Imagine that you and your friend live on opposite sides of a hill. You realize that he left his bowling ball at your house and that you need to return it.
In order to do so, you will need to carry it up to the top of the hill. Once there, you could (at least if you were lazy) simple set the ball down and let it roll to your friends door.
In this process, you will need to put in some energy (moving the ball to the top of the hill) and then some energy will be released (the ball could certainly do some work on your friends house when it gets there.Exothermic reactions
But, there are several possibilities. The first is that you live near the top of the hill and that your friend lives near the bottom. In that case, much more energy is released (when the ball hits your friend's door) than was put in (when you carried it up the hill).
If we imagine a “shortcut” through the hill, you could simple roll the ball down the hill to your friend's house. That means that, overall, the process releases energy (even though, in reality it takes in and gives off energy).Endothermic Reactions
The other possibility is that your friend is nearer to the top of the hill than you are and that, therefore, much less energy is releases than was put it.
Now, if we imagine a shortcut through this hill in this case, you would have to put in energy to get the ball back to your friend.
In this case the process takes in energy overall (even though energy is both taken in and given off). This leads to graphs of heat of reaction.
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