Final answer:
A reaction has a positive ΔG when the products have a higher free energy than the reactants, indicating a non-spontaneous process that requires energy input to proceed.
Step-by-step explanation:
When asking which reactions have a positive Δrxn, we are looking for reactions where the free energy change (ΔG) is positive. This typically means that the reaction is non-spontaneous under standard conditions. In chemistry, the change in Gibbs free energy (ΔG) can be determined using the expression ΔG = ΔGproducts - ΔGreactants, where ΔG is the difference between the free energies of the products and the reactants.
The given hypothetical reaction A(g) + 2B(g) → 2C(g) would have a positive ΔG if the free energy of the products (2C) is greater than the free energy of the reactants (A and 2B). In practical terms, this could be imagined as needing to input energy to make the reaction proceed. For example, endothermic reactions where the enthalpy change (ΔH) is positive and the entropy change (ΔS) does not provide enough 'compensation' in the form of increased disorder, can result in a positive ΔG at a given temperature according to the equation ΔG = ΔH - TΔS.
Understanding the concept of Gibbs free energy change helps predict whether a chemical reaction will occur spontaneously. Reactions with positive ΔG values are important for various applications where external energy inputs are required, such as in electrochemical cells or in biological systems where reactions are driven by enzymes or other energy-coupling mechanisms.