Energy to break bonds is less than energy release to form bonds.
is what type of reaction?

The reaction is a exergonic reaction.

Explanation:

An exergonic response is a synthetic response where the adjustment in the free vitality is negative demonstrating an unconstrained response. For procedures that occur under steady weight and temperature conditions, the Gibbs free vitality is utilized though the Helmholtz vitality is utilized for procedures that happen under consistent volume and temperature conditions.  

In exergonic reaction, during bond breakage energy is required whereas the formation of the bond releases energy. Although exergonic responses are said to happen immediately, this doesn't infer that the response will occur at a perceptible rate.

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The density of water is 1 g/cm cubed and in order for the pebble to sink its density has to be greater than the density of water and in order for the twig to float its density has to be less than the density of water