1.Which description best summarizes the steps that take place during black hole formation, in the correct order?(1 point)

a. A massive star depletes its nuclear fuel; gravity overpowers the star; supernova occurs; core of star collapses.

b. Supernova triggers fusion reactions; massive star implodes due to extreme radiation; volume and mass of star increase rapidly.

c. Supernova explodes a massive star; remaining core expands; nuclear reactions diminish the effects of gravity.

d. A small star gains mass through fusion reactions; core of star collapses; increased radiation triggers a supernova.

2.Which of these characteristics of a star make it most likely to become a black hole? Select the two correct answers.(1 point)

a. Its mass is equal to that of the sun.

b. It is dying.

c. Its mass is greater than 20 times the mass of the sun.

d. It has an abundant supply of hydrogen.

1) a.A massive star depletes its nuclear fuel; gravity overpowers the star; supernova occurs; core of star collapses.

2)

b.It is dying.

c.Its mass is greater than 20 times the mass of the sun.

Explanation:

1)

A black hole if the final result of the gravitational collapse of the core left behind by a supernova explosion.

In order to become a black hole, we must have a very massive star; these kinds of stars burn their nuclear fuel (hydrogen and helium at first) very quickly, as their temperature is very high. When they run out of hydrogen and helium, they start burning heavier elements (carbon, oxygen, ect.), until reaching the iron stage; the nuclear fusion of iron is not energy convenient, so it does not occur.

At this point, the gravitational force is no longer balanced by the radiation pressure, so gravity causes the collapse of the star on itself; as a result, a huge explosion occurs, called supernova.

At this point, if the mass of the initial star was large enough, the explosion leaves behind a black hole, which is an extremely dense and massive core whose gravity is so strong that even light cannot escape from it.

2)

In order to become a black hole, a star must have two characteristics:

- It must be dying. In fact, as we described in part 1), a black hole is produced after the supernova explosion, which is the final stage of the life of a supermassive star (it occurs when the star has run out of nuclear fuel, and nuclear fusion is no longer occurring in its nucleus).

- Its mass is greater than 20 times the mass of the sun. In fact, only stars whose mass is very large (several times the mass of the Sun) can explode in a supernova and later become a black hole. Stars with a smaller mass become instead a neutron star, or if their initial mass was very low (comparable to that of the Sun), they become white dwarfs, without passing through the supernova stage.

answer: the enthalpy of the overall chemical reaction is +87.88 kj or +88 kj.

according to hess's law, the overall enthalpy of a reaction is the difference between the sum of enthalpy values of all the products and the sum of enthalpy values of all the reactants.

δh(reaction) = ∑nδh(products) - ∑nδh(reactants)

δh(reaction) = [δh +δh] - [δh]

δh(reaction) = [1(-287.02) + 1(0) ] - [1(-374.9)] kj = +87.88 kj

here, δh (pcl3) = -287.02 kj, δh(cl2) = 0, δh(pcl5) = -374.9 kj

therefore, enthalpy of the overall reaction is +87.88 kj or +88 kj.