Cooling the Wort
At the end of the boil, cooling the wort is very important. While it is above 130F, bacteria and wild yeasts are inhibited. It is very susceptible to oxygen damage as it cools though. There are also sulfur compounds that evolve while the wort is hot. If the wort is cooled slowly these di-methyl sulfides can dissolve back into the wort causing cabbage or cooked vegetable flavors in the final beer. The objective is to rapidly cool the wort to below 80F before oxidation or contamination can occur. Here is one preferred method for cooling the wort.
Place the pot in a sink or tub filled with cold/ice water that can be circulated around the hot pot. While the cold water is flowing around the pot, gently stir the wort in a circular pattern so the maximum amount of wort is moving against the sides of the pot. If the water gets warm, replace with cold water. The wort will cool to 80F in about 20 minutes. When the pot is still warm to the touch, the temperature is close enough.
Pour the reserved 2.5 gallons of water into the sanitized fermenter. Pour the warm wort into it, allowing vigorous churning and splashing. Oxidation of the wort is minimal at these temperatures and this provides the dissolved oxygen that the yeast need to reproduce. Combining the warm wort with the cool water should bring the mixture to fermentation temperature. It is best for the yeast if the pitching temperature is the same as the fermentation temperature. For Ale yeasts, the fermentation temperature range is 65-75F. (The temperatures mentioned are not absolutely critical and a thermometer is not absolutely necessary, but is nice to have.)
Note: Do not add commercial ice to the wort to cool. Commercial Ice harbors lots of dormant bacteria that would love a chance to work on the new beer. Bottled Drinking Water is usually pasteurized or otherwise sanitized to inhibit contamination.