In the reaction 2h2 +o2 = what coefficient should be placed in front of h2o to balance the reaction

2.
You need to look at how many atoms of each element you have on the one side and make it equal to the other.
You have 4 hydrogen atoms on the left side. On the right you have 2. To get from 2 to 4 you must multiply by 2.
To check this you can see you have 2 oxygen atoms on the left and 1 on the right. This again needs to be multiplied by 2.

Cr(oh)3 -> cr(3+) + 3oh-

Exothermic,  physical change after it is already baked