Balancing chemical equations please help me?

1.a) why is the following equation not balanced? N2+H2(arrow)NH3 b)The following attempt was made to balance the equation.Why is it incorrect? N2+H3(arrow)N2H3 2.Balance the following chemical equations. a)Na+Cl2(arrow)NaCl b)CO+O2(arrow)CO2 c)Al+Br2(arrow)AlBr3 d)H2+Cl2(arrow)HCl e)Mg+HNO3(arrow)Mg(NO3)2+H2.Please number/letter off your answers thanks