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During a chemical reaction, energy is conserved and must flow between the system (reactants and products) and the environment. Understanding how energy is absorbed and released helps us comprehend the nature of endothermic and exothermic reactions. For example, the reaction of hydrogen (H₂) with chlorine (Cl₂) to form hydrogen chloride (HCl) involves breaking and forming bonds, which causes energy flow within the system. Given this understanding, which statement best explains how energy flows in a system during a chemical reaction, considering that energy is neither created nor destroyed (law of conservation of energy)?

A. It is released to the environment to break bonds, then absorbed from the surroundings when forming bonds.
B. Reactants absorb energy that is then released by the products.
C. It is absorbed from the environment to break bonds, then released back into the surroundings when forming bonds.
D. Reactants release energy that is then absorbed by the products.

1 answer

To determine which statement best explains how energy flows in a system during a chemical reaction, let's break down the process of bond breaking and bond forming in the context of energy changes.

In a chemical reaction:

  • Breaking bonds in reactants requires energy, which is typically absorbed from the surroundings.
  • Forming bonds in products releases energy back into the surroundings.

Given this flow of energy, we can analyze the statements provided:

A. "It is released to the environment to break bonds, then absorbed from the surroundings when forming bonds."

  • This is incorrect because breaking bonds requires energy to be absorbed, not released.

B. "Reactants absorb energy that is then released by the products."

  • This generally describes the process accurately, but doesn't specify that energy is absorbed for breaking bonds and released for forming bonds.

C. "It is absorbed from the environment to break bonds, then released back into the surroundings when forming bonds."

  • This statement correctly describes the energy flow: energy is absorbed to break bonds and released when bonds are formed.

D. "Reactants release energy that is then absorbed by the products."

  • This is incorrect because reactants typically absorb energy to break bonds.

Among these options, C is the most accurate explanation of how energy flows in a system during a chemical reaction, adhering to the law of conservation of energy.