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Do you know how some reactions have two arrows, like such?

http://upload.wikimedia.org/math/2/9/4/294bdf253cd...

Depending on the conditions surrounding a reaction, these ''double arrowed'' reactions can be said to be dominantly going FORWARDS OR BACKWARDS depending on what is going on in the environment around these reactions. Each unique reaction is said to be exothermic or endothermic, where all endo/exo reactions you will see with these double arrows.

Reactants=Substances On left of reaction

Products=Substances on right of reaction

ENDOTHERMIC=+ΔH Value

For endothermic reactions, the environment is COLDER on the products side, as the products have absorbed the heat energy, thus giving them an increase in energy, or an increase in ΔH, therefore (+.)

EXOTHERMIC=-ΔH Value

For exothermic reactions, the environment is HOTTER on the products side. When you think exothermic think EX-->Exit-->Releasing Energy.

This energy is released in the environment, therefore making the environment hotter. As the reaction has released energy, the products themselves are COOLER, meaning they have less energy, therefore a decrease in ΔH, therefore (-)

You have to make sure to not get mixed up between the temperature of the environment and the temperature of the substances themselves. ΔH talks about the change in heat energy of the SUBSTANCES of the reaction, not the ENVIRONMENT of the reaction.

So as we know that the products are HOTTER for endothermic reactions, we can conclude that HEATING an endothermic reaction will cause it to go ''forwards'' or ''towards the products'', as this is where the substances themselves are hotter.

The opposite goes for exothermic reactions. For these, we know the products are COOLER, as the heat energy is released into environment, so we conclude that heating an exothermic reaction will cause it to go ''backwards'' or ''towards the reactants'', as this is where the substances themselves are hotter.

When the ΔH is larger(+ or -), we say that there has been a larger change in heat energy of the reaction. So think of a bomb going off(exothermic as energy released), the ΔH for this reaction will be very high as there was a large transfer or heat energy from the reaction from the reactants, out into the environment(hardly any energy left in products)

Note that other factors like pressure and concentrations of products/reactants can change the direction in which these reactions occur.

Is that kind of what you were after???

Good luck

re-read some bits its good for revision and i made a few errors which are now fixed