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Presentation On Reversible Reactions

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Published in: Chemistry | Science
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This PPT is suitable for the IGCSE and O Level Chemistry students. It only provides theoretical background of the topic. The worksheets and topical questions will be discussed during the tutoring sessions.

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  1. Reversible Reactions Chemistry (IGCSE / O-Level)
  2. What will you learn in this lesson? • Reversible reactions • Reversible hydration and dehydration of salts • Chemical equilibria • Chemical equilibria and reaction conditions
  3. Reversible Reaction A chemical reaction that can go depending on the conditions that reverse the reaction The symbol of reverse reaction is either means backwards, or forwards, products can interact to For e.g., formation of ammonia in industry using Haber's process + 3H2(g) Dehydration of hydrated salts CoCb.6H20 coC12 + 6H20
  4. Reversible Hydration of Salts Thermal decomposition of salts such as hydrated copper (Il) sulfate, CuS04.5H20 results in the dehydration of salt CuS04.5H20(s) light blue cr»täds -E4 white powder The dehydration of copper (Il) sulfate can be reversed by slowly adding the water to white anhydrous powder. The reaction is Adding water backto dehydrøtedcooøer(ll) sulfate, strongly exothermic.
  5. Reversible Hydration of Salts (contd.) Dehydration of cobalt (Il) chloride is similar to copper (Il) sulfate COCL Adding water to blue anhydrous cobalt (Il) chloride turns the solid pink. So, it is a reversible reaction .6H20 e CoC12+6H20 pink blue This property of hydrated salt is used to detect the presence of water in any experiment Purity of water can be tested by measuring the boiling point of water i.e. 100 oc Erthe cf using The to pink.
  6. Reversible Hydration of Salts (contd.) I powder? b in parta be testfor? c Wtiteabalancedsymbol equation for the reaction in part m 2 thata eoloud$5iüguid was pure water? chloridefrom blue chlorideis
  7. Chemical Equilibria A simple reversible reaction where we can see both reactions take place at the same time For e.g., sublimation of ammonium chloride (NH4Cl). When warmed in test tube, the white solid is decomposed to ammonia and hydrogen chloride However, on the cooler surface of the upper part of the tube, the white solid is re-formed:
  8. Chemical Equilibria (contd.) Closed System: A system where none of the reactants or products can escape the reaction mixture or the container where the reaction is taking place —e CA(S) OC02tg) of a dosed and for fre decomposition of calcium carbonate, If no gas is produces as the result of the reaction, equilibrium may be maintained in an open system such as flask
  9. Chemical Equilibria (contd.) Dynamic Equilibrium: Two chemical reactions, one the reverse of the other, taking place at the same time, where the concentrations of reactants and products remain constant because the rate at which the forward reaction occurs is the same as that of the reverse reaction At particular temperature, in a closed system we get the same fixed concentrations of hydrogen, iodine and hydrogen iodide whether we start from hydrogen iodide or hydrogen/iodine mixture
  10. Chemical Equilibria (contd.) Position of Equilibrium: Tells how far a reaction has gone in favor of reactants or products. If the concentration of products is greater, the position of equilibrium is to the right in favor of product If the concentration of reactants is greater, the position of equilibrium is to the left in favor reactants
  11. Chemical Equilibria (contd.) An equtlbrtum reaction has four particular features under constant conditions: • It's dynamic: are continuously being changed to products and products are continuously betngchanged beck to reactarts • the forward and reverse (backward) reactions occur at the same rate • the concentrations of reactants and products (the position of equilibrium) remain constant • it rewires a closed sysÄm. Analogy of dynamic equilibrium: Figure 9.7: Dynamic equilÜtum: a: the person and b: the aWearto be stationary.
  12. Chemical Equilibria and Reaction Conditions Le Chatelier's Principle: When the change is made to the conditions of a system in dynamic equilibrium, the system moves so as to oppose this change The change can be made in temperature, pressure and concentrations of reactants or products
  13. Chemical Equilibria and Reaction Conditions (contd.) Effect of Temperature on the Position of Equilibrium: Consider the following exothermic reaction, -197 kYm01 Increasing the temperature would encourage the system to absorb energy and position of equilibrium shifts to the left (in favor of reactants). Raising the temperature, in this case, favors the endothermic change where the heat is taken in For endothermic reactions, increasing the temperature shifts the position of equilibrium to the right (in favor of products). Decreasing the temperature, in this case, favors the exothermic change where the heat is given out
  14. Chemical Equilibria and Reaction Conditions (contd.) Effect of Pressure on the Position of Equilibrium: Changes of pressure only effects the presence of gas in a chemical reaction. Increasing the pressure moves the equilibrium to the side where volume is smaller (position of equilibrium is moved to right in the given equation) 2SOa(g) 3 moles of = 3 volumß 2 moles of = 2 vrhN1es This happens because increased pressure compresses the gas, pushing the gas molecules into a smaller space. The system responds to counteract that and shifts to the side that involves fewer gas molecules. 2H1(g) 2 Of gas 2 Of There is no effect of increasing the pressure because volumes are equal
  15. Chemical Equilibria and Reaction Conditions (contd.) Effect of Concentration on the Position of Equilibrium: CHaCO(h1 + CHaCOOC,Hs + H.O ethu»ic acid ethyl eåanoate water Addition of few drops of concentrated sulfuric acid dehydrates the chemical reaction and removes some of the water from the reaction. So, the equilibrium is shifted to the right to restore the concentration of water.
  16. Chemical Equilibria and Reaction Conditions (contd.) The addition or removal of a reactant or product will affect the position of equilibrium in any reversible reaction temperature catalyst Increasing the temperature makes the reaction move in the direction that takes in heat (the endothermic direction). Decreasing the temperature makes the reaction move in the direction that gives out heat (the exothermic direction). This only affects reactions involving gases. Increasinq the pressure shifts the equilibrium in the direction that produces fewer gas molecules. Decreasing the pressure shifts the equilibrium in the direction that produces more gas molecules. Increasing the concentration of one substance in the mixture makes the equilibrium move in the direction that produces less of that substance. Decreasing the concentration of one substance in the mixture makes the equiibriurn move in the direction that produces more of that substance. Using a catalyst does not affect the position of equilibrium, but the reaction reaches equilibrium faster.

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