Hess's Law and Bond Enthalpies

Enthalpy and Hess' Law

What will we learn?

• Enthalpy recap

• Bond formation

• Hess' Law definition + calculations

• Enthalpy and heat of formation/reaction/combustion calculations

• Energy cycle diagrams

• Bond energy ​

• Photosynthesis + respiration as examples

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 (through incomplete and complete combustion)

• Reaction 1 = complete combustion

• and Reaction 1 = Reaction 2 + Reaction 3 (incomplete combustion)

• 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

Multiple Choice

How do you calculate the Enthalpy of Reaction?

• ΔH = ΔH_products - ΔH_reactants

• ΔG = ΔH -TΔS

• ΔT = q / mC

• E = mc²

Calculating from standard enthalpies 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

Open Ended

​Example:

Open Ended

Open Ended

Heat of combustion

• enthalpy change is heat within the bonds, and heat of combustion is heat released from the bonds 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

​Bond Energy

• Energy required to break a chemical bond

• The enthalpy change associated with ​breaking a bond

• Questions involve getting a set of bond energy data and then using them to calculate ​either total bond energy or change in enthalpy

• low bond energy = more reactive ​

​

​

​Estimating ΔH from bond energies

​Summary of calculating enthalpy changes:

1. Heats of formation for singular species

   1. sum of enthalpy formation of products - sum of enthalpy of formation of reactants

2. Heats of formation for whole reactions

   1. using the forward and reverse reactions to calculate

   2. sum of enthalpy formation of products - sum of enthalpy of formation of reactants

3. Bond energy data

   1. bonds broken - bonds formed

   2. reactants - products

​

​

​

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