How to tell if reactions will be spontaneous?

How do you tell if a reaction will be spontaneous or not? I have a test today, would really appreciate an explanation! Thanks

Answer:
Look at the change in Gibbs free energy of the reaction if it is below 0, then the reaction is likely to occur (spontaneous). If change in Gibbs free energy of the reaction is 0 then the reaction is in equilibrium and if change in Gibbs free energy of the reaction is greater than 0 the reaction is unlikely to occur (non spontaneous).
If a man forgets his wife's birthday, you can bet he'll get a spontaneous reaction. Meaning he doesn't have to add any more energy to the situation for it to happen.
The guy above is right. But how do you know the value for the Gibbs Free Energy (dG)?
dG=dH-TdS, where dG is the gibbs free energy, dH is the enthalpy, T is the temperature in K, and dS is the entropy change.

If dG<0, spontaneous
dG>0, non spontaneous
dG=0, system is in equilibrium
////////////////////////////// GIBBS FREE ENERGY //////////////////////////////...

These sorts of questions use a "standard heats of formation" table and if the sum of the heats is negative, the reaction is spontaneous.

/////////// example (iron smelting)

2Fe2O3 + 3C -> 4Fe + 3CO2

What is happening above is that the iron ore is being "taken apart" and carbon dioxide is being "put together". One other thing to take into account is that pure elements (carbon and iron) have zero energy (they are neither created nor distroyed), so they drop out of the equation.

The standard "heats of formation" table lists the energy required to put things together:

C + O2 -> CO2 (energy is -94)
Fe + O2 -> Fe2O3 (energy is -196)

Note however that the iron ore is being taken apart, not put together. In other words the reaction is reversed, so reverse the sign and one has:

C + O2 -> CO2 (energy is -94)
Fe2O3 -> Fe + O2 (energy is +196)

The iron ore equation is not balanced, but it dosen't matter. This is because we are only interested in how much energy is involved in the Fe2O3 itself. Where our interest really lies in in the initial equation:

2Fe2O3 + 3C -> 4Fe + 3CO2

which states 2 molecules of ore are being taken apart and three molecules of carbon dioxide are being put together. Therefore, the time has come to sum everything up:

Total energy = 2*(+196) + 3*(-94) = 392 - 282 = +110

The answer is positive, so this means the reaction is not spontaneous. In other words, heat has to be added to the reaction to get it to work which incidently, is why iron ore and carbon are heated in a big furnace.

FYI: "activation energy"

Everybody knows lumps of coal (carbon) do not spontaneously ignite if they are exposed to air. The "heat of reaction" is negative, but all this really means is that once the coal starts burning, it will continue to do so. The heat added to get it to burn is what might be called the activation energy. In terms of something known as "gibbs free energy" it is the "entropy" term. Entropy is the amount of heat that has to be added to a system before any useful work can be extracted from it. Gibbs energy was used to calculate the efficiency of steam locomotives, and the entropy of a locomotive is the amount of heat needed to bring the water in the engine to its boiling point. Once this is achieved, any additional heat is then easily turned into mechanical energy.

Last Note: The proof that iron smelting is not spontaneous is the fact that heat has to be added continiously to the blast furnace or the smelter will shut down. This is done by forcing oxygen into the hot furnace. The carbon (coke) is hot enough such that its activation energy (entropy) is overcome and it can burn spontaneously, forming heat. The heat then drives the iron ore to react with additional carbon to form liquid iron metal. The overall reaction is such that the spontaneous combustion of coke drives the non-spontaneous formation of iron from iron ore.

hope this helps

Roger L. Sieloff
gibbs (del G) should be negative

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