If the value of ΔH is positive for a reaction, it means that the reaction is?

Step 1: Analyze the question The question asks what a positive enthalpy change (ΔH > 0) tells us about a reaction.

Step 2: The concept ΔH = H(products) − H(reactants). It is the heat change of a reaction at constant pressure.

• ΔH negative (−): The products have less energy than the reactants. Heat is released to the surroundings, so the reaction is exothermic.
• ΔH positive (+): The products have more energy than the reactants. Heat is absorbed from the surroundings, so the reaction is endothermic.

Step 3: Check each option

• A. Exothermic: This needs ΔH < 0, so it is wrong.
• B. Endothermic: Heat is absorbed, so ΔH > 0. This is correct.
• C. Spontaneous: Spontaneity depends on Gibbs free energy (ΔG = ΔH − TΔS), not on ΔH alone. A positive ΔH can still be spontaneous if TΔS is large enough, so this is wrong.
• D. Slow: The rate of a reaction depends on activation energy, not on ΔH, so this is wrong.

Step 4: Answer A positive ΔH means the reaction absorbs heat from its surroundings. It is endothermic, which is option B.

Tip: Think of the sign as what happens to the system's energy. Positive means the system gains heat (endo = in, like "entering"). Negative means heat exits (exo = out, like "exit"). Also remember that ΔH tells you about heat, not about speed (kinetics) or spontaneity (ΔG).