Which of the following reactions is endothermic? h2(g) + ½o2(g) → h2o(g), δh = -57.82 kcal ½n2(g) + o2(g) + 8.1 kcal → no2(g) ½n2(g) + 3/2h2(g) → nh3(g) + 11.0 kcal c(diamond) + o2(g) → co2, δh = -94.50 kcal

the reactions: 1/2n2(g)+ o2(g) ⇒ no2(g) +8.1kcal and 1/2n2+3/2h2(g)⇒ nh3(g)+11.0kcal are both endothermic, that is, they react by gaining energy thus the positive enthalpy notation.

(newtons first law)

conservation of momentum law states :

momentum before collision = momentum after collision

momentum p (kg-m/s)= mass * velocity

say moving car=1 000 kg, velocity = 10 m/s then p = 1 000*10= 10 000 kg-m/s

say still car = 2 000 kg, velocity = 0 m/s then p = 2 000* 0 = 0 kg-m/s

total momentum prior to collision = 10 000 + 0 = 10 000 kg-m/s

momentum after collision = 10 000 kg-m/s

mass now = 1 000 kg+2 000 kg = 3 000 kg

10 000 = 3 000 kg* velocity m/s

10 000/3 000 = velocity

3.33 m/s = velocity after collision

an impulse is a force applied over time , momentum is exclusive of external forces and a perfect collision is implied