[nicsam]

Sorry if this is a bit obvious. I'm just wondering how to interpret enthalpy chnage of reaction so, if i have a reaction of A + B = C and the enthalpy change of reaction is 10 kj mol and then i replace  one reactant so A + D = C and the enthalpy changes to 100 kj mol. i can say that they are both endothermic in the forward direction.but can you say that one reaction is less likely? or more likely? if they are both thermodynamically unfavourable from an endothermic perspective how does it matter by the amount they differ?

[Corribus]

First, you have to define what you mean by "less likely". Less likely to what?  Strictly speaking, the enthalpy change only determines how much heat is absorbed or evolved during a reaction. Per unit amount of starting materials, a more endothermic reaction will absorb more heat from the surroundings than a less endothermic reaction. From enthalpies alone, that's really all you can conclude. If you want to start getting an idea of whether a reaction will spontaneously produce more or less reactants, you also have to introduce the statistical concept of entropy, which, when combined with enthalpy, results in a quantity called the Gibbs energy. This is sort of a measure of the potential energy of a chemical system. The Gibbs energy change for a hypothetical reaction gives you some sense of whether a reaction is likely to proceed in the forward direction spontaneously at a given temperature. The entropy changes for your two reactions may be completely different, and it's impossible to say without additional information which of the two reactions will be more spontaneous. All things being equal, a more endothermic reaction will tend to be less spontaneous, but if the more endothermic reaction also has a huge positive entropy change, this can compensate for the enthalpic unfavorability and still result in a spontaneous reaction.

Even beyond all that, a more thermodynamically favorable reaction is not necessarily "more likely" to happen, because thermodynamic favorability is only loosely related to reaction kinetics. A spontaneous reaction will occur... well, spontaneously, but this doesn't say anything about how long it will take to reach equilibrium. It is tempting to conclude that a more favorable reaction will also happen faster, but this isn't always the case. Combustion of gasoline is an extremely favorable reaction (thermodynamically) but by itself is so kinetically unfavorable that it won't happen without an input energy (spark). A lot also depends on the conditions under which a reaction takes place. Evaporation of water is spontaneous at many temperatures, but you know it happens faster for some types of containers than others (e.g., a shallow dish versus a deep cup), even if the environmental conditions are the same.