IB2/U1: Thermochemistry

heat (Q)

thermal energy:

• what is it

• what are its properties

the form of energy that is transfered between objects or systems because of a temperature difference

energy within a system that’s created by the random motion of molecules and atoms (KE)

what is a spontaneous process?

processes that occur without the addition of energy besides activation energy (no need for continuous energy input)

what is entropy (S)

units

a measure of the degree of disorder in a system

• entropy increases when there are more ways for energy to be dispersed or spread out 

• units: J/K mol

explain the 2nd law of thermodynamics: a reaction is spontaneous if there is an overall increase in entropy of the system and surroundings (universe) and how does this show that spontaneity is a balance between entropy and enthalpy?

its insufficient to just know the entropy change of the system OR the surroundings, if you want to figure out if a process is spontaneous. for example, the system can decrease in entropy, but if the surroundings increase enough in entropy, than there can still be a net increase. the increase in entropy in the surroundings is usually done through the release of heat from system to surroundings.

what is thermochem?

energy changes in changes of matter

Heat (Q) vs Thermal energy and temp

thermal energy: the total energy within a system because of the random motion of molecules and atoms: KE+PE (the forces between particles). this energy can be transferred, and when it is, its transferred as heat

• heat: thermal energy that is transferred between objects or systems of different temp hot—> cool

• temp: the average kinetic energy of the particles in a sample of matter

system vs surroundings

types of systems:

• open

• closed

• isolated

system: the actual reaction itself, the substances undergoing the change

surroundings: the environment around the system

types of systems: affects exchange between the system and the surroundings

open: exchange energy and matter

closed: exchange energy but not matter

isolated system: cannot exchange matter or energy

enthalpy

enthalpy is the amount of heat energy contained in a substance which is stored in chemical bonds

• it cannot be measured directly → we can only measure the change in enthalpy

• the enthalpy of a substance is from its particles kinetic and potential energy

what is an enthalpy change

it is the amount of heat energy released or taken in per mole of substance during a physical or chemical change

examples of enthalpy change:

• enthalpy change of formation ∆Hf

• enthalpy change of combustion ∆Hc

• bond enthalpy

• the change in energy when 1 mole of a compound is formed from its elements in their standard states

• the change in energy when 1 mole of a substance is burnt completely in O2

• bond enthalpy is the change in energy when one mole of gaseous covalent bonds is broken (energy change of bond breaking/reforming)

  • bond reform = -bond break (bond break >0 always)

what is Hess’s Law

the enthalpy change during a chemical change is independant of the intermediate steps taken (state function)

as long as you have the same reactants and end at the same products, the enthalpy change is the same

RP = RI + RP

using Hess’s law, how can you find the enthalpy of combustion ∆Hc without changing the directions of the arrows?

recall that Hess’s law states that intermediate steps dont matter as long as you have the same reactants and products so instead of switching the signs, just switch the path way, starting with the elements of formation as the reactants. 

how would you solve for the enthalpy of formation if you were only given the enthalpy of combustion for C, H₂ and C₆H₆?

*we always set up the hess cycle using whatever enthalpies we’ve been given as our intermediate step

enthalpy of vaporization

why is this important for bond energies?

the energy change when one mole of a liquid is boiled to form a gas

• this is important because if we are given a substance in liquid form, we not only need to find the bond energies for the substance but also add the enthalpy of vaporization to account for the energy needed to overcome IMFs to get to a gas state

potential energy of particles

the potential energy of particles comes from the intermolecular forces between the partciles because of their reltive position to each other

kinetic energy of particles

partciles have KE because of their movement

Enthalpy: unit and symbol

It is measured in KJ/mol 

The symbol for enthalpy is ΔH

what conditions are needed to measure the enthalpy change?

compounds need to be in their standard states and under standard conditions: 298K, 101KPA, 1 mol/dm3 solution

system vs surrounding

the system is the reaction mixture while the surrounding is everything around it

standard enthalpy change of a reaction

enthalpy change when molar quantities of reactants in their standard states react to form products in their standard states states under standard conditions

calorimetry

calorimeter

the process of measuring energy changes during a phyiscal or chemical process

• calorimeter: is an insulated reaction vessel where the change in temperature of the surroundings can be measured

specific heat capacity ( c)

specific heat capacity of water

it is the amount of heat needed to raise 1g of a substance by 1 K

water has a very high specific heat capacity! it needs a lot of energy to change its temperature! c = 4.184 J/gK

calorimetry assumptions + limitations

1. we assume no heat is being transferred between the calorimeter and the outside environment (making it an isolated system) but the system is rarely isolated, it may be closed though

2. we assume that the heat absorbed or released by the calorimeter is negligible → not true because sometimes we account for the heat absorbed by the calorimeter in calculations

3. we assume a dilute aqueous solution has a density and c equal to water (obvs not true) → d= 1g/cm3 c=4.184 J/gK

errors in calorimetry to mention

• heat loss to atmosphere

• heat absorbed by calorimeter

• incomplete combustion: because its less efficient and releases less heat

• not actually reacting at SATP (standard conditions)

enthalpy (H)

enthalpy change (∆H)

if heat is the transfer of thermal energy from warmer to colder substances, than enthalpy is the total thermal energy in a subtance. but we do not have a way of measuring this directly!

enthalpy change is the flow of thermal energy into or out of the system per mol

equations of enthalpy change

∆H = H(products) - H(reactants)

∆H = -Q/n

∆H = n∆Hx

molar enthalpy ∆Hx

x = subscript for the type of change (ie. comb, vap, neut)

this is the enthalpy change for 1 mol of a substance so the units are J/mol

this is different from the standard enthalpy of combustion. to calculate this, you will be calculating an enthalpy change for some amount of substance other than 1 mol using:

∆H = n∆Hx → you can use mol ratios to manipulate the molar enthalpy to help get other values

enthalpy change of neutralization ∆H_n

_{this is the energy change when 1 mol of water is formed from the reaction of an acid with a base under standard conditions} 

_{so the units are: KJ/mol H2O}

thermochem is the study of energy changes due to changes in matter. matter changes can be ___(3)?

changes in matter can be:

• physical: substance changes in appearance not identity (same formulas)

• chemical: new substance produced (new formulas) with new properties - same elements rearranged

• nuclear: formation of completly new elements because there is a change to the nucleus of atoms

bond dissociation energy (bond enthalpy)

the energy required to break one mole of a chemical bond in a specific compound in gaseous state 

is bond enthalpy positive or negative

bond enthalpy is always positive because bond breaking requires energy

average bond energy

average bond energy is the enthalpy change when 1 mol of a chemical bond in gaseous state are broken and averged over multiple similar compounds

how is bond energy affected by the bond order: single vs double

bond order?

bond order: the amount of bonds per atom = #bonds total/ # bond domains

as bond order increases, the atoms are held closer together because more electrons are shared between them: high bond order = shorter bond length

more shared electrons = stronger bonds and stronger attraction between atoms = more energy needed to break bond

what is the ozone layer

this is a region of earths stratosphere that absorbs most of the Sun’s harmful UV radiation

its made of high concentrations of ozone (O3)

Which molecule has a higher bond order? Why?

Which bond should require more energy to break (greater bond enthalpy)? 

Which bond is more stable?

1. O2 has a higher bond order 2 vs 1.5

2. because O2 has a higher bond order, it has more electrons being shared between atoms = stronger attraction of atoms to the localized electrons = stronger bond and it will take more energy to break the bond 

3. higher bond order= stronger bond= lower PE = more stable

explain the chapman cycle

*note: short wavelength = high energy

1. because O2 has a higher bond order, it takes more energy to break (its broken by higher energy UV rays) → homolytic fission of O2 forms O radicals

2. O radicals react with O2 to MAKE O3

3. O3 has a lower bond order so it can be broken by lower energy UV rays → this makes an O radical and O2

4. O3 reacts with O radical to make 2 O2

this cycle is the natural formation and depletion of O3 where O3 is formed then used up to mkae O2

explain CFCs and Ozone depletion

CFCs are very stable in the lower atmosphere because their C-Cl and C-F bonds are pretty stable

1. when CFCs reach the stratosphere, theyre exposed to high energy UV radiation which creates chlorine free radicals 

   1. the C-Cl bond breaks because the C-F bond is stronger because F is more EN and it is a smaller atom which increases the strength of the C-F bond

2. once formed, Cl radicals start reacting with O3 to form O2 and more Cl radicals:

1⃣ Cl• + O₃ → ClO• + O₂
 (chlorine radical reacts with ozone, destroying one ozone molecule)

2⃣ ClO• + O• → Cl• + O₂
 (the radical is regenerated and can destroy more ozone)

This is a catalytic cycle — the chlorine radical isn’t used up.
A single Cl• can destroy thousands of ozone molecules = depletion of O3

WHY doesnt Cl radicals react with O2 then? to remake O3?

• The chlorine radical has enough energy to break the weaker O–O bonds in ozone (1.5 order) but not the stronger double bond in O₂.

explain the crystal lattice around ionic compounds

in ionic compounds, ions are held in a lattice structure of alternating ions:

• each ion is completely surrounded by ions of the opposite charge

• the attraction between ions is called the electrostatic attraction which are very strong and have very high melting points

• very brittle 

explain lattice enthalpy ∆H_lat

so bond enthalpy is the amount of energy needed to break 1 mole the bonds of covalent bonds in gaseous molecules

LATTICE ENTHALPY:

Enthalpy change when one mole of a solid ionic compound is converted to its gaseous ions under standard conditions

• lattice enthalpy refers to ionic compounds not the individual bonds (C-H bonds)

• it represents the strength of electrostatic attraction between -ions and +ions

bond enthalpy (covalent!) general equation

XY(g)→X(g)+Y(g)

*watch the states!

ex. H–Cl(g)→H(g)+Cl(g) ΔH=+431 kJ/mol

compare bond enthalpy vs lattice enthalpy:

• type of substance

• enthalpy reaction

• endo vs exo?

• depends on what 2 main factors?

Property	Bond Enthalpy	Lattice Enthalpy
Type of substance	Covalent molecules	Ionic compounds
Process described	Breaking 1 mol of covalent bonds	Formation or breaking of ionic lattice
Energy change	Endothermic (breaking bonds)	Endothermic
Units	kJ/mol	kJ/mol
Depends on	Bond order, polarity	Ionic charge, ionic size
Example	H–Cl bond: +431 kJ/mol	NaCl lattice: –787 kJ/mol

🔥 Summary:

• Bond enthalpy → strength of a specific bond in a molecule.

• Lattice enthalpy → strength of ionic attractions in a solid crystal.

what are the 2 factors determining lattice enthalpy

1. ionic charge:

• higher charges = stronger electrostatic attraction

• this leads to increased lattice enthalpy

2. ion size

• smaller ions have a stronger force of attraction because the distance between their nuclei to the shared electrons is less

can we directly measure lattice enthalpies?

No. lattice enthalpies cannot be measured directly

so instead we use born-haber cycles which are special energy cycles used to indirectly calculate lattice enthalpies

what is the enthalpy of atomization?

when 1 mol of gaseous atoms are formed from an element in its standard state:

Na (s) → Na (g)

define ionization energy

define electron affinity

the energy needed to remove 1 mole of electrons from 1 mole of gaseous atoms

Na _(g) → Na⁺ _(g) + e^-

the energy released when 1 mol of electrons are added to 1 mol of gaseous atoms

Cl _(g) + e^- → Cl^- _(g) 

enthalpy change of solution ∆H_sol

enthalpy change when 1 mol of solute dissolves to form a solution of infinite dilution (aka a solution with a lot of excess water)

in an enthalpy change of solution, the ions in an ionic compounds dissociate to get ions with an aqeous state. the intermediate step is getting the ions in gas state → this is lattice enthalpy!

∆H_sol = ∆H_lat + ∆H_hyd

*enthalpy of hydration is turning the gaseous state ions into aq state

a combustion reaction involves what 2 important reactants?

fuel: which is a combustible substance

O2 gas

produces metallic or non metallic oxides

combustion of metals

when metals react with O2 to form metallic oxides, ex. the formation of MgO. this combustion reaction can produce lots of heat and light making it exothermic

what is rusting?

when less reactive metals combine with O2 to form metallic oxides ex. CuO. so rusting is a type of combustion reaction

can combustion take place without O2?

no

combustion of non metals

when non metals react with oxygen to form non-metallic oxides

combustion of organic compounds (carbon based compounds)

organic compounds undergo complete combustion reactions that only produce CO2 and H2O 

its exothermic because even though it takes energy to break the bonds between atoms in the fuel and the bonds in a molecule of O2, the products are more energetically stable than the reactants

what is incomplete combustion?

• somewhat limited O2?

• very limited O2?

when fuels undergo combustion in low oxygen environments they produce H2O and CO2 but other byproducts. less O2 means CO and C can be formed. 

water in solid fuels can cause incomplete combustion

• somewhat limited O2 → CO + H2O

• very limited O2 → C + H2O

what is the reaction for the formation of CO2?

is it complete or incomplete combustion?

what about the formation of CO?

C + O2 → CO2, this is a complete combustion reaction because the products dont include C, CO

the formation of CO is incomplete: 2C + O2 → 2CO

what are the 3 main categories of fossil fuels?

goal, crude oil, natural gas

coal: solid

crude oil: liquid

natural gas: gas

how can we separate the components of fossil fuel mixtures?

the components can be separated by their boiling points → fractional distillation

fossil fuels can be compared in terms of the amount of CO2 produced (this affects the greenhouse effect)

the amount of CO2 produced can be measured in the mols of CO2 per MJ of energy or

it can be measured as the volume of CO2 produced per mass (g) of fuel

energy released per unit mass → energy density by mass

units?

all fossil fuels release energy when they undergo combustion, but the amount varies depending on the fuel type. the amunt of energy can be compared per mole in thermochemical equations for a specific substance undergoing combustion.

the energy per unit mass is measrued in MJ/kg (amount of energy released per kg) or some other unit of mass 

what is the specific energy of a fuel

∆H_fuel / M_{molar mass} → the amount of energy produced in a complete combustion reaction per unit of mass (ex. g)

difference with energy released per unit mass:

• specific energy is a theoretical property of a fuel 

• energy released per unit mass: is an experimental or practical value taken from combustion measurements 

order of specific energy of fossil fuels from least to most

coal, crude oil, natural gas

what are the consequences of incomplete combustion (3)

• less energy released per mole of fuel compared to compelte combustion

• C and CO are harmful by products

• unreacted fuel is fire hazard

which fossil fuel has the greatest tendency to undergo incomplete combustion?

coal has the greatest tendency to to undergo incomplete combustion because there are impurities like nitrogen, sulfur and other metals that can react with Oxygen → makes other byproducts 

when these impurities consume O2, there might not be enough O2 left to react with the C in the coal leading to incomplete combustion

the larger a hydrocarbon molecule, the more easily it will undergo incomplete combustion. 

• the harder something is to fully oxidise (like large molecules!) increases its tendency to easily undergo incomplete combustion reactions 

what fossil fuel is least likely to undergo incomplete combustion?

crude oil and natural gas mostly consist of hydrocarbons but crude oil has longer chains which require more O2 to oxidize completely so its more likely to undergo incompelte combustion than shorter hydrocarbon chains.

what is the relationship between CO2 levels and greenhouse effect

the greenhouse effect occurs when certain gases like CO2 absorb infrared radiation that has been emitted from the earths surface after being heated by the suns rays. this increases the solar energy trapped within the atmosphere leading to increased temperatures.

_____

 is the fossil fuel that produces the most carbon dioxide per unit of energy and 

_____

 is the fossil fuel that produces the least amount per unit of energy during complete combustion.

coal, natural gas

The complete combustion of coal only produces one product, CO₂, (C+O2 → CO2) and produces the least amount of energy per mole of CO₂. Crude oil and natural gas produce CO₂ and H₂O, but longer hydrocarbon chains in crude oil have a greater C:H ratio than shorter hydrocarbon chains, therefore natural gas produces the least amount of CO₂ per unit energy.

renewable vs non renewable energy sources

green vs renewable

A non-renewable energy source is consumed in a chemical or nuclear process and cannot be renewed at a rate equal to or faster than it is consumed.

green energy: reduced environmental impact

what are biofuels

how are they produced

biofuels are fuels derived from biological sources 

modern biofuels are derived from plant material that is transformed via physical and chemical processes to produce usable fuels 

• biofuels are produced through the breakdown/decomposition of larger organic molecules into combustible short chain hydrocarbons 

how do biofuels release energy?

When plants photosynthesise, the carbon in carbon dioxide is converted to glucose. Biofuels release energy when combusted from carbon that was originally absorbed by the plant as carbon dioxide. Carbon fixation is the process by which inorganic carbon (CO₂) is converted into organic compounds such as glucose.

advantages and disadvantages of biofuels

advantages:

• the plants used to make biofuels must absorb CO2 when performing photosynthesis which offsets the carbon emissions of the biofuel when combusted

• they are more likely to undergo complete combustion because they are short chain hydrocarbons

• takes less resources to refine biofuels which is less harmful to the environment (no need to mine or fractionally distill in their production)

cons:

• uses lots of plant material which takes land for agriculture

• combustion of biofuels still emits GHGs

how is coal and crude oil formed?

Coal, was formed from the remains of prehistoric plants that grew in vast swamps. When the plants died, they became buried under layers of mud and sand. Over millions of years, heat and pressure, combined with the anaerobic conditions, converted the decaying plant material into coal. The reduction of biological compounds that contained carbon, hydrogen, nitrogen, sulfur and oxygen led to the formation of coal, oil and natural gas. Crude oil is formed from the remains of marine organisms.