First Law of Thermodynamics

# First Law of Thermodynamics - Click to edit Master subtitle...

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Unformatted text preview: Click to edit Master subtitle style First laws of Thermodynamics • Energy and forms of energy- Energy-work-heat- System-surrounding-universe- work- heat • Laws of conservation of energy: The first law- Internal energy- Statement of the first law- Internal energy – work – heat- Partial differential definition of internal energy F Enthalpy- definition- relation to internal energy- partial differential definition of enthalpy l Thermochemistry- Hess’s law- Bond enthalpies- Enthalpies of formation- Temperature dependent Energy and forms of energy • Energy-work-heat Put Zn metal inside a container and pour in HCl (aq), H 2 gas will be generated according to reaction . Zn (s) + 2HCl (aq) ZnCl 2 (aq) + H 2 (g ) Case A If we put a movable lid on the container, the lid will be raised due to pressure generated by gas. h If the lid has mass m there will be the work done w = F) l = mgh Case B If we put a non-movable lid on and place the container in an ice bath, we will see ice melted. The ice bath temperature is raised. The heat is transferred from the container. Energy is the capacity to make change. change in position work change in temperature heat Energy from reaction comes from chemical bonds F System – surrounding -universe system Surrounding universe universe – everything, physical universe system - part of universe which have special interest surrounding – the rest of universe There can be energy and matters transferring from and to system. According to energy and matter exchange, systems can be classified into opened system : both energy and matters are exchanged (with surrounding) closed system : only energy can be exchanged isolated system : neither energy nor matter are exchanged • Work motion against an opposing force- mechanical work F m F l w = -F& l Opposing force has the same amount with force F but opposite sign (-F)- gravitational work w= -mgh Opposing force = -mg (gravitation) h- electrical work w = φ dq φ & electric potential q & charge- expansion work (PV work) A P1,V1,T1 P2, V2, T2 opposing force = P ex 1 l W = -F l = -(PexA) l l = -Pex &V = external pressure- Process work l T=0 constant T q=0 constant q p=0 constant p isothermal process adiabatic process isopiestic process reversible process can move back to original position (slow and infinitesimal change) Irreversible process cannot move back to original position (fast and large change)- Irreversible work (wirrev) P e x Pi,Vi, T Pf,Vf, T Pin = internal pressure Pin > P ex Pin = P ex wirrev = -P ex V = -P f V- Reversible work (w rev ) > To be reversible P ex must almost be equal to P in all the time > One-step expansion from V i V f where V f >> V i is not possible Ø Reversible expansion is multi-step process and for each step P ex P in = P (P ex varies) Ø To guarantee that P ex P in in each step, the piston should move by very small distance The charge in gas volume should be as small as possible or &V 0 = dV Ø For each step dw = -P ex dV = -PdV Ø For overall expansion Only true for ideal gas and isothermal expansion For ideal gas, one can replace P by P = nRT/V - Reversible VS irreversible work...
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First Law of Thermodynamics - Click to edit Master subtitle...

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