FBISE (NBF) Chemistry XI (HSSC-1)Exercise Chapter # 06

  1. Indicate the effect of pressure and temperature on solids.?

There are little affected by changes in temperature and pressure. In solids, molecules are tightly packed, Therefore, pressure has no effect on them. However, increase in temperature increases their volume to some extent. It is because of decrease in intermolecular forces.

  1. What do you know about hexagonal closest packing?

When two layers of spheres are placed over each other than the second layer has two types of holes in it, tetrahedral holes and octahedral holes. If third layer of spheres is placed directly above the tetrahedral holes then this third layer is identical to bottom layer. This is called hexagonal close packing (hcp) arrangement. It is usually written as ABAB or 1212 pattern.

  1. Write a note on Dispersion effect.

The distortion of electronic cloud of a substance is known as dispersion effect. This produces polarizability in the molecule. The polarizability of an atom or molecule is a measure of the ease with which electron charge density is distorted. Bigger atoms have more electrons and bigger electron cloud than small atoms. In bigger atoms, the outer electrons are more loosely bound. They can shift towards another atom more readily. Thus, polarizability increases with increased atomic and molecular size.

  1. There are some changes in matter. Which of the changes require us to provide energy and which gives out energy?
  2. a) Solids -> Liquids

This is melting process. This change requires energy called heat of fusion. It is because intermolecular forces of solids should be broken to convert them into liquid.

  1. b) Liquid -> Gas

This is vaporization process. This change requires energy called heat of vaporization. It is because intermolecular forces of liquids should be broken to convert them into gas.

  1. c) Gas -> Liquid

This is condensation process. This change releases energy since gas is at higher energy state than liquid so it is cooled to convert into liquid state.

  1. d) Liquid -> Solid

This is freezing process. This change releases energy since liquid is at higher energy state than solid so it is cooled to convert it into solidstate.

  1. Explain each of the statements below.
  2. a) You can smell a fish from across the road. This is due to the diffusion process. The vapors from the fish diffuse

into air and reach you. So you can smell the fish.

  1. b) Sugar dissolves faster in hot water than in cold water. The solubility of sugar in water is an endothermic process. Therefore when temperature is increased, the intermolecular forces among sugar molecules break faster. Thus sugar dissolves faster in hot water than in cold water.
  2. c) Why is the temperature and pressure important when giving the density of oxygen and nitrogen?

According to the equation, d=PM/RT, the density of a gas is directly proportional to the pressure and inversely proportional to the temperature. Thus temperature and pressure should be mentioned for  densities of gases.

  1. Explain why the particles in solid ice stick together and those of steam not even when they get very close (collision)?

In solid ice particles have strong intermolecular forces therefore they stick together. They only show vibrational motion. In steam, particles have weak intermolecular forces and have high translational kinetic energy. Therefore, even when they collide they do not stick together.

  1. When long bridges are constructed, the roadbed is made in sections with spaces between the sections. Why must be done so?

It is because the roadbed section may expand due to increase in temperature during summer. Therefore, spaces are made between the sections to provide some space for expansion. This avoids any damage due to expansion.

  1. Explain each of the following situations in terms of energy produced or used.
  2. a) You feel cold immediately after getting out of a swimming pool, but you feel comfortably warm when you have dried yourself.

After getting out of swimming pool, a thin layer of water is present on the body. This water evaporates by taking heat from the body, therefore you feel cold immediately. When the body is dried up, then it equals its temperature to the surrounding there you feel warm.

  1. b) Skin exposed for just a short time to steam can suffer severe burns whereas, skin exposed to hot water for the same length of time suffers only a mild burn or none at all.

Since steam is produced by supplying heat of vaporization to the boiling water, therefore steam molecules contain more energy than hot water molecules. Thus, steam causes severer burns than hot water.

 

 

 

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