Thermodynamics

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  • Created by: LBCW0502
  • Created on: 24-02-18 13:39
What is thermodynamics?
Branch of science concerned with heat in relation to energy and work. Defines macroscopic variables that characterise materials. Describe average behaviour or very large numbers of microscopic constituents and laws derived from statistical mechanics
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What is energy?
The capacity to do work (thermodynamics is concerned with conservation of energy and energy transfer)
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What are the two forms of energy?
Heat (chaotic) and work (organised)
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In chemistry, what does energy determine?
What molecules can form, what reactions can occur and when equilibrium is reached
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What is the relevance to pharmacy?
Able to predict properties
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Give examples of properties which can be predicted
Solubility/stability of drug molecules. Stability of drug formulations. Phase behaviour. Partitioning. Micellisation. Drug-receptor interaction. Chemical reactions (e.g. equilibrium composition, yield)
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What are the four thermodynamic functions?
Internal energy (U), enthalpy (H), entropy (S) and Gibbs free energy (G)
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What is the zeroth law of thermodynamics?
There is a unique scale of temperatures (all heat is of the same kind)
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What is the first law of thermodynamics?
The energy of an isolated system is constant
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What is the second law of thermodynamics?
When two systems are brought into thermal contact, heat flows spontaneously from high temperature to low temperature. Entropy of an isolated system increases during any spontaneous change or process
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What is the third law of thermodynamics?
All perfect materials have the sae entropy at T = 0 (at S = 0). At higher temperatures, S is always positive. It is impossible to cool any system to T = 0 (temperature in Kelvin)
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What is a system?
The region of interest e.g. reaction vessel characterised by properties such as temperature, pressure, volume, composition
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What are the surroundings?
Region outside the system, sometimes where we make our measurements. This is separated from the system by a boundary
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What is an open system?
Both energy and matter can be exchanged between the system and its surroundings
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What is a closed system?
Only energy can be transferred
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What is an isolated system?
Neither energy nor matter can be exchanged
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Describe features of equilibrium
State of a system in which properties have definite, unchanged values as long as external conditions are unchanged e.g. thermal, mechanical, chemical. Two systems in thermal equilibrium will be in equilibrium with a third system
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How can energy be transferred?
As heat or work (or both). A system interacts with its surroundings by exchanging heat and work
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Describe features of heat
Transfer of energy due to difference in temperature between system and surroundings (-q/transfer from system to surroundings which decreases U, +q/transfer from surroundings to system which increases U). Achieves disorderly motion in surroundings
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What does the zeroth law define?
The property of temperature and describes its behaviour (the property of temperature exists in every thermodynamic system in equilibrium). Equality of temperature is a necessary condition for thermal equilibrium (regardless of unit e.g. Kelvin)
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Describe features of work
Any other means of changing the energy of a system. Achieve uniform motion in surroundings. Motion against an opposing force
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What is the expression for work?
Work = distance x opposing force
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What is the unit for energy?
Joules (J). 1 kg m^2s^-2
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Describe the work of expansion with constant pressure
W = -F delta z, W = - (external pressure x area) x delta z, W = -(external pressure) x change in volume. (e.g zinc and hydrochloric acid)
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Describe features of internal energy
Total energy of a body's components. Locked up kinetic and potential energy. Energy cannot be seen or measured. Only changes are measured. Change in U = U of final state - U of initial state
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What is an exothermic reaction?
A process which releases energy as heat
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What is an endothermic reaction?
A process which absorbs energy as heat
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How does the first law of thermodynamics relate to internal energy?
Energy of an isolated system is constant. Energy cannot be created or destroyed. Energy converted or transferred. Change in U = (d)w + (d)q
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What is a state function?
E.g. U - depends on the current state not the path followed
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What is an intensive property?
Property which doesn't depend on the number of molecules (due to only focusing on moles). Um = U/n. E.g. temperature, permittivity, molar heat capacity
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What is an extensive property?
Property which depends on the number of molecules. E.g. volume
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What is heat capacity?
The amount of energy as heat dq that a material or body must absorb for its temperature to increase by dT. Cm = C/n
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What is the significance of enthalpy, H?
Most system perform expansion work. Calculating the change in internal energy is difficult. Use enthalpy to get rid of expansion work term
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What is the definition of enthalpy, H?
H = U + pV (delta H = q at constant pressure)
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Is H an extensive or intensive property?
Extensive
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Is Hm = H/n an extensive or intensive property?
Intensive
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Is H a state function?
Yes
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Can you relate delta H to Cp?
Cp = q/delta T
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Does an endothermic reaction result in the increase or decrease in enthalpy?
Increase
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What are the uses of enthalpy?
Dissolution of a drug, formation of micelles, chemical reactions, absorption onto solids, hydration of a solute, phase transitions (melting, freezing, condensation)
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What is thermochemistry?
Branch of thermodynamics which studies heat produced by physicochemical processes in particular allowing the calculation of enthalpies of chemical reactions
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What is a phase?
A specific state of matter that is uniform throughout its composition and its physical state
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What is a phase transition?
Conversion of one phase of a substance to another phase
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What is a physical change?
E.g. vaporisation of a substance is an endothermic process because heat must be supplied to bring about the change.
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What is the enthalpy of vaporisation?
The energy must be supplied at a constant pressure per mole of molecules that are vaporised
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What is the standard state of a substance?
The pure substance at exactly 1 bar
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What are the standard conditions?
Pressure of 1 bar (10^5). Temperature (298K) and specify phase (solid/liquid/phase)
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What is the standard reaction enthalpy?
Changing in enthalpy that occurs when reactants in their standard states change into products in their standard states (e.g. formation of water, different crystalline forms)
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What is the enthalpy of vaporisation?
Change from liquid state to a gas state (higher enthalpy of vaporisation for water - hydrogen bonding). Fusion < vaporisation
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What is the enthalpy of fusion?
Melting. Smaller values (less energy required). Solid - closer together (smaller step when converted to liquid). Liquid - molecules close (far apart when converted to a gas)
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What is the enthalpy of sublimation?
Change from solid to gas. Enthalpy of sublimation = total of enthalpy of fusion and enthalpy of vaporisation (at the same temperature)
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Is the change in delta H in the forward reaction equal to negative of that of the reverse reaction?
Yes
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Is the delta H of the direct reaction equal to the delta H of the indirect reaction?
Yes
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What is Hess' Law?
The standard enthalpy of an overall reaction is the sum of the standard enthalpies of the individual reactions into which the reaction may be divided
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What are some changes spontaneous?
Matter and energy tends to become disordered
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What is entropy?
A measure of disorder of matter and energy (link to second law - the entropy of the universe tends to increase)
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What is the criterion for a spontaneous process?
Delta S total = (delta S system + delta S surroundings) > 0
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What are the properties of entropy?
It is a state function, it is an extensive property. Units of J/K. At T = 0, S = 0 (crystals) at higher T, S > 0
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Describe the features of entropy
The change in entropy of a substance is equal to the energy transferred as heat to it reversibly divide by the temperature at which the transfer takes place
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What is reversibility?
Ability of an infinitesimal change in a variable to change the direction of a process (smooth, careful, restrained) - very close to equilibrium
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What is the effect of temperature?
Temperature is responsible for the level of disorder. Higher temperature, lower entropy (takes into account initial level of disorder)
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How does S vary: when a gas expands, with temperature, with a phase transition (e.g. melting)?
S increases, S decreases, S increases for sublimation and melting but S decreases for condensation and freezing
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What is the entropy of fusion (melting)?
Transfer into system (sample melts). Transfer out of system (sample freezes). At constant pressure q = delta H. Delta fus S = q rev / Tm = delta fus H / Tm
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What is the entropy of vaporisation?
Delta vap S = q rev / T vap = delta vap H / T vap
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What is the Gibbs free energy?
G = H - TS (at constant temperature and pressure delta G = -T delta S total)
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What are the properties of Gibbs energy?
A spontaneous process leads to an increase in entropy in the universe (>0). This leads to a decrease in Gibbs free energy
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Why do water and oil not mix?
Oil droplets/hydrophobic and water molecules/hydrophilic. HCs in oil cannot form H bonds with water molecules. Disrupts strong H bonds between water molecules. Decrease in S, increase in G. Phase separation
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What does the value of delta G for a process give?
The maximum amount of non-expansion work extracted from the process at constant temperature and pressure
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What is non-expansion work?
Any other other than that arising from expansion of the system e.g. electrical, mechanical
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Describe features of chemical equilibrium
Condition for spontaneous change at constant temperature and pressure (delta G < 0). A constant temperature and pressure, a reaction mixture tends to adjust its composition until its Gibbs energy is a minimum - relation to yield
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What is chemical potential (mu)?
Partial molar Gibbs energy (used in derivative)
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What is the equation for the standard reaction Gibbs energy?
Delta r G prime = - RT lnK (proof)
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If the products are dominant, K > 1 (ln K), G < 0, is the reaction thermodynamically feasible?
Yes
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Is the reactants are dominant at equilibrium, K < 1 (-ln K), G > 0, is the reaction thermodynamically feasible?
No (also look at thermodynamic criteria of spontaneity)
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What is the effect of temperature?
Approximation - standard enthalpy change of reaction and standard entropy change of reaction do not vary with temperature - formation of Van't Hoff equation
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