Catalysts are substances that speed up chemical reactions without being used up in the overall reaction. They matter because many useful reactions would be too slow at normal temperatures and pressures without them. Catalysts are essential in industry, biology, pollution control, and energy technology.
A catalyst works by giving reacting particles a more efficient way to turn into products.
The key idea is that a catalyst provides an alternative reaction pathway with a lower activation energy. On an energy profile diagram, the catalyzed pathway has a smaller energy peak, but the reactants and products start and end at the same energy levels as before. This means a catalyst does not change the overall energy difference, ΔH, or the equilibrium position of a reversible reaction.
Catalysts can be homogeneous, in the same phase as the reactants, heterogeneous, in a different phase, or biological catalysts called enzymes.
Understanding Chemistry: Catalysts
At the particle level, a reaction only happens when particles collide in a suitable way. They need enough energy to break or weaken old bonds, and their atoms must be arranged so new bonds can form. A catalyst helps with these difficult steps.
In a heterogeneous reaction, molecules often attach to a solid surface at special active sites. Attachment can bring reactants close together, hold them in a useful orientation, or stretch a bond so it breaks more easily. The products then leave the surface, freeing the active site for more reactant particles.
The surface of a solid catalyst matters much more than its total mass. A finely divided solid has a large surface area, so it exposes more active sites. This is why catalyst powders, porous pellets, and honeycomb structures are common.
In a car catalytic converter, exhaust gases flow over metals such as platinum, palladium, and rhodium. These metals help turn harmful carbon monoxide, nitrogen oxides, and unburned fuel molecules into less harmful gases.
The catalyst works best only within a suitable temperature range. A cold engine produces more pollution because the converter has not warmed up enough.
Enzymes show how precise biological catalysts can be. Each enzyme has an active site with a particular shape and chemical environment. A matching substrate binds there for a short time.
Weak attractions hold it in place, while parts of the enzyme may bend bonds or move charged particles. This makes formation of products more likely. Enzymes control reactions in digestion, respiration, photosynthesis, and DNA copying.
Their shape is important. High temperatures or very acidic or alkaline conditions can change an enzyme's shape. This is called denaturation, and the enzyme may then stop working.
A catalyst is not always unchanged at every moment. During the reaction, it can form temporary bonds or intermediate substances. It is regenerated by later steps, so it is present again at the end overall.
This helps explain why catalysts can still become less effective. Poisoning happens when another substance sticks strongly to active sites. Sulfur compounds can poison some metal catalysts.
Catalysts may also lose surface area when particles clump together, or become coated with unwanted material. When studying reaction rates, separate the effects of catalyst amount, surface area, concentration, temperature, and pressure. Each can change the observed rate, but they do so for different physical reasons.
Key Facts
- A catalyst speeds up a reaction without being consumed in the overall chemical change.
- Catalysts lower activation energy: Ea,catalyzed < Ea,uncatalyzed.
- A catalyst provides an alternative reaction pathway, not extra energy for the reaction.
- Catalysts do not change ΔH: ΔH = Hproducts - Hreactants.
- Catalysts do not change the equilibrium constant K, but they help equilibrium be reached faster.
- Arrhenius equation: k = Ae^(-Ea/RT), so a lower Ea usually gives a larger rate constant k.
Vocabulary
- Catalyst
- A substance that increases the rate of a chemical reaction without being permanently consumed.
- Activation energy
- The minimum energy particles need to reach the transition state and react.
- Reaction pathway
- The sequence of steps and energy changes that connects reactants to products.
- Heterogeneous catalyst
- A catalyst that is in a different physical phase from the reactants, often a solid surface reacting with gases or liquids.
- Enzyme
- A biological catalyst, usually a protein, that speeds up specific reactions in living organisms.
Common Mistakes to Avoid
- Saying a catalyst is used up is wrong because a catalyst is regenerated by the end of the overall reaction, even if it temporarily forms intermediates.
- Drawing the catalyzed products at a different energy is wrong because a catalyst lowers the activation energy but does not change ΔH.
- Thinking catalysts make more product at equilibrium is wrong because catalysts do not change the equilibrium constant, only the speed of reaching equilibrium.
- Confusing homogeneous and heterogeneous catalysis is wrong because homogeneous catalysts are in the same phase as the reactants, while heterogeneous catalysts are in a different phase.
Practice Questions
- 1 An uncatalyzed reaction has an activation energy of 80 kJ/mol. A catalyst lowers the activation energy to 45 kJ/mol. By how many kJ/mol is the activation energy reduced?
- 2 For a reaction, Hreactants = 120 kJ and Hproducts = 70 kJ. Calculate ΔH. If a catalyst is added, what is the new ΔH?
- 3 A powdered solid catalyst makes a gas reaction faster than a single solid lump of the same catalyst. Explain why the powder is more effective using collision and surface area ideas.