The Q value of a nuclear reaction A + b → C + d is defined by

Q = [ mA + mb– mC – md]c2


Where the masses refer to the respective nuclei. Determine from the given data the Q-value of the following reactions and state whether the reactions are exothermic or endothermic.


(i) 11H + 31H → 21H + 21H


(ii) 126C + 126C → 2010Ne + 42He


Atomic masses are given to be


m (21H) = 2.014102 u


m (31H) = 3.016049 u


m (126C) = 12.000000 u


m (2010Ne) = 19.992439 u

i) The given equation is:

Hence, according to given data and given definition of Q value in the question


Q value = [m(1H1) + m(3H1)-2 × m(2H1)] × c2


= (-0.00433u × c2) =


-0.00433 × 931.5 MeV


= -4.0334 MeV


The negative value of Q value suggests this reaction is endothermic.


ii) The reaction is :


According to previous solution Q value,


Q = [2 × m(12C6)-m(20Ne10)-m(4He2)] × c2


= (0.004958u) × c2


= 0.004958 × 931.5


= 4.6183MeV


Here, the Q value is positive hence reaction is exothermic.


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