W3-Thermodynamics.pptx
Transcript
PY4040: Introduction to Physical Chemistry for Pharmacy TW4: Thermodynamics I Dr Gemma Shearman 1 Learning Outcomes • To appreciate the importance of thermodynamics to Pharmacy • State the first law of thermodynamics • Define and carry out simple calculations involving heat, work and interna...
PY4040: Introduction to Physical Chemistry for Pharmacy TW4: Thermodynamics I Dr Gemma Shearman 1 Learning Outcomes • To appreciate the importance of thermodynamics to Pharmacy • State the first law of thermodynamics • Define and carry out simple calculations involving heat, work and internal energy • Identify the differing types of systems (and surroundings) • Calculate work done for: o expansion of gases against a constant pressure o isothermal reversible expansions of ideal gases 2 Useful for Pharmacy? Knowledge of thermodynamics is important in a number of areas of pharmacy: • • • • Drug-receptor interactions Drug stability Drug dissolution i.e. solubility (vs. dissolution rate = KINETICS) Transfer of drugs across membranes / other partitions e.g. blood/gas partition coefficients describe how easily anaesthetics pass from gas into the blood 3 Case study: Ritonavir Ritonavir: an antiretroviral medication used to treat HIV (with others) Entered the market in 1996 (in oral capsule form) BUT was removed (temporarily) in 1998 as it was found to undergo conversion from one polymorph (form I) to another (form II)! Form II was more thermodynamically stable BUT much less soluble (and hence less bioavailable)… not good. 4 Thermodynamics? Thermodynamics considers the different forms of energy involved in any process of change and the behaviour of macroscopic systems in terms of certain basic quantities, such as pressure, temperature, etc. energy? 5 Molecular energy All chemical energy changes can be related to the potential and kinetic energies of the molecules present. Potential energy can essentially be considered as ‘stored energy’. Here, the weight has been lifted against the earth’s gravitational field, hence it has acquired potential energy. The stored energy is then released in the form of kinetic energy (which relates to objects in motion) as it falls. 6 First law of thermodynamics Crucially, although the energy has been transferred into different forms, the total amount of energy is the same before, after and during the process… i.e.: Energy can neither be created or destroyed. This is the law of conservation of energy i.e. the 1st law of thermodynamics. 7 Internal energy From before: All chemical energy changes can be related to the changes in potential and kinetic energies of the molecules present. Q. In a molecule how are these manifested? Nuclear energy Inter / intramolecular forces Kinetic energies Potential energies 8 Internal energy The internal energy of a molecule is the total contributions from all the energies... Q. In a molecule how are these manifested? Nuclear energy Inter / intramolecular forces Kinetic energies Potential energies 9 Back to the 1st law of thermodynamics The law of conservation of energy: Energy can neither be created or destroyed. is very important in thermodynamics, because we must be certain that we can account for all the energy that flows into or out of a system… BUT, we need to define what a system is! 10 SURROUNDINGS Systems Systems: We refer to the object of our study as the system. Everything that is not the object is the surroundings. Energy is then transferred between the system and its surroundings via the boundary. SURROUNDINGS BOUNDARY SURROUNDINGS SYSTEM SURROUNDINGS 11 Systems Systems are classified into three groups according to the degree of contact with the surroundings. They are either: • Isolated systems - no energy or mass can be transferred to or from the surroundings. • Closed systems - energy can be transferred to or from the surroundings but not mass. • Open systems - both energy and mass can be transferred to or from the surroundings. 12 Spot the system (each box contains 1 girl) Surroundings System MATTER / ENERGY (WORK / HEAT) ? WORK / HEAT ? ? 13 Heat and Work The energy of a closed system can only be changed by transferring energy to or from the system as work or as heat. Strictly, both WORK and HEAT are modes of transfer of energy. 14 Image taken from: http://academy.justjobs.com/caption-contest-12/ Your turn… Q. It is proposed that the temperature of water (initially at RT) could be raised by either (a) vigorous stirring or (b) heating. Which option is true? A. B. C. D. Only (a) Only (b) Both (a) and (b) Neither (a) or (b) 15 Heat Heat, q: Heat, or thermal energy, can be thought of as relating to the motion of atoms and molecules… the more thermal energy a substance has, the more rapidly the atoms and molecules within it will move. Heating is the process of transferring energy as a result of a temperature difference between the systems and its surroundings. 16 Heat The most common of units of heat are “calorie” The calorie (or gram-calorie) is the amount of heat required to raise one gram of water through one degree Celsius (e.g. from 14.5 oC to 15.5 oC). The conversion factor is: 1 cal = 4.184 J or 1 kcal = 4184 J Q. My Twix bar (pk. of 2) states the calorie content is 124 kcal. How much is this in Joules? A. 124 x 4184 = 518816 = 519 kJ (to 3 sf) 17 Work Work, w: A process that can be used directly to move an object a certain distance against an opposing force i.e. Work = Force x Distance If a force of 50 N acts on a body over a distance of 6 m then the work done is: 50 x 6 = 300 N m = 300 J 18 Change in Internal Energy We now define and as follows: • is the energy supplied to the system as work • is the energy supplied to the system as heat Hence: 19 Your turn… positive vs. negative signs In each of the scenarios below, identify whether the energy of the system (here, the person in each case) is increasing or decreasing… i.e. identify whether and/or is positive or negative for each picture! 20 Work: chemical systems Many chemical reactions (e.g. drug decomposition) produce gaseous products. It is therefore important to consider EXPANSION work – the work done when a system expands against a constant external pressure, pex. Work done (by the system) = force x distance = (pex A) x distance = pex Ah = pex DV w = - pex DV (remembering that work is work done TO the system) 21 Work: chemical systems If a system expands or contracts due to a change in number of gas molecules (assuming temperature remains constant i.e. isothermal and the external pressure also remains constant) then the work done (to the system) is: w = - pex DV = - D(n)RT (from pV=nRT) So, as an example, if a liquid drug (2.45 moles) evaporated (at atmospheric pressure and room temperature), we know there is expansion work done caused by an increase of 2.45 moles of gas (from liquid), so: w = - D(n)RT = -2.45 x 8.314 x (25+273.15) = -6073 J = -6.07 kJ 22 Work: chemical systems So, as an example, if a liquid drug (2.45 moles) evaporated (at atmospheric pressure and room temperature), we know there is expansion work done caused by an increase of 2.45 moles of gas (from liquid), so: w = - D(n)RT = -2.45 x 8.314 x (25+273.15) = -6073 J = -6.07 kJ i.e. energy in the form of work has been lost from the system. Q. Can you therefore also say that the internal energy of the system is lowered following the process of evaporation? 23 Minimum work Let’s imagine that 1 L of a gas, initially at a pressure of 10 atm, is allowed to expand into a volume of 10 L in an evacuated space (assuming a constant temperature). Q. What is the work done on the gas? Hint: what is the external pressure?? 24 Maximum work When an ideal gas expands reversibly without any change in temperature, its internal energy must remain constant i.e. . NB. pressure here is not constant. The work done: This is only applicable for reversible systems! 25 Work: a summary (for chemical systems) Work is something done TO a system (so in terms of sign, think about whether the system has gained or lost energy). Use: for expansion (or contraction) work at constant external pressure. Use the variation: w = - D(n)RT if there is a change in no. of moles of gas. If the external pressure is equal to 0, so is the work done! Use: ONLY if the question states ‘reversible’ and does not mention a constant pressure. 26 Reading list • ‘Chemistry3’ by Burrows, Holman, Parsons, Pilling and Price 27
