Equilibrium (SAC)

What is an equilibrium reaction?

• The forward and reverse reactions are occurring simultaneously and at the same rate

• The concentration of the reactants and products will always remain at a constant value (doesn’t mean they are the same)

• Other conditions eg temperature, pressure , remain at constant levels

• A reaction will only reach equilibrium if it is in a CLOSED system (nothing escapes)

Equilibrium constant (K Value)

K= [products]/[reactants] → raise to the power of coefficient

• The units for K is M^x, which is derived from first and second index laws (subtract indices)

What does the equilibrium constant tell you

• Gives an indication of the EXTENT OF THE REACTION

*refers to how many of the reactants were converted into products when a reaction reached equilibrium

• If the extent of a reaction was very low, only a small number of reactants would have been converted to products when equilibrium was reached

• A low K shows a reaction that has occurred to a small extent

• A high K shows a reaction that has occurred to a large extent

• The equilibrium constant gives NO INDICATION OF THE RATE OF REACTION.

Concentration Fraction/ Reaction Quotient (Q)

• The fraction [products]/[reactants], when the system is not at equilibrium

2 uses:

• Can show whether or not a reaction is at equilibrium

• If a reaction is not at equilibrium it can show whether the reaction will need to move in the forward or backwards direction in order to attain equilibrium

Manipulating equations and K value

• If an equation is reversed, the K value will be the inverse of the original K value (1/K). The units will also become 1/original M

• If the coefficients in a balanced equation are doubled, the new K will be the square of the original

• If coefficients are halved, new K will be sqr root of original K

Le Chatelier’s Principle

If an equilibrium system is subjected to a change, the system will adjust itself to partially oppose the effect of this change.

*Note: the adjustment only occurs until the equilibrium constant is back to the original

Adding extra reactant

• The system partially opposes this change by favouring the forward reaction

Adding extra product

The system partially opposes this change by favouring the reverse reaction

Removing reactant

The system partially opposes this change by favouring the reverse reaction

Removing product

Partially opposed by the system by favouring the forward reaction

Increasing/decreasing pressure of a gaseous system

Will favour the forward or reverse reaction depending on which side of the equation has more moles of gas.

Increasing pressure by adding inert gas

• No shift in reaction

• This is because the inert gas has increased the overall pressure in the container but not the partial pressure of the reactants and products (the pressure that each gas is individually exerting in the system)

Diluting an aqueous system

• Where reactants + products are dissolved in water

• The overall concentration of ions in the system will DECREASE → system will favour whichever reaction that has more moles of ions/particles.

Increasing temperature

• If forward reaction exothermic, then the reverse reaction is favoured (decrease in [product] and increase in [reactants]; New K value smaller)

• If the forward reaction is endothermic, the forward reaction is favoured (increase in [product] and decrease in [reactants]; New K value greater)

Decreasing Temperature

• If forward reaction is endothermic, then the reverse reaction is favoured (decrease in [product] and increase in [reactant]; New K value smaller)

• If forward reaction is exothermic, then the forward reaction is favoured (increase in [product] and decrease in [reactant]; New K value greater)

Adding a catalyst

• the rate of both the forward and reverse reactions increased to the same extent; reaction remains in equilibrium and there is no shift in direction of reaction

Concentration Time Graphs (adding/removing product/reactant)

• A spike up/down for whichever reactant product is removed/added

• The lines eventually come together at equilibrium again, but ensure it is a partial opposition and according to mole ratios

Concentration/Time graphs (increasing/decreasing pressure by increasing/decreasing volume)

• Every single gas’ concentration spikes up or down (height of spike depends on the initial concentration of the reactant/product and how much the volume was altered by)

• All of them come together at equilibrium again, ensure it is a partial opposition and according to mole ratios

Concentration/Time graphs (increasing pressure by adding inert gas)

• The system does not favour either the forward or reverse reactions, so system remains at equilibrium (straight line)

Concentration/Time graphs (increasing/decreasing pressure by increasing/decreasing volume WITH SAME AMOUNT OF MOLES ON EACH SIDE)

• A steep increase or decrease for all gases, depending on increase or decrease in concentration

• Then the gases each continue a straight line at equilibrium

Concentration/Time graphs (diluting the aqueous system)

• The concentrations of all reactants + products will drop steeply

• Will come back to equilibrium (keep in mind number of moles of each side in order to increase total number of particles)

Concentration/Time graphs (increasing/decreasing temperature)

*A CHANGE IN TEMPERATURE IS THE ONLY CHANGE THAT WILL NOT CAUSE AN INSTANTANEOUS CHANGE TO THE CONCENTRATION OF THE REACTANTS AND PRODUCTS (NO STEEP INCREASE/DECREASE)

*Rounded graphs only → gradual changes that occur due to LCP, mole ratios still apply when deciphering how big the curves are

Concentration/Time graphs (adding a catalyst)

• No shift in equilibrium = remains a straight line

Yield of a reaction

• Yield refers to the amount of product obtained during a chemical reaction

• For equilibrium reactions, yield can be increased by causing the reaction to shift in the FORWARD DIRECTION

• proportion of products compared to proportion of reactants in an equilibrium system: % yield

Rate vs Time graphs (adding/removing product/reactant)

• If added, the rate of the forward/reverse reaction will have an spiked increase in rate and the other reaction will meet it at equilibrium

• If removed, the rate of the forward/reverse reaction will have a spiked decrease in rate and the other reaction will meet it at equilibrium

Colour change experiments using LCP to determine if a reaction is exo or endo

• Keep in mind that COLOURLESS reactants/products DO NOT cause a colour to become less intense/more colourless. These particles do not effect the colour of the mixture as they cannot be seen.

Rate Yield conflict

• Yield of the reaction refers to how much product is made from a given amount of reactants

• The rate of a reaction refers to how quickly the products are made

• Both of these factors need to be optimised in order to make an industrial process economically viable.

• Eg an exothermic equilibrium; a low temp would optimise the yield of products but very low temps lead to decrease in reaction rates… → GREEN CHEM PRINCIPLE: CATALYSIS: If a catalyst is used, the reaction can occur at an acceptably fast rate using a moderate temperature, which is low enough to have an acceptably high yield of product. (THIS RATE YIELD REACTION in terms of temperature only applies to EXO reactions)

Green Chem Principles

Catalysis: allows a reaction to occur at lower temp, less energy = less electricity needed = less fossil fuels need to be combusted = reduction in production of CO2 emmissions = good for environment.

*also makes non-renewable resources like fossil fuels last longer

(Catalysis can make the production of chemicals by industry more sustainable + economic benefits as less electricity is needed meaning the process can operate at a lower cost)

Design for energy efficiency: minimising the amount of energy, mainly in the form of electricity, used to produce a given amount of chemicals

*Many reactions involved in industrial chem are exothermic; heat released can be used to provide energy for different parts of the industrial process (can be used to generate electricity). This heat would otherwise be released into the environment as “waste heat” → can harm living organisms eg fish if it is released into water. Waste heat used to produce electricity means less fossil fuels are burnt = less greenhouse gas emmissions + less money spent on electricity

non-renewable

If something is non-renewable, it is used at a rate that is much faster than the rate at which it is replaced by nature

sustainable

If something is sustainable, it is able to meet the energy needs of the present day and the energy needs of the future, without causing harm to the environment or individuals.