Question

which equation represents an oxidation-reduction reaction?
1.HCl+KOH-->KCl+H2O
2.4HCl+MnO2-->MnCl2+2H2O+Cl2
3.2HCl+CaCO3-->CaCl2+H2O+CO2
4.2HCl+FeS-->FeCl2+H2S

Answer 1

The easy, but painful when you're just learning, way is to go through methodically and write the oxidation state of EACH atom, then compare on both sides to see if anything has changed. For example, I'll do the first one BUT I can't write over or under the atoms/ions. I'll do it step wise.
HCl + KOH ==> KCl + H2O
1. HCl. H is +1; Cl is -1.
2. KOH. K is +1; O is -2; H is +1
3. KCl. K is +1; Cl is -1.
4. H2O. Each H is +1; O is -2
Compare and you will see there were no changes; therefore, #1 is not a redox reaction.
Do you know how to determine the oxidation state? Here is a good site to get you started.
http://www.chemteam.info/Redox/Redox-Rules.html