Chemical Reactions and the Mole Concept

Physical and Chemical Changes in Matter

A physical change is a transformation that does not vary the nature of matter. Before and after the change, the matter is represented by the same chemical formula. A chemical change is a transformation in which the nature of matter varies. Before the change, the substance is represented by one chemical formula, and afterwards, by a different formula.

The Collision Theory of Reactions

Collision theory: Chemical reactions occur when molecules of reactants collide and break. Atoms that have been released are rearranged to form new molecules.

Measurement of Mass: The Mole

The mole: A mole of atoms is the amount of a chemical element that is equivalent to its atomic mass in grams. In the SI system, the mole is the unit for the amount of substance.

One mole of a substance: The formula of a compound indicates each element's atoms forming the molecule, or the equivalent of a molecule if it is a crystal. The molecular mass of a compound is obtained by summing the atomic mass of each of its elements.

A mole of a substance is an amount equivalent to its molecular weight in grams. One mole of a substance contains 6.022 × 10²³ (N_A) molecules of the substance, or its equivalent in the case of crystals.

Chemical Equations and Combustion

The chemical equation is a symbolic representation of a chemical reaction.

Combustion: These are chemical reactions in which a fuel combines with molecular oxygen to produce carbon dioxide, water, and energy (CO₂ + H₂O). They are exothermic because they release heat.

Observable Evidence of Chemical Reactions

• Detachment of light: When a magnesium (Mg) ribbon burns, it reacts with oxygen (O₂) in the air and turns into a powdery substance called magnesium oxide (MgO).
• Heat release: If you add a solution of hydrochloric acid (HCl) to a solution of sodium hydroxide (NaOH), the temperature on the thermometer rises.
• Formation of gas: When you add a strong acid to certain metals, hydrogen gas (H₂) is released.
• Formation of white smoke: Bring a bottle containing a concentrated ammonia solution near a bottle containing a concentrated hydrochloric acid solution. Uncover the two bottles and you will see white smoke in the air; this is the ammonium chloride that was formed.
• Formation of a solid: To a clear solution of lead nitrate, add another transparent solution of potassium iodide. A yellow solid is formed, which settles to the bottom over time: it is lead iodide.
• Dissolution of a solid: Marble is calcium carbonate (CaCO₃). If you pour acid on marble, bubbling occurs and the solid disappears over time. In this reaction, the solid dissolves and a gas, carbon dioxide (CO₂), is evolved.