Tension and emotion are both countable and uncountable nouns, much like "fruit" or "wine".
There is a lot of fruit in the bowl.
There are a lot of different kinds of fruit in the world.
There is wine from Australia in this shop.
There are wines from all over the world.
Tension : the (GENERAL) feeling of being so nervous or worried that you cannot relax
Tensions : the (SPECIFIC) feelings caused by a lack of trust between people, groups, or countries who do not agree about something and may attack each other
Emotion : a (GENERAL) feeling that you experience, for example love, fear, or anger
Emotions : The (SPECIFIC) feelings that you experience, for example love, fear, or anger