volume

• The gas molecules have no intermolecular attraction 

• The average distance between gas molecules is very great 

• The collision of the gas molecules is elastic

• There is a statistical distribution of velocities of the gas molecules. 

Ideal gases that follow these above features obey the ideal gas law which is expressed as shown below.

Real gases approximate the behavior of ideal gases at low pressure and moderate temperature. Real gases deviate from these features however because: 

• They have significant molecular volumes which becomes an issue at high pressures as molecules are closer together

• Real gases experience intermolecular forces of attraction and this becomes significant at low temperature and high pressure. 

Noble gases exhibit near ideal behavior, but still have some forces of intermolecular attraction that allow them to be liquefied with compression.