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Reaction rate is the rate at which a chemical reaction takes place. - PowerPoint PPT Presentation


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( 8a) The rate of reaction is the decrease in concentration of reactants or the increase in concentration of products with time. Rate = change over time. Example Test Question C 3 H 8 (g) + 5 O 2 (g) → 3 CO 2 (g) + 4 H 2 O(g) + heat

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Reaction rate is the rate at which a chemical reaction takes place.


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(8a) The rate of reaction is the decrease in concentration of reactants or the increase in concentration of products with time.

Rate = change over time

Example Test Question

C3H8(g) + 5 O2(g) → 3 CO2 (g) + 4 H2O(g) + heat

Which is an acceptable method to measure the rate of the reaction?

(a) the rate of consumption of C3H8

(b) the rate of consumption of O2

(c) the rate of production of CO2

(d) All of the above

Can you justify your answer?

  • Reaction rate is the rate at which a chemical reaction takes place.
  • It is measured by the rate of production of product or disappearance of reactant.
  • Usually, the rate decreases gradually as the reaction continues. The rate is zero when the reaction is complete.
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(8b) Reaction rates depend on such factors as concentration, temperature, and pressure.

  • To start a chemical reaction, molecules must collide with a certain amount of energy.
  • As reactant concentration increases, rate increases. As reactant concentration decreases, rate decreases.
  • As surface area increases, rate increases. As surface area decreases, rate decreases.
  • If working with gases, as pressure increases, rate increases. As pressure decreases, rate decreases.
  • As temperature increases, rate increases. As temperature decreases, rate decreases.
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(8c) What is the role a catalyst plays in increasing the reaction rate?

  • Activation Energy is the minimum amount of energy required to start a chemical reaction.
  • A catalyst is a substance that increases the reaction rate when it is added to a reaction mixture.
  • Catalysts increase the reaction rate by lowering the activation energy.
  • A catalyst is not a reactant or a product because it is not used up or changed during the reaction.
  • An enzyme is a protein that acts as a catalyst. It is a biological catalyst.

The new reaction pathway should have the same “reactants” and “products” energy level. The hill of activation energy should be smaller.

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(9a) Use Le Chatelier's principle to predict the effect of changes on an equilibrium system.

Le Chatelier’s Principle: For a reversible process at equilibrium, when conditions of concentration, temperature, or pressure are changed, the reaction shifts in a direction that will counteract the stress and restore equilibrium.

Example #1 exothermic reaction: N2(g) + 3H2(g) ↔ 2NH3(g) + heat

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(9a) Use Le Chatelier's principle to predict the effect of changes on an equilibrium system.

Le Chatelier’s Principle: For a reversible process at equilibrium, when conditions of concentration, temperature, or pressure are changed, the reaction shifts in a direction that will counteract the stress and restore equilibrium.

Example #1 endothermic reaction: N2O4(g) + heat ↔ 2NO2(g)

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(9b) Equilibrium is established when forward and reverse reaction rates are equal.

  • Chemical equilibrium is a state of balance in which the rate of a forward reaction equals the rate of the reverse reaction and the concentrations of the products and reactants stop changing.
  • The amounts of products and reactants do not have to be equal.
  • Only reversible chemical reactions can reach equilibrium. Reversible reactions are shown with a double arrow:
  • Ex: H2SO4  2H+ + SO42
  • When this reaction is in equilibrium, H2SO4 is produced and broken down at equal rates.

Example Problems:

2N2(g) + 6 H2O (l) ç4NH3(g) + 3O2 (g)

1) Write the forward reaction:

2N2 (g) + 6 H2O (l) 4NH3(g) + 3O2 (g)

2) Write the reverse reaction:

4NH3(g) + 3O2 (g) 2N2(g) + 6 H2O (l)

3) At equilibrium, does the concentration of water have to equal the concentration of oxygen?

Answer: No, the concentrations of products and reactants do not have to match. But, the rate of the forward and reverse reactions will be the same.