Surface energy for different crystallographic planes
You can think about this problem from the definitions of surface energy.

The surface energy quantifies the disruption of inter-molecular bonds that occurs when a surface is created


The composition or structure of a surface depends on how the bulk crystal has been cut.
The electronic structure (or number of dangling bonds) depends on surface composition 
The reactivity of a surface depends on the electronic structure.


Imagine a battenberg cake:




The cake has been cut horizontally and the surface is comprised of tasty red and yellow squares.
If however we had cut the cake vertically straight down the middle, you would only see a red line next to a yellow line (Cut 1).
If you cut vertically and diagonally across the cake, you might only see either a purely red or purely yellow surface. In the case of the example image, you would only see a red surface (Cut 2).


I hope this example shows that (by relating the coloured squares to atoms), different cuts result in different surfaces with different properties. 

When calculating the surface energy of a system:


You need to calculate the bulk crystal energy.
You need to calculate the energy of the surface slab. Make sure your model is converged with respect to slab thickness. The central layers should be representative of the bulk crystal. Look at magnetic change, charge difference, degree of ionic relaxation.
You then calculate the difference between the slab energy and the bulk crystal energy.


Note: In periodic calculations, a surface slab is constructed by cutting the bulk in a specified plane and introducing a vacuum gap. This vacuum gaps should be sufficient that the surface does not interact with its periodic mirror images. Around 15 Angstroms. 

There are a number of codes that can help in the construction of surface slabs (METADISE, Materials Studio, Vesta)

If you wish to calculate the energy of a surface (and assuming you are using a periodic DFT code), I would highly recommend the paper by Wenhao Sun and Gerbrand Ceder (https://www.sciencedirect.com/science/article/pii/S003960281300160X)

Note: be careful of magnetic influences.