PPFA-Enthalpy (Thermodynamics 2)

What is enthalpy (H) ?

Heat changes at constant pressure

What is an Isobaric process?

It is a constant pressure process so delta P =0

Note - this means that at constant pressure (dP=0) the change in enthalpy is equal to the heat absorbed or released

Why is enthalpy useful?

- many lab processes are performed at constant temperature ,essentially at 1 atm

What are the different rates of change (DIFFERENTIALS) used and what do they mean ?

d- a tiny change in a variable eg . one step taken when walking

delta - overall change

eg. the whole distance start to finish

Give some examples of differential symbols and there meanings?

INFINITESIMAL = EXTREMELY SMALL

dp= an infinitesimal change in pressure

dv= an infinitesimal change in volume

FOR ENTHALPY :

- dH = an infinitesimal change

- delta H = H(final)- H(initial)

What type of function is Enthalpy (H) ?

Enthalpy is a new energy state variable

ΔH = Qp -change in heat at constant pressure

NOTE- to make it state it is measure at constant pressure

What type function is dH?

dH = δQ- heat transferred

and it is a Path- dependent function

What is the difference between Δ (Delta) vs δ (Lower case Delta Sign)?

Δ (capital delta): Represents THE TOTAL CHANGE in a state function between two states

Example: ΔH = H(final) - H(initial).

State functions include enthalpy, internal energy, volume — they depend only on the initial and final states, not the path taken.

δ (lower case delta): Represents an infinitesimal amount of a path-dependent quantity like heat (Q) or work (W). SMALL CHANGES FOR PATH DEPENDENT VARIABLE

NOTE- You cannot assign a total Q or W without specifying the process path, so we use δQ and δW for small increments.

What is the Equation that links enthalpy to internal energy ?

𝞓E=Q+W this can be written as :

𝞓E=δQp+δWp. - small work change at constant pressure

NOTE- Qp =𝞓H

so write as

𝞓E=ΔH+δWp

small delta -small change

sub letter p = at constant pressure

What is the equation that links work and internal energy ?

note - work =f x d (dx)

force = pressure x Area

work = pressure x area x distance

since area x distance = dv (change in volume)

when volume changes the system does work equal to -p·dV

EQUATION: δW = -p·dV

What doe the sign of dV depend on (+-) ?

- Whether system expands or contracts

- expands (system does work on surroundings ) = energy leaves w<0

-Compression (surroundings does work on the system) =energy enters w>0

What is the equation that links Enthalpy (H) to Work (W) and Internal energy (𝞓E)?

at constant pressure :

derived from-

𝞓E=δQp+δW p

EQUATION :

𝞓E = ΔH -P ΔV

What is a Isometric process ?

A process under constant volume conditions ( also known as Isochoric ΔV = 0)

When do internal energy changes in a system occur at constant volume?

- only happens via HEAT

example: heating solutions in a rigid container

What is the equation that links enthalpy at constant volume conditions?

at constant pressure :

W=0 ,ΔV = 0

𝞓E = ΔH -P ΔV

-when constant pressure p=0

- EQUATION : 𝞓E =ΔH

What is the correlation /relationship between enthalpy and internal energy ? (GRAPH)

-Enthalpy (H) is always greater than internal energy (E) of the system

-the difference increases with temperature

𝞓H ≌ 𝞓E ( almost equal )

graph ( S shape graph

What are some examples where enthalpy can be applied in Pharmacy?

Enthalpy changes occur in :

-dissolution of a solute

-the formation of micelles

-chemical reaction

-adsorption on to solids

-vaporisation of a solvent

-hydration of a solute

-neutralisation of acids and bases

-melting or phase transition

-freezing of solutes

What is an endothermic reaction/process?

ΔH is positive when heat is absorbed/taken in ⟶ (endothermic)

What is a exothermic reaction/process?

ΔH is negative when heat is released/given out ⟶ (exothermic)

Where does change in heat come from?

-Chemical reactions involve breaking old bonds and forming new ones.

-Energy is absorbed when bonds are broken and released when new bonds are formed ( because the bonds are more stable so release energy in process).

How do you calculate heat of reaction from bond energies (ΔH)?

estimated using average bond energies:

ΔH reaction =∑(energies of bonds broken)−∑(energies of bonds formed)

A 52-year-old woman experiencing hot flushes during menopause comes to the pharmacy. She has bought a chemical cold pack (ammonium nitrate) and asks:"If I keep this pack in the freezer, will it work better when I have a hot flush?

-no need to freeze

as they could lose the separation between the two reservoirs

how it works ?

-squeezing parts separates the reservoirs so gives a cooling effect

-ammonium nitrate mix with water = endothermic -absorb heat from surroundings =producing the local cooling effect

This ointment (Vaseline) feels very hardin the tub, but when I put it on my skin, it suddenly softens. Is something wrong with it ?

Change energy from the body absorbed so changes phase of ointment - liquid crystalline phase = soft

has a hydrocarbon mixture- broad temperature range

LINKED TO ENTHALPY OF PHASE TRANSITIONS

What is specific heat capacity (Cp) ?

Cp is a measure of how much heat energy (q) a substance must absorb to increase its temperature by 1 K at constant pressure.

Why is specific heat capacity (Cp) useful in Pharmacy ?

Knowing the cp helps in :

-designing efficient drying/coating processes

-maintaining stability during storage ,transport

-optimising energy use in manufacturing and sterilisation

NOTE- it can tell us how RESISTANT a substance is to temperature change when heat is added at constant atm pressure

How do you calculate Cp?

Cp=(∂ H│∂ T)p

-Change in heat divided by change in temperature at constant pressure-

∂ = partial differential (because H is a function of T and p).

What is the heat of neutralisation when you use a strong base and strong acid ?

- this value is constant for all strong acids - strong base reactions because they are completely ionised in water

ΔH = -13.6 kcal

What is the heat of neutralisation when you use a weak acid and a weak acid ?

-this value is less negative than -13.6 kcal

-because part of the heat released goes into ionising the weak electrolyte as weak acids are only partly ionised

What does heat of neutralisation take into account?

The measured enthalpy includes both ionisation + neutralisation steps

NOTE- this can be used to help differentiate between strong and weak acids/bases

What is Phase transition?

Phase transition = change of matter from one physical state to another (solid → liquid).

-this involves change in enthalpy ,due to molecular rearrangement requiring /releasing energy

How do you measure Phase changes

-Differential Scanning Calorimetry (DSC) measures temperature and heat flow during phase transitions,

- by having a reference ( empty to measure temp change )

-linking these changes to enthalpy variations

- detects exo,endo

What are further applications of enthalpy in Pharmacy ?

❑ Precipitation -ReactionsThermochemical data help predict if a reaction, like forming a precipitate, is energetically favourable.

❑ Heat of Mixing

-Used to study energy changes when two solutions are combined, important in drug solubility studies.

❑ Calorific Value of Foods

-Thermochemistry is used to determine the energy content of foods and nutritional formulations by calculating the number of calories released during combustion.

Summary :

-Enthalpy change (ΔH) represents heat transfer at constant pressure; +ΔH =endothermic, -ΔH = exothermic.

❑ ΔH links heat flow to internal energy and work, crucial for describing chemical and physical processes

.❑ Applications: Used to analyse reactions, phase transitions, and stability of systems in thermodynamics.