5 Extent of chemical reactions

what are reversible reactions?

• equilibrium arrows in which the reactants re-from into products

what are irreversible reactions?

occurs only in the forward direction; reactants do not reform from products

what is the content of reactions?

• the amount of product formed at equilibrium

what is yield?

the amount of product formed at equilibrium

what is an equilibrium reactions?

• a dynamic reaction where the forwards and reverse reactions are equal - same speed - cancel each other out - concentration becomes constant

• must be a closed system

for a system to be at dynamic equilibrium…

• concentration must be constant

• pressure of the system is constant

• temperature of the system is constant

• forward/reverse occur at the same time

what is the extent of a reaction?

• the degree to which the reactants are converted to products and describes the equilibrium position

how do you calculate percentage yield?

percentage yield = actual / theoretical X 100

what is the equilibrium constant?

• a degree of conversion from reactants to products in an equilibrium reaction, that can only be affected by temperature

compares the ratio of concentration:

• high K: significant reactions → products conversion significant extent - more products

• low K: little conversion of products → reactions low extent - more reactants

Do catalysts have effect on extent?

• catalysts have no effect on the extent, they only alter the rate of reactions. Equilibrium position is not changed.

How can equilibrium reactions be represented graphically?

• rate V time graphs

• concentration V time graphs

what is Le Chatlier’s principle?

• the position of equilibrium shifts to counteract the change 3

Describe what occurs in a rate V time graph through the example below:

A(g) ← → B(g) + 2C(g)

• Rates of reaction depends on concentration. As A is being used up → concentration drops, therefore rate of reaction drops

• As concentration B and C increase → the rate of reaction increases until it reaches equilibrium

what are concentration V time graphs?

• graphs with concentration in mols per litre on the y axis and time as the x axis and highlights how different substances have different gradients due to different molar ratios between them

How are adding/subtracting components in a concentration v time graph displayed?

• the change is sudden in one of the substances

If there’s an increase in concentration in either the reactant or product, the component being increased will gradually decrease while the other part of the reaction will gradually increase until both sides reach equilibrium → making up for the gained concentration

If there’s an decrease in concentration in either the reactant or product, the component being decreased will gradually increase while the other part of the reaction will gradually decrease until both sides reach equilibrium → making up for the lost concentration

How are temperature changes displayed on concentration v time graph displayed?

• the change is a gradual decrease or increase in all substances

• treat heat as either a reactant (for endothermic reactions) or a product (for exothermic reactions)

In an endothermic reactions, the heat is a reactant, so if you increase the temperature you’re increasing amount of reactant, and therefore the forward reaction is favoured

If the temperature is decreased your decreasing the amount of reactant, and therefore the backwards reaction is favoured

In an exothermic reactions, the heat is a product, so if you increase the temperature you’re increasing the amount of product, and therefore the backward reaction is favoured

If the temperature is decreased your decreasing the amount of product, and therefore the forward reaction is favoured

what is the relation between K value and temperature?

• only factor that affects the k constant

• if the reaction is exothermic then the K value decreases as temperature increases

• if the reaction is endothermic then the K value increases as temperature decreases

How are volume and pressure changes displayed on concentration v time graph displayed?

• if there’s a decrease in volume (increase in pressure) then all components will increase and shift to the side with fewer gaseous moles

• if there’s an increase in volume (decrease in pressure) then all components will decrease and shift to the side with more gaseous moles

what is the reaction quotient?

• can be calculated at any stage during a reaction

• when at equilibrium the reaction quotient is equal to the equilibrium constant

what is the equilibrium constant, K?

• concentrations of the ports divided by the reactants at equilibrium → can only be calculated at equilibrium → fixed temperature

• can only be affected by temperature

what are the units of K dependent on?

• if you double the coefficients, you square K

• if you reverse/flip the reaction then K is the reciprocal

• if an equation is halved, you will square root it

when is Q and K used?

• Q is used when not at equilibrium

• K is used when at equilibrium

• if Q<K then there is excess in the reactants and a forward reaction is favoured

• if Q>K then there is excess in the products and a backward reaction is favoured

Is equilibrium maintained in a closed system or an open system?

• equilibrium can only be maintained in a closed system

what occurs in an open system?

• Generally, a gas product is present gas in the open system with the system trying to replace the lost gas by shifting towards the product side, therefore only R → P - non-reversible

what must experimenters consider when producing equilibrium products?

• producing at an acceptable rate to meet market demands

• ensuring that you maximise yield by using Le Chatliers principle

How do you calculate the initial/final concentrations?

• though the “ICE” table

n - mole ratio

I - initial concentration

C - change → consumed = - produced = +

E - concentration at equilibrium