Reactions, equations and the mole

Atoms are rearranged in a chemical reaction, never created or destroyed. Every balanced equation is a statement of that fact: the same atoms appear on both sides in the same numbers, only differently connected.

Balance by adjusting coefficients, the numbers in front of formulas, never subscripts inside them. Changing a subscript changes the substance itself, which is the single most common beginner error.

• Synthesis: two or more substances combine into one.
• Decomposition: one substance breaks into several, often driven by heat.
• Single displacement: a more reactive element takes the place of a less reactive one in a compound.
• Double displacement: two compounds exchange partners, frequently producing an insoluble precipitate.
• Combustion: a fuel reacts with oxygen, releasing energy; complete combustion of a hydrocarbon gives carbon dioxide and water.
• Neutralisation: an acid and a base produce a salt and water.
• Redox: electrons are transferred, one species oxidised and another reduced.

Atoms are far too small to count individually, so chemists count in moles. One mole is Avogadro's number of particles, about 6.02 times ten to the twenty third, chosen so that one mole of a substance has a mass in grams equal to its relative formula mass.

That definition is what turns a balanced equation into a calculation. Coefficients give ratios in moles, so converting mass to moles, applying the ratio, and converting back to mass answers almost every quantitative question in introductory chemistry.

When reactants are not supplied in the exact ratio, one runs out first and caps the product. Find it by converting each reactant to moles and dividing by its coefficient; the smallest result limits the reaction. Everything else is in excess.

Theoretical yield is what the calculation predicts. Actual yield is what you collect, and it is almost always less, due to incomplete reactions, competing side reactions and losses during transfer and purification. Percentage yield is actual divided by theoretical, times one hundred.

Breaking bonds requires energy; forming bonds releases it. If forming releases more than breaking consumed, the reaction is exothermic and the surroundings warm. If not, it is endothermic and the surroundings cool.

Reaction rate is a separate question from energy change. Rate rises with higher concentration, higher temperature, greater surface area and the presence of a catalyst. Collision theory explains all four: reactions need collisions that are both frequent enough and energetic enough, and a catalyst works by providing a route with a lower activation energy, without being consumed.