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7. Equilibrium > Equilibrium > Flashcards

Equilibrium Flashcards

1
Q

What conditions are essential to reach equilibrium?

A
  • The forward and reverse reaction occur at the same rate
  • It requires a closed system
  • The concentrations of reactants and products at equilibrium are constant
  • No changes in macroscopic properties (temperature, pressure, density, colour)
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2
Q

Why do the forward and reverse reactions need to occur at the same rate?

A
  • This is a form of dynamic equilibrium
  • Reactants are continuously changed to products and they are continuously changed to reactants
    Draw graph
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3
Q

Why do the concentrations of reactants and products need to be constant?

A
  • This is because the rate of the forward and backward reactions are equal
  • Equilibrium can be approached from two directions
  • In the forward reaction the concentrations of products increase, in the forward reaction the concentrations of reactants decrease
  • At equilibrium concentrations are constant, not necessarily equal
    Draw graph
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4
Q

Why does equilibrium require a closed system?

A
  • The reactants and products cannot escape from the reaction vessel
  • In open systems, matter can escape
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5
Q

What is physical equilibrium? What conditions are required?

A
  • Physical changes in a closed system
  • E.g. rate of evaporation of the water is greater than the rate of condensation
  • It is a dynamic equilibrium
  • Rates of forward and backward reaction equal
  • Volume of liquid at equilibrium is constant
  • Closed system
  • No macroscopic properties
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6
Q

What do you calculate at equilibrium for any reaction?

A
  • The equilibrium constant Kc using the equilibrium concentrations of reactants and products
  • Need to also write the equilibrium constant expression:
    aA + bB ⇌ cC + dD (a,b,c,d represent stoichiometric coefficients)
  • A change in temperature changes the value of the Kc value
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7
Q

How do you write an equilibrium constant expression?

A
  • Products on the top, reactants on the bottom
  • Square brackets, show concentrations (mol/dm^3) of the reactants and products at equilibrium
  • Powers are stoichiometric coefficients
    Check book
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8
Q

What does the value of Kc determine?

A
  • Value of Kc at a particular temperature is a useful indication as to how far a reaction had gone toward completion (on the right side)
  • High value, reaction almost complete, most reactants converted to products. At equilibrium, higher concentration of products than reactants
  • Low value, forward reaction barely proceeded, only small proportion of reactants formed to products (lies to the left)
  • Magnitude is temperature dependent
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9
Q

How does the value of Kc reflect the position of the reaction?

A

Check book

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10
Q

How can the equilibrium constant be manipulated?

A

Structure of equilibrium expression depends on the direction it is written is (products on left or right side). Occur in a reversible reaction:
- Reversing the equation
- Changing the coefficients in the balanced equation
- Reactions in sequence

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11
Q

How does reversing the equation manipulate Kc?

A
  • Reversing the reaction, the value Kc will be the inverse/reciprocal of the value for the forward reaction
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12
Q

How does changing the coefficients in balanced equations manipulate Kc?

A
  • If coefficients are doubled, Kc is squared
  • If coefficients are halved, Kc square root taken
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13
Q

How does reactions in sequence manipulate Kc?

A
  • Value of Kc is equal to the product of the individual values for the different steps
    Kc = Kc1 × Kc2 × Kc3
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14
Q

Only the temperature can change the value of Kc.

A
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15
Q

What is the reaction quotient?

A
  • Qc is a measure of the (relative) amounts of reactants and products for a reaction that has not yet reached equilibrium
  • Predicts in which direction a reaction will proceed to reach equilibrium
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16
Q

How do you calculate Qc?

A
  • Using the same formula as for Kc (check book)
  • However, in this case non-equilibrium concentrations of reactants and products are used
17
Q

What do you calculate with the Kc and Qc values?

A
  • Comparing the value of Qc and Kc, indicates the direction in which the reaction must proceed to reach equilibrium
  • When Qc < Kc reaction is not yet at equilibrium, proceeds to the right
  • When Qc = Kc reaction at equilibrium
  • When Qc > Kc reaction is not yet at equilibrium, proceeds to the left
18
Q

What does Le Chatelier’s principle determine?

A
  • Allows to predict in which direction the position of equilibrium will shift
  • System will respond to minimize the effect of the change, applied at a state of equilibrium
19
Q

What changes can be made to a reaction at equilibrium?

A
  • Concentration
  • Pressure of gases
  • Temperature (also change the value of Kc)
  • (Catalyst)
  • Shift the equilibrium position to the left or right
20
Q

What does changing the concentration do to a reaction at equilibrium?

A
  • Decreasing concen. of products, increase concen. of reactants, move to the right
  • Decrease concen. of reactants, increase concen. of products, move to the left
    Draw graphs in book
21
Q

What does changing the pressure do to a reaction at equilibrium?

A
  • Only effects equilibrium if reactants and products are in gaseous states
  • Pressure can be changed by adding or removing gaseous reactant or products
  • Changing the volume of the system
  • V ∝ 1/P
  • Adding an inert gas will have no effect on the equilibrium position
22
Q

In what direction must the reaction shifts when the pressure is changed?

A
  • Increase pressure, shift to side with less gas moles
  • Decrease pressure, shift to side with more gas moles
  • Reactions with same number of gaseous molecules on each side, changes in pressure will have no effect
  • Use ‘gaseous molecules’ term, use coefficients of these molecules
23
Q

What does changing the temperature do to a reaction at equilibrium?

A
  • Consider the enthalpy change (∆H)
  • When the temperature is increased, the reaction will shift to the endothermic side
  • When the temperature is decreased, the reaction will shift to the exothermic side
  • In the exam, they will tell you which direction of the reversible reaction is exothermic/endothermic
24
Q

What happens during a reaction, when the temperature is changed?

A
  • If the temp. is increased, the position of equilibrium will shift to the endothermic side (direction of either forward or backward reaction)
  • When forward reaction is endothermic, the added heat will be absorbed or when exothermic temperature is increased
25
Q

How is the value of Kc changed when the temperature is changed of a reaction?

A
  • In exothermic reaction, temp. increase shifts position of equilibrium to the left, value of Kc decreases
  • In endothermic reaction, temp. increases shifts the position of equilibrium to the right, value of Kc increases
26
Q

What effect does adding a catalyst have on a reversible reaction?

A
  • Increases rate of both forward and reverse reaction, equally
  • Catalyst does not change the position of equilibrium or the value of Kc of a reaction
  • Catalysts used since state of equilibrium can be established more quickly, economic benefit
27
Q

What did Le Chatelier’s principle contribute to the modern industry?

A
  • Allows chemists to adjust reaction conditions, produce increased yield of the product, increase profitability of the process
28
Q

What is the Haber process?

A
  • Since there is an overall high demand for nitrogen and ammonia contains it, it is used in fertilizers, in nitric acid and explosives
  • N2 (g) + 3H2 (g) ⇌ 2NH3 (g )
  • The process made is possible for huge amounts of nitrogen to be produced
  • Forward reaction exothermic, hence low temp. high pressure. Find compromising temperature, economically viable, otherwise too slow
29
Q

What is the Contact process?

A
  • Used to produce sulfuric acid (H2SO4), very high demand
  • Used in fertilizers, paints, detergents, soaps
  • 2SO2 (g) + O2 (g) ⇌ 2SO3 (g )
  • Forward reaction also exothermic
  • High pressure not used, since too costly, danger of explosion

7. Equilibrium (1 decks)

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