Internal Energy, Enthalpy, and Hess's Law

flash cards from lectures 3,4 of thermo

Describe what is internal energy

Internal Energy, U, is a measure of all energy reserves in a system

U is constant in a closed system

What is the equation for ΔU

ΔU = U final - U initial

Define enthalpy change of formation and its most stable state

Enthalpy changes of formation, ΔfH, When 1 mol of substance is formed from elements in their reference states, at a stated temperature (usually 298.15 K). The reference state is the most stable at STP

What are bond dissociation energies

Bond Dissociation Energy (BDE) is defined as the enthalpy change when a specific bond in a compound dissociates (is separated to infinite distance) 

What are mean bond energies

Bond Energy (BE) or mean bond energy refers to the mean of the BDE values of similar bonds within a specific compound. 

What are tabulated average bond energies

Tabulated Average Bond Energies (ABE) usually apply to a type of bond across a range of related compounds.

For example C-H has an ABE of +410kJ mol^-1.

This applies value is used for any type of molecule with this bond. As such, any equation using these values are estimations. 

What are state functions

values which depend on the current state of a substance.

What does a state function depend on

A state function depends only on the current state of the system, as defined by the temperature and pressure. 

Any change in state function only depends on initial and final result

How does Enthalpy, H, being a state function explain Hess’s Laws

Standard enthalpy change for a reaction is the sum of the standard enthalpy changes for the the reactions which it can be divided.

As such the pathway in which a reaction happens does not matter as long as it is the same reaction the enthalpy change is the same.

What is the zeroth law

"If two systems are each in thermal equilibrium with a third, they are in thermal equilibrium with each other" 

What’s the difference between ΔU equations for open system and in a Bom calorimeter

In open systems its ΔU = q + w as both heat and work interact with the system and can enter and exit

In closed systems its ΔU = q as w=0 and heat does not escape due to insulation

Describe how we find Energy Densities and its units

-Energy densities (in MJ kg^-1) 

• Energy produced = -Δc H = 393 kJ mol^-1 

• Energy density = energy produced/RMM 

• More energy per weight 

Describe how we find energy densities by volume and its units

-Energy densities by volume (in MJ m^-3) 

• Energy Density x Density of molecule = density by volume 

• More energy per volume 

Describe the advantages of hypergolic fuels for rockets

Clear advantages being : 

• No need for air 

• Liquid at 290 k, easy for storage 

• Near-instant conversion of chemical energy to heat and work 

• No need for ignition control 

Describe disadvantages for hypergolic fuel for rockets

Clear disadvantages being : 

• Highly corrosive and toxic 

• Needs more research to fully understand