Question

which chemicals are present in crackers and how do these chemicals react and burst?

Answer 1

chemical present in crakers are lead,magnesium, cadmium and sulpherdioxide
When the fuse on a firecracker is lit and burns, it sets off a chemical reaction.
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If the gunpowder is confined in some way, the gas is not released harmlessly. Instead, it explodes.

In a bullet, the shell confines the gunpowder so that the high pressure gas released by the chemical burning reactions propels the bullet down the barrel of the gun.

In firecrackers or other fireworks, the cardboard shell of the firecracker explodes with a loud popping sound when the chemical burning reaction releases the gas.

Answer 2

Firecrackers contain flash powder (a mixture of an oxidizer such as potassium chlorate or perchlorate and powdered aluminum or magnesium) or black gunpowder in a paper tube. An attached fuse ignites the flammable mixture, which burns explosively, producing gases that rapidly build up pressure and burst the container. Aluminum and magnesium components produce brighter flashes.