Hess's Law: Combining Reactions

Define Hess's Law

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volume_mute

Which statement best expresses Hess's law?

If chemical equations are combined, their enthalpy changes are also combined.

The enthalpy change of a reaction is always zero.

Enthalpy depends only on the reaction's speed.

Heat and work are always equal in a reaction.

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volume_mute

Correct Answer

Explanation

Manipulating Thermochemical Equations

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volume_mute

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Operation

Double all the coefficients

Reverse the equation

Add two equations together

Effect on ΔH

double ΔH

add the two ΔH values

change the sign of ΔH

Match each operation on a thermochemical equation to its effect on ΔH.

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volume_mute

Correct Answer

Explanation

Reverse a Reaction

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If ΔH for C₂H₄ + H₂ → C₂H₆ is −65.6 kJ, what is ΔH for C₂H₆ → C₂H₄ + H₂?

−65.6 kJ

0 kJ

+65.6 kJ

−131.2 kJ

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Correct Answer

Explanation

Scale a Reaction

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volume_mute

If ΔH for C₂H₄ + H₂ → C₂H₆ is −65.6 kJ, what is ΔH for 2C₂H₄ + 2H₂ → 2C₂H₆?

−65.6 kJ

−131.2 kJ

+131.2 kJ

−32.8 kJ

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Explanation

Reverse and Scale

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volume_mute

The ΔH for C₂H₄ + H₂O → C₂H₅OH is −44 kJ. What is ΔH for 2C₂H₅OH → 2C₂H₄ + 2H₂O?

+44 kJ

−88 kJ

−44 kJ

+88 kJ

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volume_mute

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Explanation

Combine Two Reactions

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volume_mute

Determine ΔH for Cu + Cl₂ → CuCl₂ given 2Cu + Cl₂ → 2CuCl (ΔH = −274 kJ) and 2CuCl + Cl₂ → 2CuCl₂ (ΔH = −166 kJ).

−220 kJ

−108 kJ

−440 kJ

+220 kJ

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volume_mute

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Explanation

Decomposition Enthalpy

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volume_mute

Determine ΔH for Fe₂(SO₄)₃ → Fe₂O₃ + 3SO₃ using formation data, given that the forward (formation) reaction has ΔH = −570 kJ.

−190 kJ

+570 kJ

+1,140 kJ

−570 kJ

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volume_mute

Define Hess's Law

Correct Answer

Explanation

Manipulating Thermochemical Equations

Operation

Effect on ΔH

Correct Answer

Explanation

Reverse a Reaction

Correct Answer

Explanation

Scale a Reaction

Correct Answer

Explanation

Reverse and Scale

Correct Answer

Explanation

Combine Two Reactions

Correct Answer

Explanation

Decomposition Enthalpy

Correct Answer

Explanation

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