Physical Biochemistry - Thermodynamics

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Thermodynamics

Thermodynamics - the science of energy transfer. This is generally applicable but molecularly non specific. Can tell you whether a reaction will take place but not tell you how fast it will occur.

Heat - is a form of energy caused by molecular motion.

Temperature - describes the propensity of heat flow from one body to another.

• Zero on the thermodynamic temperature scale is the zero of molecular motion.
• Measured in Kelvins (K) whereby 0 celsius = 273.15K

System - collection of matter under study

Surroundings - environment in contact with system that may influence its behaviour

State function - anything that depends on state of system and not how it got there.

• Temperature is an intensive state function as it does not depend on the size of the system.
• Mass is an extensive state function as it depends on the size of the system.

Isolated system - no matter or energy can move in or out of the system

Closed system - energy can move in and out of the system but matter cannot

Open system - matter and energy can both move in and out of the system

• Both work (w) and heat (q) are forms of energy and are therefore expressed in Joules (J)
• Most things that happen involve changes in both heat and work

Work (w) = -force x distance

= -mass x accel x distance

• Note minus sign - work performed on a system by the surroundings is positive, work performed by a system on surroundings is negative

Work is defined by the force acting against the process - so external pressure (Pext) needs to be considered.

• So now, w= - Pext ΔV

1st Law of Thermodynamics -  the algebraic sum of all energy changes in an isolated system

Energy cannot be created or destroyed, but merely transferred.

Seven main forms of energy - electrical, gravitational, chemical, radiation, thermal, mechanical, nuclear.

Electrical to mechanical - motors

Gravitational to mechanical - falling weight

Thermal to radiation - light bulb

Nuclear to thermal - atomic fission

Different forms of energy are interconvertible.

Internal energy (U) - U = w + q

In an isolated system ΔU = 0, all the energy inside will remain constant.

Calorimetry (measurement of heat change) does not take into account of PextΔV so cannot tell us the change in internal energy ΔU, instead it measures enthalpy change ΔH.

• ΔU = ΔH - PΔV

numerically ΔU and ΔH are very similar as PextΔV tends to be a small number.

Enthalpy,