Based on the activity series, determine whether each of these reactions will occur

more reactive metal can displace less reactive metal from its salt spontaneously
where as less reactive metal can not replace more reactive metal from its salt spontaneously
reactivity of these metals in decreasing order is
Cr > Ni > Pb >Ag > Au

spontaneous
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2Ag+(aq) + Cr(s) --> 2Ag(s) + Cr2+(aq)
Cr is more reactive, Hence it displace Ag+
Au3+ (aq) + 3Ag(s) --> Au(s) + 3Ag+(aq)
Ag is more reactive hence it diplaces Au3+

Non spontaneous
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Ni2+(aq) + Pb(s) --> Ni(s) + Pb2+(aq)
Pb is less reactive So it can not displace Ni2+