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Factors Affecting Reaction Rates

Learn about factors affecting reaction rates, catalysts, inhibitors, equilibrium, reversible reactions, and calculating equilibrium constants. Understand the dynamic balance in chemistry. Explore homogeneous and heterogeneous equilibria. See examples and calculations.

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Factors Affecting Reaction Rates

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  1. Factors Affecting Reaction Rates

  2. Factors Affecting Reaction Rates • There are several factors affect reaction rates • Concentration • Surface area (Particle size) • Temperature • Catalysts • Inhibitors

  3. Catalysts • A catalyst is a substance that increases the rate of a chemical reaction without itself being used up in the reaction. • A catalysts provide an alternative energy pathway for the reaction. • The different pathway lowers the activation energy allowing more molecules to overcome the activation energy and produce products at a faster rate

  4. Potential Energy H A lower activation energy allows more molecules to overcome the activation energy, speeding up the reaction

  5. Inhibitor • An inhibitor is a substance that slows down, or inhibits reaction rates. • Uses • A preservative • A weed killer

  6. Equilibrium : A State of Dynamic Balance

  7. Reversible Reactions • When a reaction results in complete conversion of reactants to products chemists say it goes to completion • Not all reactions go to completion. They appear to stop because they are reversible • Reversible reactions can occur in both the forward and reverse directions

  8. Reversible Reactions cont. . . • Forward N2 + 3H2 2NH3 • The reactants are N2 and H2 • Reverse N2 + 3H2  2NH3 • The reactant is NH3 N2 + 3H2⇆ 2NH3 The forward and reverse reactions are happening at the same time.

  9. Chemical Equilibrium • Chemical equilibrium is a state in which the forward and reverse reactions balance each other because they take place at equal rates. • Rateforward reaction = Ratereverse reaction • This does notmean the concentrations of the products and reactants are the same • Equilibrium is a state of action, not inaction. This process is dynamic; dynamic equilibrium.

  10. http://www.youtube.com/watch?v=JsoawKguU6A

  11. Equilibrium Expression and Constant (Keq) • Law of chemical equilibrium states that at a given temperature, a chemical system may reach a state in which a particular ratio of reactant and product concentrations has a constant value known as Keq or equilibrium constant.

  12. Equilibrium Constant Keq • Keq is the ratio of product concentrations to reactant concentrations. • A large Keq, Keq > 1 means the products are favored over the reactants • A small Keq, Keq < 1 means the reactants are favored over the products

  13. Homogenous vs. Heterogeneous Equilibrium • Homogenous equilibrium means all reactants and products are in the same physical state • Ex: H2(g) + O2(g)  H20(g) • Heterogeneous equilibrium is when the reactants and products are in more than one physical state. • Ex: H2O(g) + C(s)  H2(g) + CO(g)

  14. Chemical Equilibrium—Keq aA + bB  cC +dD products [C]c [D]d reactants [A]a [B]b Keq = = **only use Keq for gases and aqueous compounds

  15. Homogenous Equilibria • Given N2(g) + 3H2(g) 2NH3(g) write the equilibrium constant expression for the reaction.

  16. Given SO2(g) + O2(g) 2SO3(g) write the equilibrium constant expression for the reaction.

  17. Heterogeneous Equilibria • Given 2NaHCO3(s) Na2CO3(s) + CO2(g) +3H2O(g) write the equilibrium constant expression for the reaction.

  18. Calculating the Value of Equilibrium Constants At equilibrium and 100°C a flask contains: [PCl5]=0.0325M [H2O]=0.025M [HCl]=0.375M [POCl3]=0.250M Calculate the Keq for the reaction PCl5(g) + H20(g) 2HCl(g) + POCl3(g)

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