[foldingstars45]

Okay, so the spec says;

"understand that ∆H, whilst important, is not sufficient to
explain spontaneous change (e.g. spontaneous endothermic
reactions)"

And I understand that enthalpy change means that because energy must be put IN to an endothermic reaction, it wouldn't happen spontaniously - but WHY is this??
Or does the spec mean that all we need to know is that enthalpy change doesn't explain why endothermic reactions don't happen spontaniously, therefore there must be something else to explain it (entropy)?

Cheeers!

[clownfish]

(Original post by foldingstars45)
Okay, so the spec says;

"understand that ∆H, whilst important, is not sufficient to
explain spontaneous change (e.g. spontaneous endothermic
reactions)"

And I understand that enthalpy change means that because energy must be put IN to an endothermic reaction, it wouldn't happen spontaniously - but WHY is this??
Or does the spec mean that all we need to know is that enthalpy change doesn't explain why endothermic reactions don't happen spontaniously, therefore there must be something else to explain it (entropy)?

Cheeers!

An endothermic reaction needs energy to be put into it, so there must be enough energy (ie heat) around to provide this energy. Remember bond breaking is endothermic and bond making is exothermic, so the reaction must be breaking more/stronger bonds than it is forming for it to be endothermic. If the reaction is taking place in a cold area then there won't be the heat energy available for the reaction to take in, and hence the reaction won't take place.

If the entropy change for the reaction is very positive, (from a given minimum temperature upwards) it can outweigh the endothermic enthalpy change and hence the reaction will be spontaneous.

Essentially you want to be using the formula for deltaG (deltaG = deltaH -TdeltaS) and making sure the value for deltaG is always less than zero.

I hope that helped a bit.