REVERSIBLE REACTIONS AND EQUILIBRIA [dchemlc]

which of the following symbols are used to indicate reversible reactions?

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Answer explanation

an arrow downwards indicates precipitation.

Which of the following reactions are not reversible?

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Answer explanation

In a dynamic equilibrium

• the rate of the forward reaction is equal to the rate of the reverse reaction.

• the rate of the forward reaction is higher than the rate of the reverse reaction.

• the rate of the forward reaction is lower than the rate of the reverse reaction.

• the rate of the forward reaction is not equal to the rate of the reverse reaction.

Answer explanation

If a reaction is in dynamic equilibrium, it is dynamic in the sense that the reactions are still continuing, but the rate of the forward reaction is equal to the rate of the reverse reaction. It is an equilibrium in the sense that the total amounts or concentrations of the various things present (reactants and products) are now constant.

Consider the reaction between nitrogen gas and hydrogen gas to make ammonia

 N_2(g) + 3H_2(g) <=====> 2NH_3(g)

If the pressure of the system was increased, what would happen to the position of equilibrium?

• Position of equilibrium will shift to the right

• Position of equilibrium will shift to the left

• Position of equilibrium would shift upwards.

• There will be no change in the position of equilibrium

Answer explanation

Increasing pressure: the position of equilibrium shifts to the side which has fewer gas molecules.

Decreasing pressure: the position of equilibrium shifts to the side which has more gas molecules.

Consider the following reaction.       

H_2(g) + I_2(g)<======> 2HI _(g)

If the pressure of the system was decreased, what would happen to the position of equilibrium?

• Position of equilibrium will shift to the right

• Position of equilibrium will shift to the left

• Position of equilibrium would shift upwards.

• There will be no change in the position of equilibrium

Answer explanation

If there is the same number of gaseous molecules on both sides of the equation, changing the pressure will make no difference to the position of equilibrium.

Consider the following reaction.

A+ 2B <=====> C + D    *H = - 100kJ/mol

If the temperature of the system was increased, what would happen to the position of equilibrium?

• Position of equilibrium will shift to the right

• Position of equilibrium will shift to the left

• Position of equilibrium would shift upwards.

• There will be no change in the position of equilibrium

Answer explanation

Increasing temperature: the position of equilibrium shifts in the endothermic direction. Decreasing temperature: the position of equilibrium shifts in the exothermic direction.

Consider the following reaction

A+ 2B -----> C + D    *H = - 100kJ/mol

The enthalpy change of the reverse reaction will be

• -100 kJ/mol

• +100 kJ/mol

• -200 kJ/mol

• +300 kJ/mol

Answer explanation

When we write a reversible reaction showing an enthalpy change, the *H always shows the enthalpy change for the forward reaction.

So, in this case *H being negative tells us that the forward reaction is exothermic. The reverse reaction will be endothermic by exactly the same amount.

Which of the following actions would have no change on the position of equilibrium?

• changing the pressure

• changing the temperature

• adding a catalyst

• changing the volume

Answer explanation

The net effect of this is that there is no change in the position of equilibrium if you add a catalyst. The catalyst is added to increase the rate at which equilibrium is reached.

 what would happen if a catalyst were added to a reversible reaction?

• the forward and reverse reactions would speed up by the same proportion

• the forward reaction would speed up but not the backward reaction

• the backward reaction would speed up but not the forward reaction

Answer explanation

Adding a catalyst speeds up reactions. In a reversible change, it speeds up the forward and reverse reactions by the same proportion. For example, if it speeds up the forward reaction ten times, it speeds up the reverse reaction ten times as well.

consider the given reaction

2NO_2(g) ------>N₂O_4(g)

(brown) (colourless)

If the pressure of the system was decreased, what would happen to the colour of the equilibrium mixture?

• The colour would fade ( be colourless)

• The colour would darken (be more brown)

• The colour wont change

• Colour would change to green

Answer explanation

If you lower the pressure, the position of equilibrium will shift to increase it again by producing more gaseous molecules. Therefore the position of equilibrium shifts to the left, and you will obtain a higher proportion of the brown nitrogen dioxide in the equilibrium mixture.