PLEASE HELP ASAP Which two reactions are slower than the others?
A. Silver tarnishes in air that contains hydrogen sulfide.
B. Lead nitrate and potassium iodide react to form a yellow
precipitate.
C. Black powder in fireworks burns explosively to produce white
light
D. Iron metal reacts with oxygen in air to form rust.

A and D takes much slower

Explanation:

Here, we want to select, out of the four given reactions, the one that is slower than the other two

The answers in these case are reactions 1 and 4 ( A and D)

The two reactions show what is called rust (as directly seen in reaction 4)

When we speak of rust, we simply mean a reaction that occurs over time

For example, non coated roofings of houses doesn’t get to change color at an instant

The color degradation that occurs is something that takes some time from the initial time they were used to roof the house

Hence, from these analogy, we can see that these reactions need an an external support to thrive or to come into existence

These external supports are natural forces and they contributing efforts occur over time and cannot be seen immediately

These reactions are thus ones that take much slower time than conventional laboratory reactions in the case of the formation of the precipitate or a reaction that requires a low flash point temperature such as that of black powder to produce such explosive effects

So in conclusion, what we are saying is that the two selected reactions are subjected to the availability of some conditions and may take time to manifest and these absolutely differentiates them from reactions that are spontaneous such as the one having an explosive effect or the other one leading to the formation of a precipitate which takes far less times

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.