_Li + __F2 → _LIF
How do I balance this?

 2Li   +   F₂    →    2LiF

Explanation:

The reaction expression is given as:

          Li   +   F₂    →     LiF

We are to balance the expression. In that case, the number of atoms on both sides of the expression must be the same.

 Let use a mathematical approach to solve this problem;

  Assign variables a,b and c as the coefficients that will balance the expression:

             aLi   +  bF₂    →     cLiF

Conserving Li: a  = c

                   F:  2b  = c

          let a = 1, c  = 1 and b  =  

  Multiply through by 2;

        a  = 2, b = 1 and c  = 2

               2Li   +   F₂    →    2LiF

answer: semi-infinite solid initially at a uniform temperature of 400 ◦c is cooled at its surface by convection. the cooling fluid at a constant temperature of 200 ◦c has a convective coefficient (ha) of 250 w/m2.k. the physical properties of the solid: k = 20 w/m.k and α = 4 * 10-5  m2/s. using δx = 0.04 m and δt = 10 s. write the finite difference equations describing the transient heat transfer of the first five δx and the temperature profile after 30 s.