All Images not calling alt text

I am having trouble with the alt text on my wordpress.com website. I am using a theme I coded myself. From what I recall, if I added alt text to an image in the Media Library it would insert into the DOM. Whether I'm mistaken or not, I do know no alt text is being added to the DOM now. Below is a code sample of a image I am calling. I do not call alt because I believed WordPress automatically added it, and if it doesn't I do not know what to put in it's place. Clarification is appreciated.

Edit: To resolve a flag of a duplicate, my question revolves around using the_post_thumbnail to call the image. I am indeed using the_post_thumbnail_url which I need to change to the_post_thumbnail but this causes the images to disappear. In the suggested duplicate question, the alt needs to be called manually where-as I am looking for it to be called automatically.

Image code sample

img src="?php the_post_thumbnail_url(); ?" class="img-responsive"

I tried adding this code to functions.php during my research but it did not work:

function add_img_title($attr, $attachment = null) {
$img_title = trim(strip_tags($attachment-post_title));
$attr['alt'] = $img_title;
$attr['title'] = $img_title;
return $attr;
}
add_filter('wp_get_attachment_image_attributes', 'add_img_title', 10, 2);

Update

After receiving feedback to change the_post_thumbnail_url to the_post_thumbnail I have decided to include the wp_query and the loop as I am having trouble calling images via the_post_thumbnail

?php 
 $query1 = new WP_Query(array(
      'cat' = 162,
      'post_type' = 'page',
      'posts_per_page' = 1
  ));

  $query2 = new WP_Query(array(
      'cat' = 162,
      'post_type' = 'post',
      'posts_per_page' = 1
  ));

  $wp_query = new WP_Query();
  $wp_query-posts = array_merge( $query1-posts, $query2-posts );
  $wp_query-post_count = $query1-post_count + $query2-post_count;
?
?php while( $wp_query-have_posts() ): $wp_query-the_post(); ?
  ?php  $id = get_the_ID(); ?
  a href="?php the_permalink(); ?"?php the_post_thumbnail( 'post-thumbnail', array( 'class' = 'img-responsive' ) ); ?/a
?php endwhile; ?