4.2 a/b/c: Enthalpy and Hess' Law

Enthalpy and Hess' Law

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Syllabus dot points

a	explain the enthalpy changes in a reaction in terms of breaking and reforming bonds, and relate this to: – the law of conservation of energy
b	investigate Hess’s Law in quantifying the enthalpy change for a stepped reaction using standard enthalpy change data and bond energy data, for example: (ACSCH037) – carbon reacting with oxygen to form carbon dioxide via carbon monoxide
c	apply Hess’s Law to simple energy cycles and solve problems to quantify enthalpy changes within reactions, including but not limited to: – heat of combustion – enthalpy changes involved in photosynthesis – enthalpy changes involved in respiration

Recap of enthalpy

• Enthalpy = measure of total energy (potential) that is in the bonds of a system

• We calculate in enthalpy changes or ΔH

• endothermic rxn = positive ΔH

• exothermic rxn = negative ΔH

Formation of Bonds

• law of conservation of energy - can't be created or destroyed, only changed in form

• requires energy to break bonds - endothermic

• releases energy when forming bonds - exothermic

• we determine net release or absorption of energy

• Hess' Law brings all of this + 4.1 together

Hess' Law Definition

• in a reaction, the enthalpy changes is always constant regardless of the steps within the reaction

• directly relates to law of conservation of energy

• reactions may have several pathways, but the overall enthalpy change will always be the same - because the amount of energy to break and form bonds will always be the same

C --> CO₂

• these are two pathways (though incomplete and complete combustion)

• Reaction 1 = complete combustion, and Reaction 1 = Reaction 2 + Reaction 3

• The enthalpy change data also supports this

Multiple Choice

Hess' Law makes use of which principle to calculate the enthalpy change of a reaction?

• The law of conservation of energy

• The law of conservation of matter

• The law that you will always find a lost item in the last place you look for it

• Murphy's law

Standard enthalpy of formation

• aka heat of formation/reaction

• ΔH°_f

• the increase in enthalpy when 1 mole of compound in its standard state is formed from elements in their standard state

• standard state = 100kPa, 1mol/L, temp depends on state

• forming an element means that ΔH°_f = 0

Calculating from heats of formation

• can either be done with just the data/equations, or with an energy cycle diagram

• both can display the same information, and often it depends on the question - or in industry it's personal preference

• both are just displaying ΔH° ₌ ΣΔH°_{f (products)} - ΣΔH°_{f (reactants)}

• must have 1 mole of product! Means you can have fractions on the left hand side, but means that you can use the data.

Multiple Choice

Using the equations below:

C_(s) + O_2(g) → CO_2(g) ∆H = –390 kJ

Mn_(s) + O_2(g) → MnO_2(s) ∆H = –520 kJ

what is ∆H (in kJ) for the following reaction?

MnO_2(s) + C_(s) → Mn_(s) + CO_2(g)

• 910

• 130

• -130

• -910

Heat of combustion

• negative of the enthalpy change for the combustion process

• because enthalpy change is heat gained, and heat of combustion is heat released when a substance undergoes combustion

• calculating is simple; use ΔH°_{C =} ΣΔH°_{f (products)} - ΣΔH°_{f (reactants)}

• Either asked to calculate it out right, or calculate the heat of formation of a reac/prod

Photosynthesis and Respiration

• both are multistep reactions, but are simplified into one step

• they are also the reverse of one another

• except photosynthesis = endothermic, and respiration = exothermic

Multiple Choice

How do you calculate the Enthalpy of Reaction?

• ΔH = ΔH_products - ΔH_reactants

• ΔG = ΔH -TΔS

• ΔT = q / mC

• E = mc²

Open Ended