you never edit the original files, that is the whole point of hooks and filters. to do anything you usually need to identify which function is responsible for the thing you want to change or the location of hook you want to add a new function too. In your case (the former) you want to edit the blog title so you need to find the function that creates it. All the functions are in the thematic folder, under extensions. thematic_blogtitle happens to be in the header-extensions.php file... along w/ everything that goes in the header part of a theme (title, description, menu, etc, etc).
you will need to override (if you are using the latest dev thematic 0.9.7.4) the existing function by dropping this into your functions.php
function childtheme_override_blogtitle() { ?>
<div id="blog-title"><span class="Chunkfive"><a href="<?php bloginfo('url') ?>/" title="<?php bloginfo('name') ?>" rel="home"><?php bloginfo('name') ?></a></span></div>
<?php }
add_action('thematic_header','childtheme_override_blogtitle',3);
however- if you look at the original code it already has a span in the blog title so idk why you really need to give it a specific class. you can target it via css just with #blog-title span
though your question about adding chunkfive as a span and then the end code (which doesnt mention chunkfive) don't jive.. this should help as you can modify the above function anyway you want.