Question

1a. Arrange BaS,  GaP,  CaO,  RbCl in order of decreasing lattice energy. (2)

1b. Arrange KBr,  InAs,  LiCl,  SrSe,  ZnS in order of increasing lattice energy. (

Answer 1

Lattice energy is a type of energy which is required to break 1 mole of Ionic solid into gaseous ions.

We know that lattice energy is directly proportional to product of charges of a compound. We also know that if product of charges of two compounds are similar to each other then we will have to consider their sizes.

Considering the above mentioned facts:

1)a) In BaS- Ba2+ and S2- ions are present. In GaP- Ga3+ and P3- ions are present. In CaO- Ca2+ and O2- ions are present. In RbCl- Rb+ and Cl- ions are present.

Since RbCl has only -1 and +1 charges , therefore product of both charges is the least amongst all other compounds it will have lowest lattice energy And also GaP has greatest charges as compared to all , it will have highest lattice energy. In CaO and BaS - since both have equal charges their relative sizes should be compared. As Ba2+ is placed below to Ca2+ in periodic table therefore Ba2+ is larger in size and similarly S2- is larger than O2-. Since BaS is a bigger compound than CaO its internuclear distance will be greater than CaO. Since lattice energy is inversely proportional to internuclear distance therefore lattice energy of BaS will be lower than that of CaO.

GaP>CaO>BaS>RbCl

1)b)In Kbr- K+ and Br- ions are present. In InAs - In+3 and As-3 ions are present. In LiCl- Li+ and Cl- ions are present. In SrSe- Sr2+ and Se2- ions are present. In ZnS- Zn2+ and S2- ions are present.

KBr<LiCl<SrSe<ZnS<InAs

( A similar explanation for the arrangement of the compounds should be considered as in first question)