Document:

<PAGE>   1

                                                                   EXHIBIT 10.7a

                             FIRST AMENDMENT TO THE
                            STERLING CHEMICALS, INC.
            AMENDED AND RESTATED HOURLY PAID EMPLOYEES' PENSION PLAN

                  WHEREAS, there is reserved to the Company the right to amend
the Sterling Chemicals, Inc. Amended and Restated Hourly Paid Employees' Pension
Plan (the "Plan"); and

                  WHEREAS, the Company deems it advisable to amend the Plan in
the manner hereafter set forth;

                  NOW, THEREFORE, the Plan is hereby amended effective as of
December 31, 1998 as follows:

                  "Notwithstanding anything in the Plan to the contrary, with
                  respect to a participant who attains age 70 1/2 after 1998
                  (other than an owner, within the meaning of IRC Section 318),
                  benefits shall not be paid or begin prior to the participant's
                  termination of employment with the Company and all members of
                  its controlled group of employers; however, in no event shall
                  the participant's accrued benefit be less than the Actuarial
                  Equivalent of the accrued benefit that would have been payable
                  as of April 1 of the year following the year the participant
                  attained age 70 1/2, had the participant terminated employment
                  on the last day of such year, adjusted for the period of
                  deferral in accordance with the applicable regulations. Except
                  to the extent required by IRC Section 401(a)(9), the Plan
                  shall not preclude such a participant from receiving benefits
                  in any of the same optional forms (except for the difference
                  in the timing of the commencement of payments) that would have
                  been available had the participant retired in the calendar
                  year in which the participant attained age 70 1/2."

                  All terms used herein that are defined in the Plan shall have
the same meanings given to such terms in the Plan, except as otherwise expressly
provided herein.

                  Except as amended and modified hereby, the Plan shall continue
in full force and effect and the Plan and this instrument shall be read, taken
and construed as one and the same instrument.

                  This instrument may be executed in several counterparts, each
of which shall be deemed an original, but all of which shall constitute but one
and the same instrument which may be evidenced by any one counterpart.

<PAGE>   2

                  IN WITNESS WHEREOF, the Company has caused this instrument to
be executed effective for all purposes as provided above.

                                    STERLING CHEMICALS, INC.

                                    By:
                                       ---------------------
                                    Name:
                                         -------------------
                                    Title:
                                          ------------------<PAGE>   1
                                                                   EXHIBIT 10.7b

                                SECOND AMENDMENT
                                     TO THE
                            STERLING CHEMICALS, INC.
            AMENDED AND RESTATED HOURLY PAID EMPLOYEES' PENSION PLAN

                  WHEREAS, there is reserved to Sterling Chemicals, Inc. (the
"Company") in Section 15.1 of the Sterling Chemicals, Inc. Amended and Restated
Hourly Paid Employees' Pension Plan (the "Plan") the right to amend the Plan;

                  NOW, THEREFORE, the Plan is hereby amended effective as of
December 17, 1998 as follows:

         1. Section 3 is amended by adding thereto a new Section 3.8 to read as
follows:

                  3.8 Window Retirement Program. Notwithstanding anything in the
         Plan to the contrary, each Window Participant who retires during the
         Window Period shall be credited with an additional five years of
         Vesting Service and Benefit Service and with an additional five years
         of age (other than for purposes of calculating a joint and survivor
         annuity for such participant).
                           For purposes of this Section 3.8, the following terms
         shall have the meanings set forth below:

                  (a) Window Participant means a participant in the Plan who (i)
         on December 17, 1998, is an active employee (or on lay off status),
         (ii) is, or prior to February 1, 1999 will be age 50 or older (iii)
         retires not later than February 28, 1999, on or after reaching age 50
         with the commencement of his Monthly Retirement Income being not later
         than March 1, 1999, and (iv) has in effect prior to the commencement of
         his Monthly Retirement Income an executed Waiver and Release. If a
         participant who has executed a Waiver and Release dies prior to its
         becoming effective or after it becomes effective but prior to reaching
         his retirement date, his Eligible Surviving Spouse, if any, shall be
         entitled to the applicable benefit under Section 9.4 or 9.5 as if the
         participant had become a Window Participant and then died.

                  (b) Waiver and Release means a form of waiver and release
         prepared by the Company under which the participant releases all
         employment related claims against the Company and its affiliated
         persons.

                  (c) Window Period means the period beginning on December 17,
         1998 and ending February 16, 1999.

<PAGE>   2

         2. Section 4.4 is amended by adding thereto a new sentence to read as
follows:

                  In addition, the reduction provided above shall not apply to a
Window Participant (as defined in Section 3.8).

                  All terms used herein that are defined in the Plan shall have
the same meanings given to such terms in the Plan, except as otherwise expressly
provided herein. Except as amended and modified hereby, the Plan shall continue
in full force and effect and the Plan and this amendment shall be read, taken
and construed as one and the same instrument.

                  This amendment may be executed in several counterparts, each
of which shall be deemed an original, but all of which shall constitute but one
and the same instrument which may be evidenced by any one counterpart.

                  IN WITNESS WHEREOF, the Company has executed this instrument
this _________________, 1999, effective for all purposes as provided above.

                                    STERLING CHEMICALS, INC.

                                    By:
                                       ---------------------
                                    Name:
                                         -------------------
                                    Title:
                                          ------------------<PAGE>   1
                                                                   EXHIBIT 10.7c

                                 THIRD AMENDMENT
                                     TO THE
                            STERLING CHEMICALS, INC.
            AMENDED AND RESTATED HOURLY PAID EMPLOYEES' PENSION PLAN

                  WHEREAS, there is reserved to Sterling Chemicals, Inc. (the
"Company") in Section 15.1 of the Sterling Chemicals, Inc. Amended and Restated
Hourly Paid Employees' Pension Plan (the "Plan") the right to amend the Plan;

                  NOW, THEREFORE, Exhibit A to the Plan is hereby amended for
participants whose Employment Termination Date is on or after December 31, 1998
as follows:

                                   "Exhibit A
                            Retirement Income Factor

The "Retirement Income Factor" applicable with respect to a participant shall be
determined based on Final Average Pay in accordance with the following table:

<TABLE>
<CAPTION>
           Final Average Pay            Retirement Income Factor
           -----------------            ------------------------
<S>                                     <C>
           Less than $35,500                        $35
           $35,500 - $36,499                        $36
           $36,500 - $37,499                        $37
           $37,500 - $38,499                        $38
           $38,500 - $39,499                        $39
           $39,500 - $40,499                        $40
           $40,500 - $41,499                        $41
           $41,500 - $42,499                        $42
           $42,500 - $43,499                        $43
           $43,500 - $44,499                        $44
           $44,500 - $45,499                        $45
           $45,500 - $46,499                        $46
           $46,500 - $47,499                        $47
           $47,500 - $48,499                        $48
           $48,500 - $49,499                        $49
           $49,500 - $50,499                        $50
           $50,500 - $51,499                        $51
           $51,500 - $52,499                        $52
           $52,500 - $53,499                        $53
           $53,500 - $54,499                        $54
           $54,500 - $55,499                        $55
           $55,500 - $56,499                        $56
           $56,500 - $57,499                        $57
           $57,500 - $58,499                        $58
           $58,500 - $59,499                        $59
           $59,500 and greater                      $60"
</TABLE>

<PAGE>   2

                  All terms used herein that are defined in the Plan shall have
the same meanings given to such terms in the Plan, except as otherwise expressly
provided herein. Except as amended and modified hereby, the Plan shall continue
in full force and effect and the Plan and this amendment shall be read, taken
and construed as one and the same instrument.

                  This amendment may be executed in several counterparts, each
of which shall be deemed an original, but all of which shall constitute but one
and the same instrument which may be evidenced by any one counterpart.

                  IN WITNESS WHEREOF, the Company has executed this instrument
this ___________________, 1999, effective for all purposes as provided above.

                                    STERLING CHEMICALS, INC.

                                    By:
                                       ---------------------
                                    Name:
                                         -------------------
                                    Title:
                                          ------------------

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