KEY2CHEM

Reversibility of Reactions

Many important chemical and physical processes are reversible. That is, the reaction can proceed in both the forward (reactants convert to products) and reverse (products revert back to reactants) directions. Reversible reactions play a major role in biological and environmental processes.

Example 1.

A glass of water at $$0^\circ C$$ contains several ice cubes. Which statement about this system is true?

A. The ice is melting.

B. The water is freezing.

C. The ice is melting and the water is freezing.

Solution

C. The ice is melting and the water is freezing.

At this temperature, the phase change between liquid and solid state ($$\require{mhchem}\ce{H2O(l) <=> H2O(s)}$$) is reversible.

Example 2.

At $$100^\circ C$$, the boiling of liquid water to water vapor is a reversible process. Which describes why the phase change is reversible?

A. The water molecules are breaking strong covalent bonds that can re-form.

B. The water molecules are breaking relatively weak intermolecular forces that can re-form.

C. The water molecules are decomposing into hydrogen gas and oxygen gas, which are in the vapor phase.

Solution

B. The water molecules are breaking relatively weak intermolecular forces that can re-form.

The relatively weak intermolecular forces can break and re-form between water molecules.

Example 3.

Nitrate ions ($$\require{mhchem}\ce{NO3-}$$) in soil can be reduced to ammonia ($$\require{mhchem}\ce{NH3}$$). Ammonia can _______ nitrate ion.

A. be reduced to

B. be oxidized to

C. not be changed back into

Solution

B. be oxidized to

Oxidation and reduction are reverse processes for electron transfer.