The heat exchange in chemical reactions is due to a change in:

The heat exchange involved in chemical reactions is due to change in the enthalpy.  

Enthalpy is a type of energy which is associated with the energy of bond in a molecule. The enthalpy in chemical reactions can be positive, negative or zero which is determined by the property of chemical reaction.

The enthalpy in endothermic is positive and in exothermic reaction is negative but the enthalpy in the reaction is never constant (loss or gain of heat).

the answer is na (sodium)

for step 7 table c on the lab "newtons law of action"   the average time it took the car to travel .25 meters   was 1.92 seconds.

the average time it took the car to travel 0.50 meters was 2.61 seconds

hope this : )