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Does a change in energy always indicate that a chemical reaction has taken place?
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8. Thermochemistry
Nature of Energy
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When hydrogen gas (H_2) reacts explosively with oxygen gas (O_2) to form water, where does the released energy originate?
A
From the conversion of hydrogen atoms into helium
B
From the formation of new bonds in H_2O, which releases energy
C
From the breaking of H_2 and O_2 bonds, which releases energy
D
From the absorption of energy from the surroundings
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Understand that chemical reactions involve breaking old bonds and forming new bonds. Energy changes during a reaction depend on these processes.
Recognize that breaking bonds requires energy input (endothermic process), while forming bonds releases energy (exothermic process).
In the reaction of hydrogen gas (H_2) with oxygen gas (O_2) to form water (H_2O), bonds in H_2 and O_2 molecules are broken first, which consumes energy.
Next, new bonds are formed in the water molecules (O-H bonds), and this bond formation releases energy.
The net energy released in the reaction comes from the fact that the energy released by forming the new O-H bonds in water is greater than the energy required to break the original H-H and O=O bonds.
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