Reversibility & Equilibrium Flashcards

All 36 cards in this deck

What is a reversible reaction?

A reaction in which the products can react to produce the original reactants.

What does the symbol $\rightleftharpoons$ mean in an equation?

e.g. $\mathrm{NH_4Cl} \rightleftharpoons \mathrm{NH_3} + \mathrm{HCl}$

The reaction is reversible — it can go in both directions.

e.g. ammonium chloride breaks down into ammonia and hydrogen chloride, which can recombine.

True or false? The direction of a reversible reaction can be changed by changing the conditions.

True. Changing conditions such as temperature can make the reverse reaction happen.

What is observed when blue hydrated copper sulfate is heated?

It turns white (anhydrous copper sulfate) and water is driven off as vapour.

How can white anhydrous copper sulfate be changed back to blue?

Add water — the reverse reaction forms hydrated copper sulfate, which is blue.

What happens when solid ammonium chloride is heated in a tube, and then the vapours cool?

On heating it decomposes to ammonia and hydrogen chloride gases; on cooling these recombine to form solid ammonium chloride.

If a reversible reaction is exothermic in one direction, what is it in the opposite direction?

Endothermic, and the same amount of energy is transferred in each direction.

True or false? In a reversible reaction, the forward reaction transfers more energy than the reverse reaction.

False. Exactly the same amount of energy is transferred in each direction, just in opposite directions.

In the reaction hydrated copper sulfate $\rightleftharpoons$ anhydrous copper sulfate $+$ water, which direction is endothermic?

The forward direction (heating to drive off water); adding water back is exothermic.

True or false? A reversible reaction that absorbs energy when it goes forwards releases energy when it goes backwards.

True. The reverse of an endothermic reaction is exothermic, transferring the same amount of energy.

When is a reversible reaction at equilibrium?

When the forward and reverse reactions occur at exactly the same rate.

What is a closed system in the context of equilibrium?

Apparatus that prevents reactants and products escaping (or entering).

True or false? At equilibrium both reactions have stopped.

False. Both reactions carry on, but at exactly the same rate, so the amounts do not change.

True or false? At equilibrium the amounts of reactants and products are always equal.

False. Their concentrations stay constant, but they are usually not equal to each other.

What happens to the concentrations of the substances once equilibrium is reached?

They stay constant, because the forward and reverse reactions occur at the same rate.

State Le Chatelier's Principle.

If a system at equilibrium is subjected to a change in conditions, the system responds to counteract the change.

What do the relative amounts of reactants and products at equilibrium depend on?

The conditions of the reaction — concentration, temperature and pressure.

What effect does adding a catalyst have on the position of equilibrium?

No effect — it speeds up the forward and reverse reactions equally, so equilibrium is only reached sooner.

What are the steps to predict the effect of a change in conditions on a system at equilibrium?

e.g. increasing the pressure on $\mathrm{N_2(g)} + 3\mathrm{H_2(g)} \rightleftharpoons 2\mathrm{NH_3(g)}$

1. Identify the change: pressure increased.
2. Decide which direction counteracts it: the side with fewer gas molecules (4 → 2), the right.
3. State the shift and its effect: equilibrium shifts right, so the yield of ammonia increases.

What are the steps to deduce from yield data whether a forward reaction is exothermic or endothermic?

e.g. the yield of product falls as temperature increases

1. Read the trend in yield as temperature rises: the yield falls.
2. A lower yield means the equilibrium has shifted to the left on heating.
3. Heating favours the endothermic direction, so the forward reaction is exothermic.

What happens to a system at equilibrium if the concentration of a reactant is increased?

Equilibrium shifts to the right — more products are formed until equilibrium is reached again.

What happens to a system at equilibrium if the concentration of a product is decreased?

Equilibrium shifts to the right — more reactants react until equilibrium is reached again.

What happens to a system at equilibrium if the concentration of a product is increased?

Equilibrium shifts to the left — more reactants are formed until equilibrium is reached again.

True or false? Removing a product from an equilibrium mixture increases the amount of reactant.

False. The equilibrium shifts right to replace the product, so more reactant is used up.

What are the steps to predict the colour change when more of a reactant is added to a coloured equilibrium mixture?

e.g. adding thiocyanate ions to $\mathrm{Fe^{3+}} + \mathrm{SCN^-} \rightleftharpoons \mathrm{FeSCN^{2+}}$ (red)

1. Note the change: concentration of a reactant (SCN$^-$) is increased.
2. Equilibrium moves right to reduce that increase.
3. Concentration of the product rises: the mixture becomes more red.

True or false? Once equilibrium is reached, adding more reactant has no effect because the rates are already equal.

False. The system is no longer at equilibrium and shifts right until a new equilibrium is reached.

If the forward reaction is endothermic, what does increasing the temperature do to the relative amount of products at equilibrium?

It increases the amount of products (equilibrium shifts to the right).

If the forward reaction is exothermic, what does increasing the temperature do to the relative amount of products at equilibrium?

It decreases the amount of products (equilibrium shifts to the left).

In which direction does the equilibrium position move when the temperature is decreased?

In the exothermic direction, so that the decrease in temperature is counteracted (energy is released).

True or false? Increasing the temperature always increases the yield of product in a reversible reaction.

False. It only increases the yield if the forward reaction is endothermic; for an exothermic forward reaction the yield falls.

Data show the percentage yield of a product decreases as temperature increases. What does this tell you about the forward reaction?

The forward reaction is exothermic (it releases energy to the surroundings).

What effect does an increase in pressure have on the position of equilibrium?

It shifts towards the side with the smaller number of gas molecules shown in the symbol equation.

What effect does a decrease in pressure have on the position of equilibrium?

It shifts towards the side with the larger number of gas molecules shown in the symbol equation.

When does changing the pressure have no effect on the position of equilibrium?

When there are equal numbers of gas molecules on each side of the equation.

e.g. $2\mathrm{HI(g)} \rightleftharpoons \mathrm{H_2(g)} + \mathrm{I_2(g)}$

True or false? Changing the pressure shifts the equilibrium position of a reaction between aqueous solutions.

False. There is no change in equilibrium position because no gases are present.

What are the steps to predict the effect of increasing the pressure on a gaseous equilibrium?

e.g. $2\mathrm{NO_2(g)}$ (brown) $\rightleftharpoons \mathrm{N_2O_4(g)}$ (colourless)

1. Count the gas molecules on each side: 2 on the left, 1 on the right.
2. Increased pressure shifts equilibrium to the side with fewer molecules: the right.
3. More $\mathrm{N_2O_4}$ forms, so the mixture becomes a lighter shade of brown.