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New Unit: Equilibrium

New Unit: Equilibrium. (the mother of all units). Dynamic equilibrium. balance between opposing forces that occur at the same rate (forward rxn at same rate as reverse rxn) mixture of reactants and products. Types : Solubility Phase Chemical.

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New Unit: Equilibrium

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  1. New Unit: Equilibrium (the mother of all units)

  2. Dynamic equilibrium • balance between opposing forces that occur at the same rate (forward rxn at same rate as reverse rxn) • mixture of reactants and products. • Types: • Solubility • Phase • Chemical

  3. Both crystallization and dissolution are occurring at the same rate – collisions are always occurring – Example, NaCl in H2O, initial dissolution caused by collisions btw water molecules and ions, but then ions are free to move, and may collide with NaCl and crystallize. NaCl(s) = Na+(aq) + Cl-(aq) - both dissolved & undissolved solute must be present. Need a supersaturated solution. Solubility Equilibrium (heterogenous system)

  4. Example – evaporation/condensation equilibrium in a closed system (since we’re dealing with gases here, system must be “closed” on all sides to achieve equilibrium) Kinetic molecular theory – as molecules evaporate into the gas phase, more gaseous molecules are available to collide /w the liquid surface & enter liquid phase again…. Solid/liquid equilibrium can also be established at a melting or freezing point. H2O(s) = H2O(l) when t = 0oC. Phase Equilibrium (heterogenous)

  5. Chemical Reaction Equilibrium (can be hetero or homogenous) • More complex than solubility & phase equilibria! • Any closed chemical system is in a state of equilibrium • Both reactants and products are present. • Note, if the system is not closed & product escapes, system cannot be at equilibrium.

  6. Example: For the following reaction; N2O4(g) = 2NO2(g)

  7. Equil. was established by placing a certain amount of N2O4(g) in a closed container. No product present initially so rxn proceeds in forward direction until equilibrium is established.

  8. Q/ If I started with 0.750 mol of reactant in a 1.0L container, what would my concentration of product be? A/ Last year, stoich assumed all rxns went to completion, therefore ratios tell me 1.50 mol of product in my 1 L container. N2O4(g) = 2NO2(g)

  9. Percent Reaction: • In a closed system, it was determined experimentally that at equilibrium, [N2O4] = 0.721 mol/L, and [NO2] = 0.0580 mol/L. • Percent reaction can be used to describe relativeamounts of chemicals present in equilibrium systems – refers to amount of product formed. • For the example above; % rxn = (actual/theoretical) x 100% = 0.0580/1.50 x 100% = about 4% N2O4(g) = 2NO2(g) 4%

  10. Note: We would get the same equilibrium concentrations if we started by placing 1.50 mol of NO2 in the container. • ** For reversible rxns in closed systems – the same equilibrium concentrations are reached whether equilibrium is approached from the forward or reverse direction. (same proportions)

  11. Percent reaction can be used to classify reactions into three broad categories: 1.No reaction – reactants are strongly favoured, <1% reaction 2.Quantitative – observed to be complete, written /w single arrow , >99% 3.Reactants favoured, <50% reaction Products favoured, >50% reaction {There are always significant amounts of both reactants and products in the system, equilibrium achieved}

  12. Four conditions that apply to all equilibrium systems; • Equilibrium is achieved in reversible processes when the rates of opposing change are equal – represented by a double arrow. • Macroscopic properties are constant. No change in properties that depend on the total quantity of matter. (e.g. colour, pressure, conc/n, pH) • Dynamic change at the molecular level, but no change at the macroscopic level. • Equilibrium can only be reached in closed systems. (including constant temperature – energy closed too) • Equilibrium can be approached from either direction. Proportions will still be the same.

  13. Using ICE Tables: • We can use ICE tables for organizational purposes for solving equil. problems. • ICE is an acronym that stands for Initial, ChangeandEquilibriumconcentrations. • Initial – concentrations of reactants and products before the reaction begins • Change – between start and equilibrium point • Equilibrium – concentration of reactants & products at equil.

  14. Example: • Formation of HF(g) at SATP H2(g) + F2(g) = 2HF(g) • If we start with 1.00 mol/L concentrations of both reactants, calculate the [] of H2 and HF at equilibrium if [F2] at equilibrium is 0.24 mol/L. • Calculate % rxn as well.

  15. Example: • Ammonia decomposes into nitrogen gas and hydrogen gas. When 4.0 mol of ammonia gas is introduced into a 2.0 L rigid container, at equilibrium, the amount of ammonia is 2.0 mol. • Determine the equilibrium concentrations of nitrogen and hydrogen gas, and % rxn.

  16. Homework • page 428 #1-3, • page 437 #6,7, • page 437 #3,4 (careful for limiting reactant stuff!!!) #3 as a class? • page 438 #8,9

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