find the reaction for Co(s)+Cl2(g)⟶

Answer 1

Co(s) + Cl₂(g) → CoCl₂(s)

It is already balanced as written: one Co on each side, two Cl on each side.

This is a synthesis (combination) reaction

A metal plus a nonmetal gives an ionic compound — the general form is A + B → AB. It is also a redox reaction: cobalt is oxidised from 0 to +2, and each chlorine is reduced from 0 to −1.

Getting the formula right is the actual work here

Chlorine reliably forms Cl⁻, so you need enough chloride to cancel the cobalt's charge. Cobalt is a transition metal with variable oxidation states — it forms both Co²⁺ and Co³⁺ — so you cannot read its charge off the group number the way you can for sodium or magnesium.

Direct reaction of the metal with chlorine gas gives cobalt(II) chloride, CoCl₂. Two Cl⁻ balance one Co²⁺.

Had the problem specified cobalt(III), the formula would be CoCl₃ and the balanced equation would become:

2 Co(s) + 3 Cl₂(g) → 2 CoCl₃(s)

So watch for a Roman numeral in the compound name — with transition metals it is the only thing telling you the charge. (In practice CoCl₃ is unstable and decomposes, which is part of why the direct reaction gives the II compound.)

Balancing, if you had started from CoCl₂ unbalanced

Write the correct formulas first, then balance with coefficients. Never adjust a subscript to make an equation balance — changing CoCl₂ to CoCl₃ does not balance the equation, it changes the substance. Here the formulas happen to balance on their own.

Why you may have met this compound already: CoCl₂ is the classic humidity indicator. Anhydrous CoCl₂ is deep blue; absorbing water gives the hexahydrate CoCl₂·6H₂O, which is pink. That is the blue-to-pink paper in desiccant packs, and heating it drives the water back off and returns the blue — a reversible hydration, not a new synthesis.