Answer by Paul Dasil for Bootstrap 3 collapse change chevron icon on click

Pure CSS, with even less code + animation.

HTML part:

<a data-toggle="collapse" href="#collapseExample"      aria-expanded="false" aria-controls="collapseExample">        Open/Close collapse<i class="fa fa-chevron-right pull-right"></i></a>

CSS:

a[aria-expanded=true] .fa-chevron-right { transition: .3s transform ease-in-out; transform: rotate(90deg);}