The question is incomplete, here is the complete question:
Write a balanced chemical equation for each single replacement reaction that takes place in aqueous solution. write no reaction if a reaction does not occur
1.) Zn + PbCl₂
2.) Cu + Fe(NO₃)₂
<u>Answer:</u>
<u>For 1:</u> The reaction does occur.
<u>For 2:</u> The reaction does not occur.
<u>Explanation:</u>
Single displacement reaction is defined as the reaction in which more reactive element displaces a less reactive element.
The reactivity of metal is determined by a series known as reactivity series. The metals lying above in the series are more reactive than the metals which lie below in the series.

For the given options:
Zinc is more reactive than lead as it lies above in the series. So, it will displace lead from its chemical equation.
The chemical equation for the reaction of zinc and lead chloride follows:

Copper is less reactive than iron as it lies below in the series. So, it will not displace iron from its chemical equation.
The chemical equation for the reaction of copper and iron (II) nitrate follows:

Condensation Theory. thought I've heard it be called the condensation-nebula theory.
Answer:
+1.03 V
Explanation:
The standard emf of the voltaic cell is the value of the standard potential of it, which is calculated by the standard reduction potential (E°).
The standard reduction potential is the potential needed for the reduction reaction happen, and it's determined by the reaction with the hydrogen cell (which has E° = 0.0V). The half-reactions of reduction of Ni⁺² and Ag⁺, are:
Ni⁺²(aq) + 2e⁻ → Ni(s) E° = -0.23 V
Ag⁺(aq) + e⁻ → Ag(s) E° = +0.80 V
The value is calculated by a spontaneous reaction, in which the cell with the greater E° is reduced (gain electrons), and the other is oxidized (loses electrons). So, Ag⁺ reduces.
emf = E°reduces - E°oxides
emf = 0.80 - (-0.23)
emf = +1.03 V
Answer:
2016.417222 hours
Explanation: you def made your goal
28800 seconds
Helium
Remember: electron filling of atomic shell ...
The element which have electron in the lowest quantum shell will have the smallest atomic radius\
He:1s1 (Helium)
H:1s2 (hydrogen)