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Edexcel A-level 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.

Edexcel A-level 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?

Edexcel A-level 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?

Edexcel A-level 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.

Edexcel A-level 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.

Edexcel A-level 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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