Thermodynamics Past Paper PDF

Thermodynamics Learning outcomes: 1. To know the four laws of thermodynamics (zero to three). 2. To understand the derivation of Gibbs free energy. 3. To understand how and why the Gibbs free energy determines the direction of reactions including the terms “exergonic” and “endergonic”...

Thermodynamics Learning outcomes: 1. To know the four laws of thermodynamics (zero to three). 2. To understand the derivation of Gibbs free energy. 3. To understand how and why the Gibbs free energy determines the direction of reactions including the terms “exergonic” and “endergonic”. Thermodynamics allows us to determine the direction in which changes due to reactions or other processes are favoured and to calculate their energy requirement or the energy made available for other processes. The Zeroth Law of Thermodynamics Initially overlooked because it seemed so obvious. “Two systems in thermal equilibrium with a third system are in thermal equilibrium with one another.” Or – “If objects are the same temperature then there is no net heat flow between them”. The Third Law of Thermodynamics. “The entropy of all systems tend towards a minimum at a temperature of absolute zero”. “Entropies are equal and zero at absolute zero”. “The lowest possible temperature is absolute zero which is 0 K or -273.15 °C”. What this means to you is that you must remember to add 273 K to temperatures in °C for thermodynamic calculations The First Law of Thermodynamics. “The internal energy of an isolated system is constant”. “Energy can change in form but cannot be created or destroyed”. Or – “The best you can do is break even.”  E = mc2 http://en.wikipedia.org/wiki/Atom_bomb If we consider a change (Δ) in a system, then according to the first law of thermodynamics, any change in the total energy of the system (U) must be accounted for by energy entering or leaving the system. Therefore the change in U (U) must equal the change in heat energy (q) plus the work done on the system (w). This gives us: U = q + w (sometimes  is used in place of Δ for small changes) Note that if the heat energy leaves the system (for example if a reaction is exothermic and heats up its surroundings), q is negative. Likewise, if energy leaves the system (i.e. it does work on its surroundings), w is negative. The first and second laws of thermodynamics are why perpetual motion machines are impossible. “We obey the Laws of Thermodynamics in this house”. http://www.cyberrock.net/homerisms.html Homer Simpson - In This House We Obey The Laws of Thermodynamic s - YouTube The Second Law of Thermodynamics. “The entropy of an isolated system tends to increase”. “Heat cannot of itself pass from a body to a hotter body” “Disorder increases”. Or – “You can’t even break even”.  The Second Law of Thermodynamics During every energy transfer or transformation, some energy is unusable, and is often lost as heat According to the second law of thermodynamics: – Every energy transfer or transformation increases the entropy (disorder) of the universe Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Entropy is most frequently described as being “disorder”, but thinking in terms of probabilities is often more useful. The right hand situation is more probable and therefore of higher entropy. Intuitively it might seem that the right hand situation is more “ordered”. However, it is clear that random movement is more likely to cause the left hand situation to change to the one on the right than for the opposite change to occur. A liquid is of higher entropy than a solid, and a gas is of higher entropy still The Second Law of Thermodynamics. “The entropy of an isolated system tends to increase”. “Disorder increases”. Osmosis One of my favourite examples of the relationship between Free Energy and Entropy is a rubber band. Chemical reactions that increase the number of molecules and species increase entropy because it is less likely that a larger number of molecules will come together for the reverse reaction. A→B+C is more likely than: B+C→A because the latter requires B and C to collide in the right way. As many of you know, processes that release heat energy (are exothermic) often occur spontaneously. However, this is not the whole story. Some endothermal or “neutral” processes occur spontaneously. http://commons.wikimedia.org/wiki/Image:NCI_iced_tea.jpg Examples include melting of ice, And expansion of a gas. Spontaneous Reactions To explain why reactions occur spontaneously we need to look at the second law of thermodynamics.  Free Energy, Stability, and Equilibrium Free energy is a measure of a system’s instability, its tendency to change to a more stable state During a spontaneous change, free energy decreases and the stability of a system increases Equilibrium is a state of maximum stability A process is spontaneous and can perform work only when it is moving toward equilibrium Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings We can develop this quantitatively if we consider the mathematical form of the Second Law of Thermodynamics. This is: S ≥ q/T Where S is entropy and T is temperature. And TS ≥ q (from multiplying both sides of S ≥ q/T by T) Note that this doesn’t stop entropy decreasing (negative S), but this can only occur if q is negative (i.e. the process is exothermic). Therefore, while a process is more likely to be spontaneous if it is exothermic so q is negative, but ultimately it is the relationship between S and q that is important. The state function H (enthalpy) is defined as the heat energy change at constant pressure*, so (from TS ≥ q) at constant pressure, this becomes: TS ≥ H *Mathematically H = U + PV TS ≥ H If we rearrange (subtract TS from both sides): 0 ≥ H - TS (from 0 + TS ≥ H) Now we define a new state function G (the Gibbs free energy) as: G = H – TS then at constant T and P: G = H - TS  0 ≥ G and we can say that G must always be negative (or zero) as otherwise the second law would be violated.  Gibbs Free Energy - G Determines the direction of change of a chemical or physical system. If G is positive, the process occurs in the opposite direction to that assumed in the calculation. If it is zero, the system is at equilibrium. A process with a negative G can do work and is described as “exergonic” from the Greek “ergon” meaning work. If the G is positive, then it is said to be “endergonic”.  Fig. 8-5 More free energy (higher G) Less stable Greater work capacity In a spontaneous change The free energy of the system decreases (∆G < 0) The system becomes more stable The released free energy can be harnessed to do work Less free energy (lower G) More stable Less work capacity (a) Gravitational motion (b) Diffusion (c) Chemical reaction SPONTANEOUS PROCESSES ARE EXERGONIC NOT EXOTHERMIC (necessarily) The American engineer, physicist and chemist Josiah Gibbs formulated this analysis of the ability of a system to do work. Although he understood the importance of what he had done, he made little effort to convince others and his achievement importance was only recognised later. Partly because mostly published in Transactions of the Connecticut Academy of Sciences, Not much read outside Connecticut Or in it. Processes can be driven by two things: 1.An increase in disorder 2.A release of heat energy In fact, the two are both aspects of the Second Law, as heat is a “disordered” form of energy with relatively high entropy. I am not going to ask you to remember or use this equation. For the reaction: A+B⇌ C+D A and B are the reactants and C and D are the products o  [C ][ D]  G G  RT ln    [ A][ B ]   o  [C ][ D ]  G G  RT ln    [ A][ B]  ΔG is a general term for free energy but ΔG° is the free energy under standard conditions – that is, with all reactants and products at a concentration of 1 M if they are in solution. Note also the “biochemical” standard states, where pH is 7.0 (a 1 M solution of H+ ions would be pH 0, not common in living systems). For G, this would be written ΔG°’ Life and Entropy Living organisms are highly ordered systems. The Second Law of Thermodynamics tells us that ordered systems tend to become less ordered. How do organisms cope with  G = H -TS A process occurs if G is negative. Entropy can decrease if the H term is sufficient to “neutralise” the S term. One example is ice freezing. The order increases, but enough heat is released for the process to occur. Note. Total entropy still increases because the heat is lost and cannot be recovered. Condensation reduces entropy (liquid water has lower entropy than water vapour) but is exothermic, So it can have a negative G How does this help? If there is a release of heat energy (negative H), then it is possible to protect ordered systems against entropy (maintaining negative S). The Sun is a Source of Free Energy http://www.solarviews.com/cap/sun/sun.htm The Second Law of Thermodynamics is not violated because a much greater entropy increase occurs in the Sun than the decrease due to life on Earth, (in time, the Sun will use all its fuel and go out). How is this energy converted to a usable form? Photosynthesis Harvests Free Energy © Brian Wilson Photosynthesis drives life on Photosynthesis converts oxidised carbon (CO2) to reduced carbon (sugar, CH2O). In an oxidising environment, energy can be released when the reduced substances are reoxidised, such as when carbon is oxidised back to CO2. This is respiration.  Biological Order and Disorder Cells create ordered structures from less ordered materials Organisms also replace ordered forms of matter and energy with less ordered forms Energy flows into an ecosystem in the form of light and exits in the form of heat Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Fig. 8-6b Products Amount of energy required Free energy (∆G > 0) Energy Reactants Progress of the reaction (b) Endergonic reaction: energy required  Fig. 8-6a Reactants Amount of energy released Free energy (∆G < 0) Energy Products Progress of the reaction (a) Exergonic reaction: energy released Fig. 8-6 Reactants Amount of energy released Free energy (∆G < 0) Energy Products Progress of the reaction (a) Exergonic reaction: energy released Products Amount of energy required Free energy (∆G > 0) Energy Reactants Progress of the reaction (b) Endergonic reaction: energy required Hydrolysis of ATP to ADP + Pi alters the position of an equilibrium by a factor of 108.  Phosphoanhydride Bond Hydrolysis The bonds between the phosphate groups of ATP’s tail can be broken by hydrolysis Energy is released from ATP when the terminal phosphate bond is broken This release of energy comes from the chemical change to a state of lower free energy, not from the phosphate bonds themselves Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  How ATP Performs Work The three types of cellular work (mechanical, transport, and chemical) are powered by the hydrolysis of ATP In the cell, the energy from the exergonic reaction of ATP hydrolysis can be used to drive an endergonic reaction Overall, the coupled reactions are exergonic Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Formation of Acyl-CoA is the first step in degradation of fatty acids. Go for this reaction is –33.6 kJ mol-1 (including hydroysis of the PPi) and is therefore strongly favoured. Without hydrolysis of the two phosphoric anhydride bonds (i.e., without ATP) Go would be +31.5 kJ mol-1 and would not occur. http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=stryer.figgrp.2210 Activation by Phosphorylation ATP can drive endergonic reactions by phosphorylation, transferring a phosphate group to some other molecule, such as a reactant The recipient molecule is now phosphorylated Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings http://www.uic.edu/classes/bios/bios100/lectf03am/sucrosepump.jpg Laws of Thermodynamics – YouTube From 2:06 MCQs 1) Which of the following statements is incorrect? a. There is a temperature at which entropy is zero b. Energy cannot be created. c. There are no upper or lower limits to temperature d. Total entropy can only increase. e. There is no heat flow between objects at the same temperature. 2) Which law of thermodynamics underlies each statement? a.There is a temperature at which entropy is zero. b. Energy cannot be created. c. Total entropy can only increase. d. There is no heat flow between objects at the same temperature. 2) Spontaneous processes are always: a.Exothermic b.Exergonic c. Explosive d.Endothermic e.Endergonic 3) What type of process is respiration ? a.Exothermic b.Exergonic c. Explosive d.Endothermic e.Endergonic 3) What type of process is ATP synthesis? a.Exothermic b.Exergonic c. Explosive d.Endothermic e.Endergonic 5) Which of the following statements are true? a.Gibbs free energy calculations allow us to determine the direction of reactions. b. Processes with a positive free energy can do work. c. Endergonic processes can be “driven” by coupling them to exergonic processes. d. Hydrolysis of ATP is an exergonic process. e. Hydrolysis of ATP creates energy for use in the cell.