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Reaction Rates and Equilibrium Notes

Moles and Grams

  • If 40 grams make one mole and you have 20 grams, then you have half a mole.

Rate of Reaction

  • Rate of reaction is change in concentration over change in time.
  • -1 in the units (e.g., mol \cdot dm^{-3} \cdot s^{-1}) indicates "per", representing "change in concentration over changing time".
  • Units of concentration is moles per decimeter cubed (mol/dm^3).
  • Time is measured in seconds (s).

Analyzing Graphs of Reactions

  • Products vs. Reactants Concentrations: If the concentration of A, B, and C is increasing over time, they are products.
  • Rate: A steeper slope on a concentration vs. time graph indicates a faster reaction rate (more product is made in a shorter time).

Collision Theory

  • Particles need to collide with sufficient energy exceeding the activation energy.
  • Consideration is also given to the orientation of the particles during collision.
  • Three key aspects:
    • Collision
    • Orientation
    • Sufficient energy

Surface Area and Reaction Rate

  • Powdering a substance increases its surface area, affecting the reaction rate.
  • Increased surface area leads to a higher frequency of collisions due to greater exposure of reacting particles.
  • Particles collide more frequently, enhancing the reaction rate.

Concentration and Reaction Rate

  • Increasing concentration increases the reaction rate.
  • With the same volume, higher concentration means more particles, leading to more successful collisions.
  • Frequency of successful collisions increases with concentration.
  • Most collisions are unsuccessful; successful collisions lead to product formation.

Pressure and Reaction Rate

  • Pressure in gases is caused by particles colliding with the container walls.
  • Increasing the number of gas particles increases pressure.
  • Kinetic energy of particles remains the same; only the frequency of collisions increases.
  • Pumping up a tire increases the number of particles hitting the tire walls more often, maintaining inflation.

Chemical Equilibrium

  • Equilibrium: Achieved when the rate of the forward and reverse reactions are equal.
  • On a graph, equilibrium is indicated when there is no change in the overall concentration of reactants and products.
  • The concentrations of reactants and products may or may not be equal at equilibrium.

Equilibrium Constant (Kc)

  • K_c > 1: Equilibrium shifts to the right, favoring product formation.
  • K_c = 1: Reactants and products are at equilibrium.
  • K_c < 1: Equilibrium shifts to the left, favoring reactant formation.
  • The value of Kc is determined by inserting the values into the Kc expression.

Interpreting Changes in Concentration

  • An equilibrium shift is indicated if the product concentration increases more than the reactant concentration decreases.
  • If product concentration is higher, it indicates reactants are being added, shifting the equilibrium to the right (product side), resulting in K_c > 1.

Effect of Temperature on Equilibrium

  • Increasing temperature shifts the equilibrium to reduce the temperature.
  • Equilibrium favors the endothermic reaction, which reduces temperature.
  • To make more product C in an exothermic reaction, reduce temperature.
  • Low temperature slows down the reaction rate.

Equilibrium vs. Rate

  • Equilibrium determines how much product is formed.
  • Rate determines how quickly the product is formed.
  • Compromise: High temperature may yield less product but produce it faster.

Effect of Pressure on Equilibrium

  • Increasing pressure shifts the equilibrium to reduce pressure.
  • The system shifts towards the side with fewer moles of gas. For example:
    A + B \rightleftharpoons C (2 moles to 1 mole)
  • Increasing pressure, equilibrium shifts to the right, favoring C (fewer moles of gas).
  • Increased pressure can be achieved by reducing the volume of the container.

Role of Catalyst

  • Catalysts have no effect on equilibrium position.
  • Catalysts increase the rate of both forward and reverse reactions, allowing equilibrium to be achieved quicker.

Equilibrium with Equal Moles

  • If the number of moles of gas is the same on both sides of the equation, changing the pressure has no effect on the equilibrium.
    A + B \rightleftharpoons C + D (2 moles on each side)

Practical Considerations

  • High pressure can shift equilibrium but is expensive and dangerous (risk of explosion).

Grams to Moles Conversion

  • Always convert grams to moles using the molar mass.
  • Example: If the molar mass is 40 grams/mole and you have 10 grams, you have 0.25 moles. (\frac{10 \text{ grams}}{40 \text{ grams/mole}} = 0.25 \text{ moles})