3.17: Know that Some Reactions are Reversible and this is Indicated by the Symbol ⇌ in Equations

REVERSIBLE REACTIONS: Reaction that can occur in both directions (reactants can react to form products, and products can react to form initial reactants)

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CHEMICAL EQUATIONS FOR REVERSIBLE REACTIONS:

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When writing chemical equations for reversible reactions, two arrows are used with each having half an arrow head (to indicate reaction can occur in both directions):

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Dehydration of Hydrated Copper (II) Sulfate:

Hydrous Copper (II) Sulfate     ⇌     Anhydrous Copper (II) Sulfate     +     Water

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Thermal Decomposition of Ammonium Chloride:

Ammonium Chloride     ⇌     Ammonia     +     Hydrogen Chloride

3.18: Describe Reversible Reactions such as the Dehydration of Hydrated Copper (II) Sulfate and the Effect of Heat on Ammonium Chloride

REVERSIBLE REACTIONS: Reaction that can occur in both directions (reactants can react to form products, and products can react to form initial reactants)

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DEHYDRATION OF HYDRATED COPPER (II) SULFATE:

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DEHYDRATION OF HYDRATED COPPER (II) SULFATE

Diagram showing the Dehydration of Hydrated Copper (II) Sulfate

EXPLANATION: Copper (II) Sulfate crystals that contain Water are known as Hydrated Copper (II) Sulfate When Hydrated Copper (II) Sulfate crystals are heated in test tube, blue crystals will turn into white powder, and colourless liquid will collect at the top This is because when Hydrated Copper (II) Sulfate crystals are heated, it loses Water of Crystallisation and turns into Anhydrous Copper (II) Sulfate (white powder) and Water (colourless liquid):   CuSO4.5H2O (s)      ⇌       CuSO4 (s)     +     5H2O (l)

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EFFECT OF HEAT ON AMMONIUM CHLORIDE:

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THERMAL DECOMPOSITION OF AMMONIUM CHLORIDE

Diagram showing the Thermal Decomposition of Ammonium Chloride

EXPLANATION: When Ammonium Chloride is heated in a test tube, white crystals at the bottom disappears and reappears further up, with colourless Gas forming in between This is because heating Ammonium Chloride causes it to decompose into Ammonia (colourless gas) - NH3+, and Hydrogen Chloride - HCl:   CNH4Cl (s)      ⇌       NH3 (g)     +     HCl (g)

3.19C: Know that a Reversible Reaction can Reach Dynamic Equilibrium in a Sealed Container

REVERSIBLE REACTIONS: Reaction that can occur in both directions (reactants can react to form products, and products can react to form initial reactants)

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DYNAMIC EQUILIBRIUM: Condition that exists when rate of forward and backward reaction in a reversible reaction mixture is equal

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Example: The Reaction Between Hydrogen Gas and Nitrogen Gas to Make Ammonia:

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• When Nitrogen and Hydrogen are the only reactants present in reaction, rate of forward reaction is highest as concentrations of both Hydrogen and Nitrogen will be at their highest
• As the reaction continues, concentrations of Hydrogen and Nitrogen will gradually decrease to form product (Ammonia), therefore the rate of forward reaction will decrease

• As Hydrogen and Nitrogen will react to increase the concentration of Ammonia (product), the rate of backward reaction will therefore increase (Ammonia can decompose to reform Hydrogen and Nitrogen)
• However, as Ammonia can decompose to reform Hydrogen and Nitrogen (reactants), this reversible reaction can therefore reach dynamic equilibrium

Assuming that the reactions occur in a sealed container where the two reactions are interlinked and none of the Gas can escape, the rate of of forward reaction and backward reaction will eventually become equal:

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3H2 (g)      +      N2 (g)      ⇌      2NH3 (g)

3.20C: Know the Characteristics of a Reaction at Dynamic Equilibrium are: the Forward and Reverse Reactions Occur at the Same Rate, the Concentrations of Reactants and Products Remain Constant

DYNAMIC EQUILIBRIUM: Condition that exists when rate of forward and backward reaction in a reversible reaction mixture is equal

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FEATURES OF REACTIONS IN DYNAMIC EQUILIBRIUM:

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• Forward and backward reactions occur at the same rate
• Concentrations of reactants and products remain constant respectively (unchanged)
• Colour of reactants and products remain constant respectively (unchanged)

Diagram:

Diagram showing Dynamic Equilibrium

3.21C: Understand Why a Catalyst Does Not Affect the Position of Equilibrium in a Reversible Reaction

CATALYST: A Substance that Increases the Rate of a Reaction, but is Chemically Unchanged at the End of the Reaction

POSITION OF EQUILIBRIUM: Position where rate of forward reaction is equal to rate of backward reaction

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EFFECT OF CATALYST ON EQUILIBRIUM POSITION:

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• Catalyst will not affect the position of equilibrium as it provides an alternative pathway requiring lower activation energy for both forward and backward reactions, therefore increasing the rate of forward and backward reaction by the same amount
• As a Result, the concentration of reactants and products is nevertheless the same at equilibrium as it would be without the catalyst

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Diagram:

Diagram showing the Effect of Catalyst on Equilibrium Position

3.22C: Know the Effect of Changing Either Temperature or Pressure on the Position of Equilibrium in a Reversible Reaction: an Increase (or Decrease) in Temperature Shifts the Position of Equilibrium in the Direction of the Endothermic (or Exothermic) Reaction, an Increase (or Decrease) in Pressure Shifts the Position of Equilibrium in the Direction that Produces Fewer (or More) Moles of Gas

EFFECTS OF TEMPERATURE:

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CHANGE	HOW THE EQUILIBRIUM SHIFTS
INCREASE IN TEMPERATURE	Equilibrium moves in the endothermic direction
DECREASE IN TEMPERATURE	Equilibrium moves in the exothermic direction

Example:

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Iodine Monochloride reacts reversibly with Chlorine to form Iodine Trichloride:

                            ICl              +              Cl2              ⇌              ICl3

                      Dark Brown                                                         Yellow

When the equilibrium mixture is heated, it becomes darker brown in colour. Explain whether the backward reaction is exothermic or endothermic

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As the equilibrium mixture has become darker brown in colour, more Iodine Monochloride (ICl) has been produced, therefore shifting equilibrium to the left As increasing temperature causes equilibrium to move in the endothermic direction, the backward reaction is therefore endothermic

EFFECTS OF PRESSURE:

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CHANGE	HOW THE EQUILIBRIUM SHIFTS
INCREASE IN PRESSURE	Equilibrium shifts in direction that produces the smaller number of molecules of gas
DECREASE IN PRESSURE	Equilibrium shifts in direction that produces the larger number of molecules of gas

Example:

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Nitrogen Dioxide can form Dinitrogen Tetraoxide, a colourless gas

                            2NO2             ⇌              N2O4

                       Brown Gas                    Colourless Gas

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Predict the effect of the increase in pressure on the position of equilibrium

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Number of Molecules of Gas on the Left    =    2     Number of Molecules of Gas on the Right  =    1 As an increase in pressure causes equilibrium to shift in direction that produces the smaller number of molecules of gas, equilibrium shifts to the right