Determine the part of speech for each word. "Imbalance" is only defined as a noun, while "unequal" can be used as an adjective, noun or adverb. Because two words are the closest in meaning when they are used as the same part of speech, look at the definition between the noun definitions of each word.
Define each of the words. "Imbalance" is defined as not having the correct proportion or not having correct equilibrium. The noun definition for unequal is someone or something that is not equal to the other. Not having balance and not being equal are two different concepts.
Use both words in a sentence with their noun definitions in mind. For "imbalance" the sentence might read: He had an imbalance in his brain that caused his medical condition. For "unequal" the sentence might read: He was looked at as an unequal when he tried to pay at the fancy restaurant. In both of these sentences, the words are used correctly.
Switch these words within the sentences you just created. The first sentence now reads: "He had an unequal in his brain which caused his medical condition." This usage does not make sense. Switching the last sentence: "He was looked at as an imbalance when he tried to pay at the fancy restaurant." This second sentence also does not make sense.