W1-Ideal gases-Gemma.pptx

PY4040: The Science of Drugs An Introduction to Physical Chemistry for Pharmacy TW1: Ideal and Real Gases Dr Gemma Shearman 1 Learning Outcomes • State and describe the states of matter, and the relevance of intermolecular forces • Understand the importance of ‘gases’ to Pharmacy • Define a...

PY4040: The Science of Drugs An Introduction to Physical Chemistry for Pharmacy TW1: Ideal and Real Gases Dr Gemma Shearman 1 Learning Outcomes • State and describe the states of matter, and the relevance of intermolecular forces • Understand the importance of ‘gases’ to Pharmacy • Define and use the ideal gas law and the relationship between volume, pressure and temperature • Explain what constitutes ideal and real gases • Apply Dalton’s law for mixtures of gases 2 Classification of matter Three (common) forms of matter: SOLID LIQUID GAS 3 The force is strong… Q. Which one of the options below determines which state of matter a substance (e.g. glyceryl trinitrate… a covalent molecule!) takes? (assume SATP i.e. 25 oC, 1 atm) i. ii. Intramolecular forces Intermolecular forces Glyceryl trinitrate (nitroglycerin) is used to prevent agina! A. Intermolecular forces NB not to be confused with intramolecular forces (such as the covalent bonds themselves) that hold the molecule together. 4 The force is strong… There are attractive (intermolecular) forces between covalent molecules which hold them together in the forms we see around us. • Dipole - dipole (incl. hydrogen bonding) • Dipole - induced dipole • Instantaneous dipole – induced dipole (London dispersion) Also ionic forces for ionic species (incl. ion-dipole)! Sodium salicylate is an NSAID Q. Which solvent is sodium salicylate more soluble in: diethyl ether or water? Why? 5 The force is strong… Q. CT question: A three-year old girl was abducted (she was pulled into a car) and murdered. Although a suspect pleaded guilty, no fingerprints were found subsequently in the car (processed only 4 days after the abduction). Why?* 6 * Noble, D. (1995), “Vanished into thin air: the search for children’s fingerprints”, Analytical Chemistry, Vol. 67, pp. 435A‐8A. Classification of matter: MACROscopic A GAS is a fluid state of matter that fills the container it occupies A LIQUID is a fluid state of matter that possess a well-defined surface A SOLID retains its own shape independent of the shape of its container In a fluid (both gases and liquids), the molecules have enough energy to overcome the attractive forces between each other. This means that they can move past one another, and the substance flows. 7 Classification of matter: MICROscopic SOLID LIQUID GAS Particles: Particles Particles: • are packed together in regular way; • vibrate on the spot. • randomly arranged but close together; • move past each other (but collide often). • are far apart; • move rapidly and randomly. 8 Some useful things to remember Some useful conversions are: Some useful constants are: C = ( + 273.15) K NA = 6.022 x 1023 mol-1 1 L = 1 dm3 R = 8.314 J K-1 mol-1 1 atm = 1.01325 x 105 Pa 1 bar = 1.00000 x 105 Pa o Some useful multiplication factors are: 10-1 10-2 10-3 deci centi milli 10-6 10-9 micro nano Figure: The mole road map Credit: CK-12 Foundation - Christopher Auyeung License: CC BY-NC 3.0 9 Gases and volatile agents Q. Why? What is the relevance to Pharmacy? R. Encountered (by pharmacists) as: • • • • Anaesthetics (mainly general) Volatile drugs Aerosol propellants Propellants for needle-free injection technologies (gas powered) https://www.youtube.com/watch?v=pYA-exOR0uM • Other therapeutic medicinal uses e.g. in microbubbles 10 The ideal gas law As gas is the simplest state the equations describing its physical properties are the most straightforward! For a perfect (or ideal!) gas the equation of state is the perfect gas law (or ideal gas law): where is the gas constant, and has a value of 8.314 J K-1 mol-1. 11 Physical parameters Clearly physical parameters such as amount (e.g. mass), volume, pressure and temperature characterise the state of the sample. (SI) units enable us to quantify each parameter, so… Parameter Mass Volume Pressure Temperature Symbol SI unit kg m3 Named unit kilogram cubic metre Pa ( = N m-2) K Pascale Kelvin 12 Using the ideal gas law Q. Suppose I have 2.000 moles of xenon (an excellent, albeit pricey, general anaesthetic!) at room temperature (298.15 K) occupying a volume of 4.000 m3. What pressure is exerted? A. N.B. Think about significant figures.... 13 Some tricks of the trade Q. The performance of gas powered needle free injectors may be affected by temperature. In one such device, the pressure of helium in the canister was found to be 8.0 MPa at 0.00 oC. What is the pressure of helium at 40.00 oC? A. BUT and are all constant so rearranging to get all the constants together gives… and thus: Rearrange for the one you want: 14 Reality… and the potential energy curve Potential energy repulsions dominant Separation IDEAL ‘zone’ attractions dominant 15 Real vs. ideal gases Repulsive forces Attractive forces Attractive forces IDEAL GASES No molecular interactions • Typically for low pressures i.e. high separation between gas molecules REAL GASES Molecular interactions • Typically for high pressures i.e. low separation between gas molecules 16 The van der Waals equation of state The ideal gas law can be modified to take account of the effect of forces of repulsion and attraction between gas molecules… This is known as the van der Waals equation of state. The two constants, and take account of the deviations from ideality, where: • • term represents the attractive forces (thus lowering the pressure), and represents the ‘excluded’ space i.e. space occupied by the gas molecules! 17 The van der Waals equation of state Typical parameters obtained for real gases are: Substance / atm dm6 mol-2 / 10-2 dm3 mol-1 /K Ammonia 4.169 3.71 239.81 Argon 1.338 3.20 87.30 Helium 0.0341 2.38 4.222 Ethane 5.507 6.51 184.2 Q. Can you see any trends? What about the size of the molecules? 18 Gas mixtures Dalton’s law: The total pressure of a mixture of gases is equal to the sum of pressures exerted by the individual gases occupying the same volume alone (aka ‘partial pressure’). Figure: Dalton’s law Credit: CK-12 Foundation - Christopher Auyeung License: CC BY-NC 3.0 19 Gas mixtures: partial pressures Partial pressures can be calculated knowing the total pressure and the number of moles of each component… e.g. for gases A and B: Where and are described as partial pressures AND so: mole fraction of A, symbol: 20 Using Dalton’s law Q. The vapour pressure of water at body temperature is 47 Torr. Which one of the following is the partial pressure of dry air in the lung when the total pressure of (wet) air is 760 Torr?  A. B. C. D. E. 47 Torr 713 Torr 0.0618 Torr 61.8 Torr 807 Torr 21 Using Dalton’s law Q. ENTONOX is a 50:50 mol % mix of nitrous oxide and oxygen. It is typically supplied in cylinders at a pressure of 137 bar. Giving your answer in Pa, what is the pressure of oxygen?  A. B. C. D. E. 68.5 Pa 274 Pa 6.85 x 106 Pa 6.94 x 106 Pa 13.9 x 107 Pa ENTONOX is a painrelieving gas mixture and is often used in childbirth! Remember to convert: Try using: 1 bar = 1.00000 x 105 Pa 22 Reading list • ‘Elements of Physical Chemistry’ by P. Atkins and J. de Paula, 6th Ed. Focus 1 (pp. 3 – 36 incl.) Further reading (for context): • ‘Physicochemical Principles of Pharmacy’ by A.T. Florence and D. Attwood, 4th Ed. Chapter 1 NB. The 4th edition of this book is the last that covers this topic! You can access this edition online through iCat. 23