find the reaction for K(s)+Cl2(g)⟶

Answer 1

2 K(s) + Cl2(g) -> 2 KCl(s)

Here is how to get there rather than just recall it, because this same two-step works for any metal-plus-nonmetal combination.

Step 1 - predict the product from the charges

Potassium is in group 1, so it loses one electron and becomes K+. Chlorine is in group 17, so it gains one electron and becomes Cl-. A compound has to come out electrically neutral, and +1 with -1 already cancels, so the formula is KCl - one of each, no subscripts.

This is a synthesis (combination) reaction: two elements go in, one compound comes out. It is also a redox reaction - potassium is oxidized from 0 to +1, chlorine is reduced from 0 to -1.

Step 2 - balance

Write the skeleton first: K + Cl2 -> KCl. Chlorine is the problem, because it exists as a diatomic molecule, Cl2, but appears singly in the product. Put a 2 in front of KCl to use up both chlorine atoms, then a 2 in front of K to match:

2 K(s) + Cl2(g) -> 2 KCl(s)

Count to confirm: 2 K on each side, 2 Cl on each side. Balanced.

Two things worth carrying forward. Never change a subscript to balance an equation - KCl2 would be a different substance that does not exist; you only ever adjust coefficients. And remember the seven diatomic elements, H2 N2 O2 F2 Cl2 Br2 I2, because forgetting that chlorine comes in pairs is the single most common way this problem gets balanced wrong.