Question

For this question, the "entropy term" refers to "-TΔS". Addition reactions are generally favorable at low temperatures because ________.

Answer 1

Lowering the temperature typically reduces the significance of the decrease in entropy. That makes the Gibbs Free energy of the reaction more negative. As a result, the reaction becomes more favorable overall.

Step-by-step explanation:

In an addition reaction there's a decrease in the number of particles. Consider the hydrogenation of ethene as an example.

`\rm H_2C\text{=}CH_2\; (g) + H_2\; (g) \stackrel{\text{Ni}^\ast}{\to} H_3C\text{-}CH_3\; (g)`.

When
`\rm H_2` is added to
`\rm H_2C\text{=}CH_2` (ethene) under heat and with the presence of a catalyst,
`\rm H_3C\text{-}CH3` (ethane) would be produced.

Note that on the left-hand side of the equation, there are two gaseous molecules. However, on the right-hand side there's only one gaseous molecule. That's a significant decrease in entropy. In other words,
`\Delta S < 0`.

The equation for the change in Gibbs Free Energy for a particular reaction is:

`\Delta G = \Delta H + (\underbrace{- T \, \Delta S}_{\text{entropy}\atop \text{term}})`.

For a particular reaction, the more negative
`\Delta G` is, the more spontaneous ("favorable") the reaction would be.

Since typically
`\Delta S < 0` for addition reactions, the "entropy term" of it would be positive. That's not very helpful if the reaction needs to be favorable.

`T` (absolute temperature) is always nonnegative. However, lowering the temperature could help bring the value of