From the tagging, it seems that you are using KOMAscript and the class scrbook. Then you change the font either with \setkomafont or \addtokomafont (see section 3.6, page 51 in the revised manual), and with the modifier chapterprefix. Try:
\setkomafont{chapterprefix}{\rmfamily\Large\bfseries}
or, to change to a serif typeface only:
\addtokomafont{chapterprefix}{\rmfamily}
To change all sectioning to the default Roman font, reverse all font changes you have done to chapter, and try
\addtokomafont{sectioning}{\rmfamily}
It seems from your comment that somebody has tampered with the definitions in your classfile. This minimal example works for me, giving me sans serif in heading and body text.
\documentclass[twoside,12pt,a4paper,pointlessnumbers,headsepline,idxtotoc]{scrbook}
\usepackage{fixltx2e}
\usepackage[UKenglish]{babel}
\usepackage{tgheros}
\renewcommand*\familydefault{\sfdefault}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{blindtext}
\begin{document}
\blinddocument
\end{document}
In your document, comment out (or delete) all usepackage{secsty} etc. that changes the headings. Or add packages and commands from you preamble to my code.
If you are going to use Helvetica as sans serif font, I suggest using the Tex-Gyre version instead, by loading the package tgheros. I have changed my MWE correspondingly.
The font TeX Gyre Pagella is based on Palatino, which was designed by Herrmann Zapf. Within the past ten years or so, Zapf has designed a sans-serif font, called Palatino Sans, to accompany Palatino. Note, though, that Palatino Sans isn't free and isn't include in any of the standard TeX distributions.
You may want to use Helvetica as your document's sans serif font. If you do so, you should probably look into scaling Helvetica down a bit as its x-height is rather larger than that of Palatino.
Best Answer
In addition to suggestions from @bdh_dtu and @egreg you might want to also add in the xspace package:
\xspacetries to be clever about inserting a space after the macro if there would normally be one but not if there shouldn't be one, such as when writing\R.(i.e. ending the line with\Rfollowed by a full stop.