Endothermic and endothermic enthalpy changes.

MattHorwood

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Can someone please explain why, when calculating enthalpy change of an exothermic reaction, the average bond enthalpy of he products is higher so a negative value is achieved. I thought exothermic reactions lost energy to the surroundings so how do they end up with a higher average bond enthalpy on the product side if energy is lost? This feels like a dumb question from an A-level student but I just don’t quite understand.

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charco

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(Original post by MattHorwood)
Can someone please explain why, when calculating enthalpy change of an exothermic reaction, the average bond enthalpy of he products is higher so a negative value is achieved. I thought exothermic reactions lost energy to the surroundings so how do they end up with a higher average bond enthalpy on the product side if energy is lost? This feels like a dumb question from an A-level student but I just don’t quite understand.

There are two different forms of energy being discussed:

1. Chemical potential energy
2. Heat (kinetic) energy

An exothermic reaction changes chemical potential energy into heat energy.

The more stable a system is the less chemical potential energy it possesses.

Strong bonds are more stable and have less chemical potential energy.

Hence in a chemical reaction bond breaking of the reactants involves endothermic change as heat energy is converted into CPE, and bond formation of the products involves change from CPE to kinetic energy.

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MattHorwood

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(Original post by charco)
There are two different forms of energy being discussed:

1. Chemical potential energy
2. Heat (kinetic) energy

An exothermic reaction changes chemical potential energy into heat energy.

The more stable a system is the less chemical potential energy it possesses.

Strong bonds are more stable and have less chemical potential energy.

Hence in a chemical reaction bond breaking of the reactants involves endothermic change as heat energy is converted into CPE, and bond formation of the products involves change from CPE to kinetic energy.

Oh I finally get it. Thank you so much!

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