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Unit 2 7 Chemical Equilibria Notes
 

Unit 2 7 Chemical Equilibria Notes

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    Unit 2 7 Chemical Equilibria Notes Unit 2 7 Chemical Equilibria Notes Document Transcript

    • Unit 2.7 - Chemical equilibria Reversible reactions • Some reactions are irreversible like… 2H2(g) + O2(g) → 2H2O(l) • There are many more, which are reversible , in other words they can easily go backwards or forwards, like… H2(g) + I2(g) ⇌ 2HI(g) • The ⇌ symbol indicates that the reaction is reversible Dynamic equilibrium • If a reaction is reversible then it does not go to completion • Both the forward reaction (reactants → products) and the backward reaction (______________ → _____________) can and do happen • If the forward and backward reactions are happening at the same rate then the concentrations of the reactants and products will stay the _____________ Dynamic equilibrium The rates of the forward and reverse reactions are equal. Therefore, there is no further change in the concentrations of the reactants and products. Graph of concentration vs. time Graph of rate vs. time Example - Formation of HI
    • For the reaction… H2(g) + I2(g) ⇌ 2HI(g) To be at equilibrium, the rate of… H2(g) + I2(g) → 2HI(g) [forward] Must be equal to the rate of… 2HI(g) → H2(g) + I2(g) [backward] Case study - The Haber- Bosch process • Perhaps the best-known reversible reaction • Fixation of atmospheric nitrogen • The product, ammonia (NH3), is used in fertilizer and explosive manufacture • Traditionally uses an iron catalyst • Temperature 350- 500 °C • High pressure 15- 25 MPa Fritz Haber and Carl Bosch reactants products N2(g) + 3 H2(g) ⇌ 2 NH3(g) ∆H = -92.4 kJ/mol • High pressure is used to increase the ___________ of ammonia • High temperature is used to increase the __________ of reaction Le Chetaliers’s principle “ If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or partial pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established.” In other words… 1. Effect of temperature Remember… Henry Le Chetalier - and you o Exothermic (-ve ∆H) reactions ___________ energy to the surroundings making them thought Fritz Haber looked scary! _________
    • o Endothermic (+ve ∆H) reactions ___________ energy from the surroundings making them ___________ Exothermic reactions Increase temperature Decrease temperature Favour the backward reaction, which is Favour the ______________ reaction, which is endothermic, and move the position of _____thermic and move the position of equilibrium towards the reactants equilibrium towards the ______________ Endothermic reactions Increase temperature Decrease temperature Favour the ____________ reaction, which is Favour the ______________ reaction, which is _____ thermic, and move the position of _____thermic, and move the position of equilibrium towards the ______________ equilibrium towards the ______________ Case study - The Haber- Bosch process N2(g) + 3 H2(g) ⇌ 2 NH3(g) Forward reaction: ∆H = -92.4 kJ/mol (exothermic) Backward reaction: • The effect of increasing the temperature would be… • The effect of decreasing the temperature would be… NB The reaction is done at a compromise temperature (350-500 °C) and uses a catalyst to achieve a commercially viable rate of reaction. Lower temperatures would give a higher yield of ammonia but the reaction would take much longer. 2. Effect of pressure Again using the Haber-Bosch process as an example: reactants products N2(g) + 3 H2(g) ⇌ 2 NH3(g) moles of gas moles of gas • As you can see there are __________ moles of gas on the left hand side of this equation • Therefore, the forward reaction has the effect of _____________ the pressure (producing more gas would increase the pressure)
    • • For the Haber-Bosch process high pressure favours the production of the _____________ ammonia as the equilibrium shifts in order to counteract the _____________ in pressure • Lowering the pressure would favour the ________________ reaction More moles of gaseous reactants Increase pressure Decrease pressure Favour the forward reaction and move the Favour the ______________ reaction, which is position of equilibrium towards the reactants and move the position of equilibrium towards the thus counteracting the initial increase ______________ More moles of gaseous products Increase pressure Decrease pressure NB This only applies to equilibrium reactions involving gases. Reducing the volume or adding more gas to the same volume can increase pressure. 3. Effect of concentration The effect of altering the concentration of one or both of the reactants applies to reactions that are carried out in solution. As the Haber-Bosch process involves a gas-phase reaction we will have to find another example: 2CrO42-(aq) + 2H+(aq) ⇌ Cr2O72-(aq) + H2O(l) Chromate(VI) Dichromate(VI) Yellow Orange Addition of excess acid Adding excess acid causes the equilibrium to shift to the __________ to oppose the change (increase in reactant ________________) and as more dichromate(VI) ions are produced, the solution turns ________________ Addition of excess alkali
    • • Increasing the concentration of reactants favours the _________________ reaction • Reducing the concentration of reactants favours the _________________ reaction Effect of catalyst • Catalysts are substances that _______________ the rate of a chemical reaction but remain unchanged themselves • They do this by providing an alternative route with a lower activation energy • They have no effect on the position of equilibrium • They speed up both the forward and backward reactions so equilibrium is reached faster Nitrogen dioxide and dinitrogen tetroxide 2NO2(g) ⇌ N2O4(g) ∆H = -57 kJ/mol brown colourless Forward reaction: Backward reaction: • The forward reaction is _______thermic • Increasing the temperature favours the _____________ reaction • Decreasing the temperature favours the _____________ reaction If we reduced the temperature we will observe… If we increase the temperature we will observe… Methane hydrate Methane hydrate (also known as methane clathrate and ‘fire ice’) is a solid substance containing methane molecules surrounded by cage-like structures of water molecules. Huge deposits of methane hydrate exist on deep-sea beds around the world. Up to ten times the current natural gas reserves may be trapped in hydrates. Methane is also a powerful greenhouse gas; any sudden
    • release could be catastrophic. The methane in hydrates is in equilibrium with gaseous methane: methane hydrate(s) ⇌ methane(g) + water(l) ∆H = +ve Effect of increasing pressure on this equilibrium… Effect of increasing temperature on this equilibrium…