5.5 Equilibria

A reaction which can go backwards and forwards

Amounts of reactants and products The conditions (such as temperature and pressure)

The enthalpy change in a reversible reaction always depends on the forwards reaction For the backwards reaction, the enthalpy change is the same number but with the opposite sign

Dehydration of copper sulfate crystals Thermal decomposition of ammonium chloride

To make the reaction go forwards: heat the hydrated copper sulfate crystals (CuSO4.5H2O), the crystals turn from blue to white as they lose all their water To make the reaction go backwards: add water to anhydrous copper sulfate (CuSO4), the white powder will absorb the water and turn blue

To make the reaction go forwards: heat NH4Cl, nothing appears to happen as both are colourless gases To make the reaction go backwards: allow NH3 and HCl to mix in the absence of heat, white NH4Cl forms immediately wherever the cold gases have met This can be demonstrated all at once: - The NH4Cl at the bottom of the tube (hot) decomposes to form invisible NH3 and HCl - The invisible NH3 and HCl cool as they move up the tube, recombining to form NH4Cl

When a reversible reaction is left in a sealed container, the forwards and backwards reactions will eventually ‘balance out’. From that point onwards, the mixture of reactants and products in the container is referred to as being in dynamic equilibrium Dynamic = the forwards and backwards reactions are still happening, just at the same rate Equilibrium = the concentrations of reactants and products remain unchanged

Walking down the up-escalator quickly enough that you aren’t actually moving

Decomposition of N2O4

If you leave N2O4 in a box, some of it breaks apart forming NO2 But then, the NO2 just made could recombine to form N2O4 again After a while, the breaking apart and recombining end up happening at the same time So the overall concentrations of N2O4 and NO2 stay the same

The resulting balance of products and reactants in an equilibrium mixture

On the right

On the left

The greater the imbalance of reactants and products, the further an equilibrium is to the ,eft or right

It is possible (but rare) that the amounts of products and reactants are equal

By observing or measuring one of the substances

• Amounts of reactants or products • Gas pressure • Temperature

Le Chatelier’s Principle ‘The equilibrium shifts to counteract any change imposed on it

• State how the change needs to be counteracted • Explain how the forwards/backwards reaction achieves that

Add/remove some of the substance from the mixture - If you add something, the equilibrium will shift to remove the extra you just added (whichever direction uses it up) - If you remove something, the equilibrium will shift to create more of it (whichever direction creates it)

Gases exert pressure on the sides of the container, and more gas means higher pressure The equilibrium can affect the gas pressure by changing the moles of gas - if you increase gas pressure, the equilibrium will shift to lower it back down (wherever has fewer moles gas) - if you decrease gas pressure, the equilibrium will shift to bring it back up (wherever has most moles gas)

Absorbing/releasing heat energy The enthalpy change tells us whether the forwards reaction is endothermic or exothermic - If you increase the temperature, the equilibrium will shift to bring it back down (wherever is endothermic) - If you decrease the temperature, the equilibrium will shift to bring it back up (wherever is exothermic)

Catalysts increase rates of both forwards and backwards reactions, they balance Therefore, adding a catalyst has no effect on the position of an equilibrium