Q: What are the five types of chemical reactions, and how do you recognize each one from its reactants?
A: The five types are Synthesis (A+B→AB, multiple reactants combine into one product), Decomposition (AB→A+B, one compound breaks into multiple products, often needing an energy input), Single Replacement (A+BC→AC+B, a free element displaces another element from a compound — governed by the activity series), Double Replacement (AB+CD→AD+CB, ions of two compounds trade partners, often forming a precipitate, gas, or water), and Combustion (a hydrocarbon fuel reacts with O2 to form CO2 and H2O — O2 as a reactant is the clearest identifying signal).
Q: Explain OIL RIG and why oxidation and reduction always occur together in the same reaction.
A: OIL RIG stands for Oxidation Is Loss (of electrons, oxidation number increases) and Reduction Is Gain (of electrons, oxidation number decreases). They always occur together because electrons lost by the oxidized substance must be gained by some other substance in the same reaction — one cannot happen without the other.
Q: Walk through the COACH method for balancing a chemical equation.
A: COACH: Count the atoms of each element on both sides; Only change Coefficients, never subscripts (since subscripts define a compound's identity); remember Atoms must be Conserved (same count on both sides); Check both sides once finished; balance Hydrogen and Oxygen last, since they tend to appear in the most compounds.
Q: State Le Chatelier's principle and explain how increasing pressure affects a gas-phase equilibrium.
A: Le Chatelier's principle states that a system at equilibrium, when stressed by an external change, shifts in the direction that partially relieves that stress. Increasing pressure (by decreasing volume) shifts a gas-phase equilibrium toward whichever side has fewer total moles of gas, since that side takes up less volume and partially relieves the increased pressure; if gas moles are equal on both sides, pressure has no effect.
Q: What four factors increase reaction rate (CATS), and which one works by a fundamentally different mechanism than the others?
A: CATS: Concentration and surface Area both increase collision frequency; Temperature increases both collision frequency and the fraction of collisions with enough energy to react; a catalyst works differently — it lowers the activation energy itself by providing an alternative reaction pathway, without changing temperature or collision frequency at all.