Catalyst
A catalyst is a substance that speeds up a chemical reaction without being used up, by providing an easier path with lower activation energy.
Definition
A catalyst is a substance that speeds up a chemical reaction without being used up, so it comes out unchanged at the end and can be reused. It works by lowering the activation energy, the energy barrier reactants must cross, providing an alternative reaction mechanism that increases the rate constant according to the Arrhenius equation, $k = Ae^{-E_a/RT}$, without altering the equilibrium constant or $\Delta G$. Homogeneous catalysts share a phase with the reactants, while heterogeneous catalysts, typically solid surfaces, adsorb reactants, weaken their bonds, and then release products; enzymes are biological catalysts that achieve enormous rate enhancements by binding the transition state of a reaction more tightly than the substrate itself.
Example
Dropping a piece of raw potato into hydrogen peroxide makes it fizz quickly because an enzyme in the potato catalyzes its slow decomposition into water and oxygen. Manganese dioxide catalyzes the same reaction, $2H_2O_2 \rightarrow 2H_2O + O_2$, turning a process that takes days into one that finishes in seconds, and the $MnO_2$ can be filtered out and reused afterward. Catalase performs this job in cells at about $10^7$ molecules per second per enzyme, roughly $10^8$ times faster than the uncatalyzed reaction, while a catalytic converter uses platinum, palladium, and rhodium surfaces to oxidize $CO$ to $CO_2$ and reduce $NO_x$ to $N_2$ in a fraction of a second.
Key Insight
A catalyst is like a ramp instead of a wall: the reaction reaches the same destination, just by an easier path, and it can never make a reaction happen that would not happen anyway, only change how fast it gets there. Because a catalyst lowers the barrier for the reverse reaction by the same amount, it speeds up the approach to equilibrium from either direction, which is why the Haber process catalyst that makes ammonia faster would also decompose ammonia faster; only the reaction conditions decide which direction wins.