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Flashcards for F325 Equilibria, Energetics and Elements

Focussing on Module 1 5.1.2 How Far?

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  • Created on: 22-04-14 11:49

The Equilibrium Constant

aA + bB ↔ cC + dD is a reversible reaction

[A] [B] [C] [D] are the equilibrium concentrations of the reactants and products

a b c d are the balancing numbers from the equation and the powers to which the reactants/products are raised to in the Kc equation

Kc = Products / Reactants

Kc = ( [C]^c [D]^d ) / ( [A]^a [B]^b )

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Concentrations/Quantities Present at Equilibrium

H2 (g) + I2 (g) ↔ 2HI(g)       

 Volume 1dm^3

Start of Reaction:

0.6 moles H2,  0.4 moles I2,  0 moles HI

End of Reaction:

0.28 moles H2

How many moles of I2 are left and how many moles of HI have been made?

0.6 moles - 0.28 moles gives 0.32 moles used

Ratio H: HI is 1:2, therefore, moles of HI made is 0.32 x 2 = 0.64 moles (HI made)

 Ratio H2 : I2 is 1:1, so moles of I2 used is just 0.4 - 0.32 = 0.08 moles (I2 used)

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The Effect of Changing Temperature on the Value of

Kc = 1 means the position of the equilibrium is halfway between the reactants and the products 

K> 1 means the reaction is product-favoured 

K< 1 means the reaction is reactant-favoured

Exothermic Reactions (ΔH -ve) As temperature increases, Kc decreases

Equilibrium yield of products on the right decreases, equilibrium yield of reactants on the left increases

The equilibrium shifts to the LEFT

Endothermic Reactions (ΔH +ve) As temperature increases, Kc increases

Equilibrium yield of products on the right increases, equilibrium yield of reactants on the left decreases

The equilibrium shifts to the RIGHT

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The Value of Kc and Catalysts


Kc is not affected by catalysts

Catalysts affect forward and backward reactions equally

They speed up the rate of reaction

And decrease the time taken to reach equilibrium, they do not affect the equilibrium

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What is Kc?

The Equilibrium Constant, Kc

Compares the concentrations of reactants and products present at equilibrium

If the value of Kc is large, the position of equilibrium is on the right, favouring the products

When the forward reaction is endothermic, Kc increases with increasing temperature

When the forward reaction is exothermic, Kc decreases with increasing temperature 

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What is k?

The Rate Constant, k

Measures the rate of a reaction

Large value of k = fast rate of reaction

k increases with increasing temperature, so the rate of reaction increases too

k can only be measured experimentally

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Compromise between Kc and k

For the Haber Process: N2(g)H2(g) ↔ 2NH2(g) ΔH = -92

Exothermic Forward Reaction

At high temperatures the value of Kc will be small

Endothermic Reverse Reaction

At low temperatures the value of Kc will be large

Compromise needed to increase value of k enough to produce a reasonable yield without the value of Kc decreasing too far

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