Potential Energy Decrease

Takakkaw Falls, Yoho National Park
Takakkaw Falls (Yoho National Park) at 284 metres is Canada's second highest waterfall.

One of the natural "driving forces" behind change is a decrease in potential energy.  You can probably think of many changes that occur of their own accord because they result in a decrease in potential energy.  Water always runs downhill, spontaneously.  Of course, we can move water back uphill, but to do so we must use an external source of energy to run a pump.  By itself, water will not run uphill because to do so would require it to increase in energy.

As water flows downhill it undergoes a decrease in potential energy.  This energy is not disappearing, but is being converted into kinetic energy of motion of the water.  However, from the point of view of potential energy, there has been a decrease in energy.  At the top of the falls the water has high potential energy.  At the bottom it has lower potential energy.  Therefore the net energy change is negative.  In the notation used to represent H for a chemical reaction,

H = Hbottom - Htop

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Since change is always defined as final state (bottom) - initial state (top) and Htop is larger than Hbottom, H must be a negative value.

One of the driving forces causing change is a decrease in potential energy, where H is negative.  Decreasing potential energy is one of the reasons chemical reactions occur spontaneously.

 

Baking bannock
A campfire is a typical spontaneous exothermic reaction.  We use the energy released by the combustion reaction to cook, and for warmth and light.  Once the reaction has started, it will continue spontaneously, as long as fuel and oxidizer are present.

In terms of chemical reactions, H is the enthalpy change.  A negative H means that a reaction is exothermic.  Since an exothermic reaction releases heat into its surroundings, exothermic reactions cause the temperature to rise.  All combustion reactions are exothermic.