QCE Chemistry Year 2016 Mark 0.00 Pages 21 Published Jan 31, 2017 12 Chemistry: Revision Notes By Sophie (1 ATAR)
Powered by TCPDF (www.tcpdf.org) Your notes author, Sophie. Sophie achieved an ATAR of 1 in 2016 while attending Shalom Catholic College Currently studying Bachelor of Science at The University of Queensland Achievements: Attended National Youth Science Forum 2016 Biology Subject Prize - 2016 Chemistry Subject Prize - 2016 Sophie says: Hi! I am an enthusiastic learner and in Years 11 and 12, I spent a lot of time making revision notes to make sure I was prepared for exams. The way you revise is completely up to you, however, I hope you can get a head start through my notes. Best wishes for your endeavours over the next years, Sophie.
The reason behind reverse reactions Two drives in a reaction to reach a state where: - The lowest amount of energy is required - The highest amount of chaos (entropy) States of lower energy - Exothermic energy released as heat, less energy in product than reactants - Endothermic heat energy taken in, more energy in product than reactants - Since lower energy is favoured, an exothermic reaction occurs more easily than the reverse If lower required energy (enthalpy) cannot explain why the reaction occurred, then chaos should. Reactions have a desire for increased chaos or entropy (randomness) - The states from lowest to highest entropy: - Solid liquid gas - So a reaction that involves the production of a gas from a liquid occurs more easily than the reverse If, in a reaction, the drives of enthalpy and entropy go in the opposite directions, the reaction favours the strongest drive. Reverse reactions Dynamic equilibrium: - Equal balance between the forward and the reverse reactions - E.g. when a solid is in contact with a saturated solution, there is a dynamic balance between the dissolution and the precipitation - Equilibrium reactions are written as below: Pb(NO 3 ) 2 (s) Pb 2+ (aq) + 2NO 3 - The two-way arrow indicates that both processes are occurring - The precipitation will occur when the concentrations of the relevant ions in the solution are greatly above what they should be in a saturated solution - This continues until the concentrations of the ions fall back to the saturated solution levels - However, at this point, ions continue to dissolve and precipitate in equal numbers. This is a reversible reaction.
Why do reverse reactions come to equilibrium? - Reaction rate decreases as the concentration of the reactants decreases (as they are used up in the forward reaction) - Initially there is no product for a reverse reaction to occur it begins once the product concentration increases - Hence, the forward reaction slows and the reverse reaction speeds up until the rates are equal - Once the rates are the same, there is no net change and therefor the reaction rates can t change the reaction reaches a dynamic equilibrium The carbon dioxide/water equilibrium CO 2 + H 2 O (l) H 2 CO 3 - This is also about the solubility of CO 2 in water - Solubility increases as the pressure of the CO 2 increases - Solubility decreases as temperature increases - These things can be regarded as moving the position of equilibrium - If the equilibrium lies to the right, at equilibrium, most of the reactant has been converted to product - If the equilibrium lies to the left, at equilibrium, most of the reactant will still be present and only a small amount converted into product Le Chatelier s Principle - A simple generalisation for predicting how an equilibrium will respond to a disturbance (in the force hahaha) - The principle is: o If a system at equilibrium is disturbed, the system will adjust itself so as to minimise the disturbance - An equilibrium is disturbed if: o The concentration of one or more of the species is changed o The total pressure upon a reaction involving gas is changed o The temperature is altered - An equilibrium involving a pure solid and an aqueous solution or a pure solid and a gas mixture is not disturbed by adding more pure solid to it. This is because the amount of solid has changed, not the concentration. Template paragraph for exam response The equilibrium is disturbed when is removed/added/changed. Le Chatelier s principle acts to reduce the impact of this disturbance by producing more/less. To do this, is/are converted to. This establishes a new equilibrium in favour of the forward/reverse reaction.
Powered by TCPDF (www.tcpdf.org) Thought process for Le Chatelier s principle problems When you re told what the disturbance is, think: what is the - Stress: What has created the disturbance - Fix: What needs to happen for the disturbance to be minimised - How: What reaction/s need to take place in order for this to happen - Effect: What is the result of this? How have the amounts of reactants/products changed? Where is the new equilibrium with respect to the old one? When you re told the desired effect: - Effect: Where is the desired equilibrium? What are the desired amounts of reactants and products? - How: What reactions need to take place in order for this to happen? - Fix: What stress could these reactions be a result of? - Stress: What is the stress required to bring about the desired result? Le Chatelier s Principle and the CO2, H2CO3 equilibrium (That almost sounds like a Harry Potter book title) Pressure change in the case of gases, this is a concentration change - If more CO 2 is added to the reaction vessel, more dissolves into the water to even out the concentrations - This results in a new equilibrium being established to the right (in favour of the forward reaction) - If the pressure of the CO 2 is decreased, more of the H 2 CO 3 will decompose in order to fix up the ratio/concentration. - The new equilibrium is further to the left (favouring the reverse reaction) Total pressure change - If the total pressure is increased, the reaction that reduces the pressure will be favoured - Because liquids can t be pressurised by gases can, some of the CO 2 will dissolve into the water, meaning that the equilibrium is shifted to the right. - Also, the number of moles of the products of the forward and reverse reactions can affect which direction will reduce pressure - E.g. 2SO 2(g) + O 2(g) 2SO 2(g) - There are 3 moles of reactants and 2 moles of product. 2 moles of gas creates less pressure than 3 moles of gas, therefore, the forward reaction would be favoured to reduce the pressure. - If the total pressure is increased through the addition of a gas that was NOT originally involved in the reaction, this would have NO EFFECT on the equilibrium position because the pressure of the original gas/es would not have changed.