5.1.2—Construct/interpret a reaction pathway diagram

Read a reaction pathway diagram with energy or enthalpy on the vertical axis and reaction progress on the horizontal axis. The reactants and products are placed at their starting and ending energy levels for the reaction direction shown.

Find ΔH from the signed vertical difference between products and reactants: products lower than reactants gives a negative, exothermic ΔH; products higher gives a positive, endothermic ΔH. Reverse the reaction and the sign reverses.

The peak represents the activated complex. The vertical distance from the reactants to the peak is the forward activation energy; for the reverse direction, measure from the products to the same peak. A catalyst provides an alternative pathway with lower activation energy but does not change the reactant/product energy difference or ΔH.

Use an ordered read-off: identify axes and reaction direction, mark reactant and product levels, calculate the signed product-minus-reactant gap for ΔH, then measure the relevant reactant-or-product-to-peak gap for activation energy. Keep energy changes and reaction rate as separate conclusions.

Do not call the peak-to-product distance the forward activation energy, infer a faster reaction from ΔH alone, or claim that a catalyst changes equilibrium energy levels. Use only distances with clearly identified endpoints and direction.