In a reaction coordinate diagram, if the products are at a lower energy level than the reactants, which statements are true about ΔH and the reaction type?

In a reaction coordinate diagram, if the products are at a lower energy level than the reactants, which statements are true about ΔH and the reaction type?

• A. ΔH is positive; the reaction is endothermic
• B. ΔH is negative; the reaction is exothermic
• C. ΔH is zero; isothermal
• D. ΔH is negative; the reaction is endothermic

Answer: (B)

When a reaction coordinate diagram shows the products at a lower energy than the reactants, the enthalpy change (ΔH) is negative, because ΔH equals the energy of products minus the energy of reactants. A negative ΔH means heat is released to the surroundings, so the reaction is exothermic. If the products were higher in energy, ΔH would be positive and the reaction would be endothermic. A zero ΔH would imply no enthalpy change, which isn’t the case here, and isothermal refers to constant temperature, not a guarantee of zero ΔH.

When a reaction coordinate diagram shows the products at a lower energy than the reactants, the enthalpy change (ΔH) is negative, because ΔH equals the energy of products minus the energy of reactants. A negative ΔH means heat is released to the surroundings, so the reaction is exothermic. If the products were higher in energy, ΔH would be positive and the reaction would be endothermic. A zero ΔH would imply no enthalpy change, which isn’t the case here, and isothermal refers to constant temperature, not a guarantee of zero ΔH.