Answer: C
Explanation:
In endothermic reactions, enthalpy is positive, and in exothermic reactions, enthalpy is negative, So, if enthalpy is positive, then it is an endothermic reaction, and hence is required for the reaction to occur.
Foreshocks and aftershocks can happen at the same time is not true.
We add up all the various atoms:
C: 55
H: 72
Mg: 1
N: 4
O: 5
55 + 72 + 1 + 4 + 5
= 137
The answer is B.
19) it's not balanced because when adding h2 and o2 u get h2o2 not h2o
20) I'm not sure