5: which two species reacts spontaneously: (a)Cu(s) + Ag+(aq) (b)Br2(I) + Cl-(aq) (c)H2O(l) + Ca+ +(aq) (d) Au(s) + Mg2 +(aq)

To determine which two species react spontaneously, we need to consider the concept of reduction potentials and the reactivity of the elements involved. Spontaneous reactions typically occur when a more reactive species displaces a less reactive one or when a species with a higher reduction potential can reduce another species with a lower reduction potential. Let's analyze each option step by step.

Analyzing the Reactions

(a) Cu(s) + Ag+(aq)

Copper (Cu) is less reactive than silver (Ag), meaning that silver ions can be reduced by copper. The standard reduction potential for Ag+ to Ag is +0.80 V, while for Cu2+ to Cu, it is +0.34 V. Since Ag+ has a higher reduction potential, this reaction will proceed spontaneously:

• Cu(s) + 2Ag+(aq) → Cu2+(aq) + 2Ag(s)

(b) Br2(l) + Cl-(aq)

Bromine (Br2) is a stronger oxidizing agent than chloride ions (Cl-). The standard reduction potential for Br2 to Br- is +1.07 V, while for Cl2 to Cl- it is +1.36 V. Since Cl- cannot oxidize Br2, this reaction does not occur spontaneously.

(c) H2O(l) + Ca2+(aq)

Calcium ions (Ca2+) are derived from a very reactive metal, calcium. However, water does not spontaneously react with Ca2+ under standard conditions. Instead, calcium would react with water to produce hydrogen gas and calcium hydroxide, but this is not a direct reaction with Ca2+.

(d) Au(s) + Mg2+(aq)

Gold (Au) is a noble metal and does not easily react with magnesium ions (Mg2+). In fact, magnesium is more reactive than gold, so this reaction will not proceed spontaneously either. The standard reduction potential for Mg2+ to Mg is -2.37 V, which is much lower than that of Au3+ to Au (+1.50 V).

Identifying the Spontaneous Reactions

From our analysis, the only spontaneous reaction occurs in option (a), where copper reacts with silver ions. The other options do not result in spontaneous reactions based on their respective reduction potentials and reactivity. Therefore, the answer is:

• Spontaneous Reaction: (a) Cu(s) + Ag+(aq)

In summary, the only pair that reacts spontaneously is copper with silver ions, while the other combinations do not yield spontaneous reactions due to the relative reactivity and reduction potentials of the involved species.