Study the reactions. C2H2 (g)+2H2 (g)→C2H6 (g)ΔH=−75 kcal
C2H4 (g)+H2 (g)→C2H6 (g)ΔH=−33 kcal
C2H2 (g)+H2 (g)→C2H4 (g)ΔH= ?

How can the enthalpy of Reaction III be calculated correctly?

A) by first multiplying the enthalpy of Reaction 2 by two and then finding the sum of the enthalpy of Reaction 1 and the enthalpy of Reaction 2

B) by first reversing Reaction 2 and then adding the enthalpy of Reaction 1 and the enthalpy of Reaction 2
C) by adding the enthalpy of Reaction 1 and the enthalpy of Reaction 2
D) by subtracting the enthalpy of Reaction 1 from the enthalpy of Reaction 2

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.

the best answer is d because the strong cohesion is caused by the ability of water molecules to undergo hydrogen bonding.

a is incorrect because when water solidifies, it becomes   ice, which expands and occupies a bigger volume for the same mass of water in its liquid state.

b is incorrect because water has high surface tension. besides bonding to itself it hydrogen bonds to other molecules at the top decreasing possibility for breaking of attractive forces.

c is incorrect because the water molecule has oxygen which is highly electronegative and so pulls the electron pair towards itself creating a dipole moment, making the molecule polar.

e is incorrect because water has strong adhesion, ability to stick to other molecules such as glass because of this molecules having stronger hydrogen bonding than water.

no, the red giant is the hottest with 2.00.000.000 degrees celsius. while the white dwarf only 25,000 degrees celsius.