First of all, diction is the author's choice of words, phrases, sentence structure, and figurative language that all work together to create the meaning of the text. Second, the narrator's voice may not always represent the author's voice and therefore may not represent the author's tone, but only the narrator's tone; although sometimes time and history have established a link between a particular author and their "narrative voice" and tone.
To prove the narrator's tone toward marriage in The Awakening, which you may presuppose to be Kate Chopin's tone, the analysis of a small sample of passages will get you started on your way to as detailed an analysis as is needed. Pertaining to language, when speaking of marriage, the narrator and/or Chopin establishes a disapproving and saddened tone by using gloomy words like "lamentable," such as when Edna decline to go to a marriage by telling her husband that weddings were "lamentable spectacle." Also, the fact of whom Edna was addressing--she was addressing her husband to whom she was once wed--enhances the disapproving, saddened tone. The example of to whom the speaker is speaking (Edna to her husband) would be part of syntax, or sentence structure, which would fall under the category of diction.
A more detailed example from diction of the tone of disapproval and sadness toward marriage relates to Chopin's authorial narrative choice of what is called middle diction (how the average educated person of the Pontellier's wealthy class would speak). Chopin uses middle diction to express negatives thoughts and feelings in correctly and calmly constructed language. For example, consider this negative string of reflections: "She could not have told why she was crying. Such experiences as the foregoing were not uncommon in her married life. They seemed never before to have weighed much against the abundance of her husband's kindness...." These negative thoughts are all expressed in a "positive" way, not a complaining, whining sort of way, through the correct employment of Chopin's diction choice.
An examples of details demonstrating the narrator's and/or Chopin's tone of disapproval and sadness toward marriage is in the detail of Edna sitting alone in the midnight hours on the veranda crying after being foolishly and ignorantly insulted by her husband who is obviously calloused and arrogant. Imagery (of the visual and tactile sort combined) conveying tone is incorporated in this same scene: Edna's peignoir (delicate robe) sleeve is so damp from wiping her flood of tears that she gives up trying to wipe them at all. [Note: Detail and imagery are part of the broader category of mood.]