A catalyst increases the reaction rate by lowering the activation energy, but it does not change itself before and after the reaction, nor does it change the position of equilibrium.
Basic Knowledge
A catalyst provides a new reaction pathway by temporarily bonding with reactants, thereby lowering Ea. A positive catalyst accelerates a reaction, while a negative catalyst (inhibitor) slows it down.
Classification: ① homogeneous catalysts(present in the same phase in a liquid phase), ② heterogeneous catalysts(solid catalysts; gaseous or liquid reactants react on the surface), ③ enzymes(biological catalysts, which are proteins).
Representative heterogeneous catalysts: Fe(Haber process), Pt(automobile exhaust catalyst), Ni(hardening of margarine), and V2O5(SO3 production in the Contact process).
Catalyst（A substance that changes the reaction rate but is not consumed）
Homogeneous catalyst（Such as aqueous H2SO4, in the same phase）
Heterogeneous catalyst（Such as platinum mesh and vanadium oxide）
Enzyme（A highly selective protein catalyst）
Catalyst poison（A substance that destroys catalytic activity, such as S or Pb）
Three-way catalyst（Simultaneously treats Pt/Pd/Rh, NOx, CO, and HC）
Further Exploration (Principles and Applications)
A catalyst does not change the position of equilibrium (because it accelerates both the forward and reverse reactions by the same factor), but its essential effect is to shorten the time required to reach equilibrium. The Haber–Bosch process(N2+3H2→2NH3) was industrialized using an Fe catalyst and supported the increase in food production in the 20th century.
Enzymes specifically recognize substrates through the three-dimensional structure of their active sites and dramatically lower Ea. A major difference from inorganic catalysts is that enzymes denature and lose activity when conditions move away from their optimum temperature and pH.
- Catalysts lower Ea
- Catalysts do not change equilibrium
- Three classifications: homogeneous, heterogeneous, and enzymes
- Haber process=Fe catalyst
- Contact process=V2O5 catalyst
- Three-way catalyst=automobile exhaust purification
- Enzymes have optimum temperature and pH
Points to Note (Common Confusions and Frequent Mistakes)
① A catalyst changes the reaction rate but not the yield(equilibrium). ② It also does not change the reaction enthalpy ΔH. ③ Because the amount of a catalyst does not change before and after the reaction, it does not appear in the stoichiometric equation. ④ Enzymes denature at high temperatures and therefore lose activity above 60℃.
Practice
- Explain how a catalyst increases the reaction rate using an energy diagram.
- What catalyst is used in the Haber–Bosch process?
- Explain why a catalyst does not change the equilibrium constant.