The open end wrench pictured above will work quite nicely to remove or install pedals.

Most people get frustrated trying to remove both pedals as one seems to come off quite easily while the other one seems frozen in place.  In a word or two, reverse threads.

Yes, one of the pedals has reverse threads and instead of going CW to tighten, you go CW to loosen.

This is what a pair of (very old) clip on pedals look like

If you look closely at the threads, they are different.

The threads on the left pedal slant down to the right while the other pedal is opposite.

Now that you know that the threads are different, what do you do?  It is simple really.  The pedal with the normal threads tightens with a CW motion on the wrench and untightens with a CCW motion on the wrench as you would expect.  The other pedal is opposite; CW loosens it and CCW tightens it.

The significant thing to remember is that the pedal on the left side of the bike is the one with the reverse threads.

This is the left side pedal with a wrench attached.  If you were going to remove it, your first thought would be to turn the wrench CCW.  That is what you do with normal threads.  This happens to be the side with reverse threads so you do the opposite to remove or tighten this pedal.  CW removes it and CCW tightens it. (Note that the bike is upside down. ;=)

Why did the bicycle makers do this?  I am glad you asked.  The original makers of bikes figured out that the left pedal would loosen up after a little bit of pedaling because the effort of pedaling was like putting a wrench on it. With normal threads the pedal loosens up, with reverse threads, the act of pedaling tightens the pedal.  Thus the solution to always losing only the left pedal, was to use reverse threads only on the left pedal.

So, now you know, The wrench treatment on the right pedal is treated as normal.  For the left pedal use the opposite wrench treatment: CW loosens, CCW tightens.