Given the following half-reactions and their standard reduction potentials,

Ni2+ + 2e− Ni E° = -0.25 V
2H+ + 2e− H2 E° = 0.00 V
Ag+ + e− Ag E° = 0.80 V
a spontaneous reaction will occur between:

a. Ni2+ and H2
b. H+ and Ag
c. Ag+ and Ni
d. Ni2+ and Ag+
e. H+ can react spontaneously with any of the species here because its reduction potential is zero

I don't know I think the answer is 3 or 4? Some help? Thank you

The answer is c.

a won't work. Why?
Ni^2+ is -0.25 with
H2 is 0 so Ecell is negative and that isn't spontaneous.

But c will. Why?
Ag^+ is 0.8
Ni to Ni^+ is +0.25
Add and Ecell is + so rxn is spontaneous.

You can go through the others to convince yourself that all are negative.

To determine whether a spontaneous reaction will occur between two species, we need to compare their reduction potentials.

The general rule is that a spontaneous reaction occurs when the reduction potential of the oxidizing agent (the species being reduced) is greater than the reduction potential of the reducing agent (the species being oxidized).

Let's compare the reduction potentials for the given half-reactions:
1. Ni2+ + 2e− Ni E° = -0.25 V
2. 2H+ + 2e− H2 E° = 0.00 V
3. Ag+ + e− Ag E° = 0.80 V

Now let's compare the reduction potentials for the possible combinations:
a. Ni2+ and H2:
Ni2+ has a reduction potential of -0.25 V, and H2 has a reduction potential of 0.00 V. Since -0.25 V < 0.00 V, it is not a spontaneous reaction.

b. H+ and Ag:
H+ has a reduction potential of 0.00 V, and Ag has a reduction potential of 0.80 V. Since 0.00 V < 0.80 V, it is not a spontaneous reaction.

c. Ag+ and Ni:
Ag+ has a reduction potential of 0.80 V, and Ni has a reduction potential of -0.25 V. Since 0.80 V > -0.25 V, it is a spontaneous reaction.

d. Ni2+ and Ag+:
Ni2+ has a reduction potential of -0.25 V, and Ag+ has a reduction potential of 0.80 V. Since -0.25 V < 0.80 V, it is a spontaneous reaction.

e. H+ can react spontaneously with any of the species here because its reduction potential is zero. However, this option (e) is not correct since it does not specifically mention any other species.

Based on the comparison of reduction potentials, the correct answer is (d) Ni2+ and Ag+.

To determine if a spontaneous reaction will occur between two species, we need to compare their reduction potentials.

The more positive the reduction potential, the stronger the tendency for reduction to occur. Therefore, in a spontaneous reaction, the species with a higher reduction potential will undergo reduction, while the species with a lower reduction potential will undergo oxidation.

Let's compare the reduction potentials of the given half-reactions:

1. Ni2+ + 2e− Ni E° = -0.25 V
2. 2H+ + 2e− H2 E° = 0.00 V
3. Ag+ + e− Ag E° = 0.80 V

Based on the reduction potentials, we can determine the following:

- Ni2+ (-0.25 V) can undergo reduction and be reduced to Ni.
- H+ (0.00 V) has the reducing agent potential of zero, so it cannot undergo any spontaneous reduction.
- Ag+ (0.80 V) can undergo reduction and be reduced to Ag.

Now, let's analyze the options given:

a. Ni2+ and H2:
Since Ni2+ has a lower reduction potential (-0.25 V) compared to H2 (0.00 V), a spontaneous reaction will not occur between Ni2+ and H2.

b. H+ and Ag:
Since H+ has a reducing agent potential of zero (0.00 V) and Ag has a reduction potential of 0.80 V, a spontaneous reaction can occur between H+ and Ag.

c. Ag+ and Ni:
Since Ag+ has a higher reduction potential (0.80 V) compared to Ni2+ (-0.25 V), a spontaneous reaction can occur between Ag+ and Ni.

d. Ni2+ and Ag+:
Since Ag+ has a higher reduction potential (0.80 V) compared to Ni2+ (-0.25 V), a spontaneous reaction can occur between Ag+ and Ni2+.

e. H+ can react spontaneously with any of the species here because its reduction potential is zero.
This statement is incorrect. H+ does not have a spontaneous reaction with Ni2+, as Ni2+ has a lower reduction potential (-0.25 V), indicating that it has a higher tendency to be oxidized.

Based on the analysis above, the correct answer is (b) H+ and Ag.