UPCAT SCI - CHEM. CHEMICAL REACTIONS

What is a chemical reaction?

A process where one or more substances are changed into new substances. Reactants are on the left of the arrow, products are on the right.

Law of conservation of matter

Matter can neither be created nor destroyed. Mass of reactants = mass of products. Proposed by Antoine Lavoisier.

Combination/Synthesis reaction

Two or more elements or compounds combine to form ONE single product. A + B → AB

Decomposition reaction

ONE compound breaks down into two or more products. AB → A + B. Opposite of combination.

Single displacement reaction

One element replaces another element in a compound. A + BC → AC + B. More reactive element displaces the less reactive one.

Double displacement/Metathesis reaction

Two reactants exchange atoms/ions with each other. AB + CD → AD + CB.

Redox reaction

Involves transfer of electrons between species. Always in pairs — one species oxidized, one reduced.

LEORA

Loss of Electrons = Oxidation = Reducing Agent

GEROA

Gain of Electrons = Reduction = Oxidizing Agent

ROD

Reduction = Oxidation number Decreases. Oxidation = oxidation number increases.

Disproportionation reaction

Special redox where ONE element undergoes BOTH oxidation AND reduction simultaneously. Same starting ox# → two different ox# in products.

Comproportionation reaction

Opposite of disproportionation. Same element with TWO different starting ox# → merge into ONE same ox# in product.

Combustion reaction

Flammable compound reacts with O2 from atmosphere producing heat.

Complete combustion

Oxygen-rich environment. Products: CO2 + H2O + heat only (for C and H compounds).

Incomplete combustion

Oxygen-deficient environment. Products: CO + C (soot) + CO2 + H2O + heat. CO is colorless, odorless, tasteless — causes CO poisoning.

Combustion of N-containing compounds

Produces NO2 (nitrogen dioxide) in addition to CO2 and H2O.

Combustion of S-containing compounds

Produces SO2 (sulfur dioxide) in addition to CO2 and H2O. SO2 causes acid rain.

Combustion of halogenated compounds

Produces HX (where X = F, Cl, Br, I) in addition to CO2 and H2O.

Balancing combustion tip

Balance in order: C first → H next → O last. O is always balanced last by adjusting O2 coefficient.

Precipitation reaction

Two aqueous solutions react to form an insoluble solid product called a precipitate.

Neutralization reaction

Acid + Base → Salt + Water. The acid's H+ and base's OH- combine to form water.

What is a limiting reactant?

Reactant that runs out first. Controls how much product forms. Reaction stops when it is depleted.

What is an excess reactant?

Reactant that remains unreacted after limiting reactant is used up. Has more moles than needed per the mole ratio.

What is a mole?

Counting unit = 6.022x10^23 particles. 1 mole of any element = its atomic mass in grams.

Moles formula

moles = mass (g) / molar mass (g/mol)

How to find limiting reactant

1. Convert grams to moles. 2. Use mole ratio from balanced equation to check which runs out first. 3. Whichever needs more than available = limiting.

How to find mass of product

1. Find limiting reactant. 2. Use mole ratio from equation to find moles of product. 3. Multiply by molar mass to get grams.