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Some reactions in my textbook specify the need for the presence of sunlight, eg-photolytic decomposition of silver bromide and substitution reaction of methane with chlorine.

What is going on in these reactions? is there something special in light or can these reactions take place if provided with another source of energy like heat?

Also, can thermal reactions(reactions requiring heat) be done in the presence of sunlight?

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There is nothing magical about light; it just brings in some energy. What's important is that it brings a huge lump of energy to one point, at once. If you'll heat the system in the hope that the heat will provide the necessary energy for the reaction, then you might or might not succeed. In the worst case, before initiating your reaction, heat will initiate some other reactions you don't want.

I expect the radical bromination of some huge branched alkane to be a particularly bad case. Put it under sunlight, and it will go smoothly; heat it, and you'll have all sorts of side reactions, fragmentations, isomerizations going on at once. Of your examples, decomposition of silver bromide is relatively "well-behaved" (after all, $\ce{AgBr}$ must decompose since it doesn't have many other things to do), and $\ce{CH4 + Cl2}$ is somewhere in between.

If you try it the other way around, it is the same: some thermal reactions can be started by using sunlight instead of heat, some can't.

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