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SQA Higher Chemistry Hess' Law

Hess' Law

This page covers the following topics:

Hess' law is used to determine reaction enthalpy change indirectly (using an alternative route of reaction)

Hess's Law 1

Hess' law can be calculated using bond enthalpies or expermental data to determine Ξ”H1 and Ξ”H2

Hess' Law 2

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Use the provided image to determine the equation used to calculate Ξ”H according to Hess' Law.

SQA Higher Chemistry Hess' Law Use the provided image to determine the equation used to calculate Ξ”H according to Hess' Law.
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What is the equation for Hess' Law when using enthalpies of formation?

SQA Higher Chemistry Hess' Law What is the equation for Hess' Law when using enthalpies of formation?
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What is the first step to using Hess' Law?

SQA Higher Chemistry Hess' Law What is the first step to using Hess' Law?
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What is the enthalpy of formation of methane? You may use the Hess cycle provided to help you. Ξ”Hc(CHβ‚„) = βˆ’892 kJ/mol, Ξ”Hc(Hβ‚‚) = βˆ’286 kJ/mol and Ξ”Hc(C) = βˆ’394 kJ/mol.

SQA Higher Chemistry Hess' Law What is the enthalpy of formation of methane? You may use the Hess cycle provided to help you. Ξ”Hc(CHβ‚„) = βˆ’892 kJ/mol, Ξ”Hc(Hβ‚‚) = βˆ’286 kJ/mol and Ξ”Hc(C) = βˆ’394 kJ/mol.
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What is the standard enthalpy of reaction for NaOH + HCl β†’ Hβ‚‚O + NaCl? Ξ”Hf(NaOH) = βˆ’469 kJ/mol, Ξ”Hf(HCl) = βˆ’92.3 kJ/mol, Ξ”Hf(Hβ‚‚O) = βˆ’286 kJ/mol and Ξ”Hf(NaCl) = βˆ’407 kJ/mol.

SQA Higher Chemistry Hess' Law What is the standard enthalpy of reaction for NaOH + HCl β†’ Hβ‚‚O + NaCl? Ξ”Hf(NaOH) = βˆ’469 kJ/mol, Ξ”Hf(HCl) = βˆ’92.3 kJ/mol, Ξ”Hf(Hβ‚‚O) = βˆ’286 kJ/mol and Ξ”Hf(NaCl) = βˆ’407 kJ/mol.
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