The First Law of Thermodynamics Conservation of Energy

  • Slides: 7
Download presentation
The First Law of Thermodynamics Conservation of Energy in Disguise

The First Law of Thermodynamics Conservation of Energy in Disguise

The Zeroth Law l l l Came after the others; that is why it

The Zeroth Law l l l Came after the others; that is why it is zero. Statement of thermal equilibrium. If A is in thermal equilibrium with B and B is in thermal equilibrium with C; then A and C are in thermal equilibrium.

The First Law l l l A Statement of Conservation of Energy. ΔE =

The First Law l l l A Statement of Conservation of Energy. ΔE = q + w E is the internal energy q is the heat put INTO the system w is the work done ON the system. Any change in energy of a system must come in one of two forms: work and heat.

Sign Conventions of the First Law l l l Always from the point of

Sign Conventions of the First Law l l l Always from the point of view of the system. Any energy INTO the system is + and any energy out of the system is -. q > 0 heat put INTO the system q < 0 heat leaving the system w > 0 work ON the system w < 0 work done BY the system

Chemists Rarely Do Work! l l Physical work that is… When pressure is constant,

Chemists Rarely Do Work! l l Physical work that is… When pressure is constant, there is no physical work. ΔE = q (if atmospheric pressure is constant) When only heat is transferred, we give it a special name.

Enthalpy Defined! l l Enthalpy (DH) is heat transferred at constant pressure! DH =

Enthalpy Defined! l l Enthalpy (DH) is heat transferred at constant pressure! DH = q (First Law of T’Dyn) It is a STATE FUNCTION – it depends only on the start and end point. Elevation is state function, distance is not.

Endothermic and Exothermic l l Endothermic = heat enters the system. (q > 0

Endothermic and Exothermic l l Endothermic = heat enters the system. (q > 0 DH > 0 ) Exothermic = heat leaves the system. (q < 0 DH < 0)