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📘 Enthalpy change tracks heat at constant pressure

Enthalpy change tracks heat at constant pressure for a stated reaction. Write the balanced equation with states before you touch a number.

3
lessons
~15 min
to learn
Adults
level
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What you’ll learn

  1. Enthalpy and Hess lawDefine enthalpy change and use Hess law sign and scaling rules.Delta H depends on state; reversing a reaction reverses the sign and scaling coefficients scales Delta H.
  2. Build and solve cyclesCombine reactions and use formation enthalpies to find a target Delta H.Cancel intermediates in aligned equations or use products-minus-reactants formation data.
  3. Check and reportInterpret signs, units, and measurement context in enthalpy results.A negative total means exothermic in the written direction; state symbols and scale must match the data.

Questions this course answers

What happens to Delta H when a reaction equation is reversed?

Reversing the process changes an energy release into an equal energy requirement, or vice versa.

Why can Hess law add several reaction enthalpies?

A state function has the same net change between fixed endpoints regardless of path.

What must happen to Delta H if every reaction coefficient is doubled?

Enthalpy change scales with the amount of reaction represented by the coefficients.

Which formula uses standard formation enthalpies?

Hess law gives the reaction value as the stoichiometric product sum minus the reactant sum.

What does a negative reaction Delta H indicate?

A negative enthalpy change means the system releases heat in the written direction.

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