Which type of reaction occurs in the following equation?
2NO2(g) + 2OH"(aq) —— NO2(aq) + NO, (aq) +H2O(1)
a combination redox reaction
a displacement redox reaction
a synthetic redox reaction
a disproportionation redox reaction

A disproportionation redox reaction  

Explanation:

1. Assign an oxidation number to every atom in the equation

2. Identify the atoms that change their oxidation number

N in NO₂: +4 ⟶ +3 in NO₂⁻

N in NO₂: +4 ⟶ +5 in NO₃⁻

3. Identify the type of change

This is a disproportionation — a reaction in which one substance is simultaneously oxidized and reduced.

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.