Hess's Law

Question 1

What is a state function?

Answer 1

• The final value does not depend on the path take to reach the values, there are different ways to get to the ‘answer’
• Enthalpy (heat content of a system) is a state function

Question 2

What is Hess’s law?

Answer 2

• The total enthalpy change in a chemical reaction is independent of the rout by which the chemical reaction takes place, as long as the initial and final conditions are the same.
• The change in enthalpy during a chemical reaction does not depend on whether the reaction proceeds in one or in many steps

Question 3

What are enthalpy cycles and when is Hess’s law applied?

Answer 3

• They represent Hess’s law in which reactants are converted into products
• There are two routs or pathways in which the final value can be obtained
• The direct route is ΔH1. The enthalpy change of the direct rout is equal to the enthalpy change for the indirect route (ΔH2 + ΔH3).
• Hess’s law is used to calculate enthalpy changes for reactions that cannot be calculated experimentally.
• Enthalpy cycles contain enthalpy of formation

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Question 4

What is the enthalpy change of formation (ΔHf)?

Answer 4

• Also known as standard enthalpy change of formation, is the enthalpy change when one mole of a compound is formed from the elements in their standard states under standard conditions.
• Enthalpy of formation values are useful as they indicate the stability of compounds in relation to their elements
• The more negative, the greater the stability of the compound
• E.g. Formation of methane

Question 5

What is the formula for the enthalpy change of formation?

Answer 5

ΔH⦵ = ΣΔH⦵f (products) − ΣΔH⦵f (reactants)

- The standard enthalpy change of formation of an element in its standard state is zero

Question 6

How to apply enthalpy change of formation and Hess’s law?

Answer 6

• It is basically vectors
• When ΔH3 = ΔH1 + ΔH2 then rearranged ΔH1 = ΔH3 − ΔH2
• The minus indicates going against the direction of the arrow

Question 7

What is the enthalpy change of combustion (ΔH⦵c) ?

Answer 7

ΔH⦵ = ΣΔH⦵c (reactants) − ΣΔH⦵c (products)

- The opposite of the enthalpy change of formation

Question 8

How to determine the enthalpy change for a reaction from multiple reactions with known enthalpy changes?

Answer 8

• Here the reactions must be rearranged and the common compounds that are on opposite side (one in reactants of the one equation) and the other in the products of the other equation
• The reactions must be ‘flipped around’ sometimes
• The common compounds are cancelled
• Whatever changes are made to the reactions (a certain amount of moles is required) the same must be done to the ΔH value.
• This includes dividing, multiplying or reversing
• See where which element is placed in the final equation

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Question 9

Watch out!

Answer 9

• When you are asked for the enthalpy change for formation but the values of the compounds are of combustion then use the formula for COMBUSTION
• The formula is chosen based on the values given