Endothermic reactions, on the other hand, absorb heat and/or light from their surroundings. For example, decomposition reactions are usually endothermic. In endothermic reactions, the products have more enthalpy than the reactants. Thus, an endothermic reaction is said to have a positive<span> enthalpy of reaction. This means that the energy required to break the bonds in the reactants is more than the energy released when new bonds form in the products; in other words, the reaction requires energy to proceed.</span>
<u>Answer: </u>
A sample initially contained 150 mg of radon-222. After 11.4 days only 18.75mg of the radon-222 in the sample remained where 3 half-lives have passed
<u>Explanation:</u>
Given, the initial value of the sample, 
= 150mg
Final value of the sample or the quantity left, A = 18.75mg
Time = 11.4 days
The amount left after first half life will be ½.
The number of half-life is calculated by the formula
where N is the no. of half life
Substituting the values,
On equating, we get, N = 3
Therefore, 3 half-lives have passed.
Answer:
a. The electronic configuration of the hydride anion is 1s2. TRUE
b. The valence electronic configuration of strontium is 4d2. FALSE
c. For a given value of l the number of possible values of ml is 2l + 1. TRUE
d. Cu+ has the same electronic configuration as Ni. TRUE
e. The magnetic quantum number is never larger than the principle quantum number (for a given orbital). TRUE
Explanation:
a. The electronic configuration of the hydride anion is 1s² is true since the hydriden anion is the hdrogen atom which has gained an electron and we will add that electron to the 1s¹ configuration of H.
b. The valence electronic configuration of strontium is 4d2 is false since Sr is an element of period 5 , therefore its valece electrons are in in period five and it has 2 electrons because Sr belongs to group 2.
c. For a given value of l the number of possible values of ml is 2l+1 is true since this number gives the magnetic orientation for the sublevel. Thus for s there is only one orientation, then ml = 2 (0 ) +1 . Por p with l equal to 1 we have three possible orientations : 2(1) + 1 =3. The d and f sublevels have 10 and 14 orientations.
d. Cu⁺ has the the same electronic configuration as Ni is true since Cu, atomic number 29, has one more electron than its neighbor Ni with an atomic number of 28. If we remove one electron from copper we are gong to have the same 28 electrons niquel has in its neutral state.
e. The magnetic quantum number is never larger than the principal quantum number for a given orbital is true since l, the magnetic quantum number can have values up to n-1, the principal quantum number.
To be used as a conductor for heat
