Progress of reaction energy changes

In the bromine reaction, shown in Fig. 2.3, we climb a much higher hill and end up in a much higher valley. The increase in potential energy—and the corresponding decrease in kinetic energy—is much larger than in the chlorine reaction more heat will be taken up from the surroundings. 

An exothermic reaction follows much the same course. (Take, for example, the reverse of the bromine reaction that is, read from right to left in Fig. 2.3.) In this case, how evcr, the products contain less potential energy than did the reactants so that we end up in a lower valley than the one we left. Since this time the new particles contain more kinetic energy than the particles from which they were formed, and hence move faster, we observe a rise in temperatufe. Heat will be given off to the surroundings. 

In any reaction there are many collisions that provide too little energy for us to reach the top of the hill. These collisions are fruitless, and we slide back to our original valley. Many collisions provide sufficient energy, but take place when the molecules areTmproperly oriented. We then climb an energy hill, but we are off the road we may climb very high without finding the pass that leads over into the next valley. 

An energy diagram of the sort shown in Figs. 2.2 and 2.3 is particularly useful because it tells us not only about the reaction we are considering, but also about the reverse reaction. Let us move from right to left in Fig. 2.2, for example. We see that the reaction 

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