6 2 reversibility and equilibrium

A reversible reaction is a reaction that can occur in both directions: the forward reaction (which forms the products) and the reverse direction (which forms the reactants).

Reversible reactions are represented using two opposing arrows, ⇌.

When ammonium chloride is heated, ammonia and hydrogen chloride are formed.

The balanced symbol equation for the reaction that occurs when ammonium chloride is heated is: NH4Cl (s) ⇌ NH3 (g) + HCl (g)

True. The reverse reaction can also be called the backwards reaction.

True. If a reaction is exothermic in one direction, it is endothermic in the opposite direction.

When hydrated copper sulfate is heated the colour change is blue to white and colourless liquid (water) collects at the top of the test tube.

Anhydrous salts are those that have lost their water of crystallisation.

The equation for the reversible reaction when copper(II) sulfate is heated is: CuSO4.5H2O ⇌ CuSO4  +  5H2O

False. The same amount of energy is transferred in the forward and reverse reactions.

The formation of hydrated copper(II) sulfate, from anhydrous copper(II) sulfate is an exothermic reaction.

Two features of a system at equilibrium are:

A closed system is a system where none of the reactants or products can enter or leave the reaction vessel.

True. Dynamic equilibrium can only occur in a closed system as none of the reactants or products are able to leave the reaction vessel.

Le Chatelier's principle states that if a change is made to the conditions of a system, the system will respond to counteract the change.

Three conditions that can change and alter the position of equilibrium are:

If there is an increase in the amount of products formed, the equilibrium has shifted to the right.

True. The position of equilibrium is said to shift to the right if the forward reaction is favoured.

If there is an increase in the amount of reactants formed, the equilibrium has shifted to the left.

If the concentration of a product is decreased, more reactants will react until equilibrium is reached.

Equilibrium shifts to the left to reduce the effect of a decrease in reactant (or an increase in the concentration of product).

In a reversible reaction, the concentration of the reactants is highest at the start of the reaction.

If the concentration of Fe3+ ions is decreased the rate of the reverse reaction increases to counteract the change, lowering the yield of FeSCN2+

An exothermic reaction is one in which energy it released to the surroundings resulting in the temperature increasing.

An endothermic reaction is one in which energy is taken in from the surroundings resulting in the temperature decreasing.

False. If the forward reaction is exothermic, the backward reaction is favoured by decreasing the temperature.

If the forward reaction is endothermic, a high temperature would increase the yield of products. This is because the forward reaction is endothermic. A high temperature would favour the endothermic pathway to oppose the increase.

True. Cooling an exothermic reaction results in the equilibrium shifting in the exothermic direction to produce more heat and raise the temperature.

If the temperature of the reaction is decreased the yield of ICl3 will increase because the forward reaction is exothermic and will be favoured to counteract the decrease in temperature.

There are two molecules of reactant in the reaction, one molecule of C2H4 and one molecule of H2O.

Changing pressure will not affect the position of equilibrium because there are the same number of molecules on each side of the equation.

Increasing the pressure would favour the forward reaction. This is because there are 4 molecules on the left and side and 2 molecules on the right hand side. An increase in pressure will favour the side with the fewest number of molecules.

Equilibrium shifts in the direction that produces the larger number of molecules of gas to increase the pressure again.

False. Increasing pressure shifts the equilibrium to favour the side with fewer gas molecules.

If pressure is decreased, the yield of product will also decrease. This is because the reverse reaction will be favoured due to having a higher number of molecules.

High pressure is needed to obtain a high yield of product. This is because to counteract the change the forward reaction will be favoured as there are fewer molecules of gas.