Figure 11. Depending of boiling-points (b.Pt) of double Hydrogen-contained compounds from nature of the second atom and presence of Hydrogen bonding

• The evaporation of sweat, used by many mammals to cool themselves, achieves this by the large amount of heat needed to break the hydrogen bonds between water molecules.

• Moderating temperature shifts in the ecosystem (which is why the climate is more moderate near large bodies of water like the ocean).

Multiple hydrogen bonds in alive objectes:

• hold the two strands of the DNA double helix together:

• hold polypeptides together in such secondary structures as the alpha helix and the beta conformation;

• help enzymes bind to their substrate;

• help antibodies bind to their antigen

• help transcription factors bind to each other;

• help transcription factors bind to DNA.

2. Why does Chemical Bond occur?

The fascinating variety of materials in the world around us is possible because chemical bonds unite atoms of the elements in so many different combinations. Some elements are found in nature only in chemical compounds. Even the atoms of elements that can be found in nature in uncombined form - such as Oxygen and Nitrogen in the air, of Gold and Copper in the Earth’s crust - do not exist as independent atoms. They are bonded together.

If their potential energy is lowered by the change, two atoms will form a chemical bond. Throughout nature, changes that decrease potential energy are favored. Books fall off desks, and the result is lower potential energy for the books. Most atoms have lower potential energy in bonds than as independent atoms.

Chemical-bond formation is often energy - releasing process. Experiments also show that the reverse - breaking chemical bonds - is often an energy-absorbing process. Atoms separated by breaking a chemical bond have a higher total potential energy than when they were bonded. Whether or not a given chemical reaction occurs spontaneously is partly dependent on whether or not forming new bonds in the products produced enough energy to break bonds in the reactants.

Covalent, ionic and metallic bond all decrease the potential energy of the combined atoms.

Practice problems

1. What are the three major types of bonds?

2. What is the role of electronegativity in the determination of the ionic or covalent character of a bond?

3. What type of bond would be expected between H and F; Cu and S; Br and I?

4. List the three bond pairs referred to in the previous question in order to increase ionic characters.

Сhapter # 5. Laboratory glassware, labware and rules of laboratory research

Vocabulary

English	Українська назва	English	Українська назва
Test-tube	Пробірка	Gooch crucible	Тигель Гуча
Beaker	Стакан	Cylinder	Циліндр (мірний)
Walter’s Crucible Holder	Держач для тиглів Вальтера	Mortar	Ступка
		Pestle	Пестик
Buchner Funnel	Воронка Бюхнера	Spatula	Шпатель
Clamp	Зажим	Pump	Пробовідбірник (хімічний)
Crystallizing Dish	Кристалізатор	Erlenmeyer’s flask	Колба Ерленмеєра
Dropper Bottle	Крапельниця	Volumetric flask	Мірна колба
Cork	Корок	Evaporation dish	Випарювальна чашка
Florence flask	Круглодонна колба	Pipette	Піпетка
Forceps	Пінцет	Filtering funnel	Воронка для фільтрування
Glass jar	Склянка

The most useful chemical glassware is presented in Fig. 12 and 13.

F igure 12. Chemical Glassware

Test tube

Beaker

Erlenmeyer Flask

Florence Flask

Volumetric Flask

Dropper Bottle

Glass Jar

Filtering Funnel

Buchner’s Funnel

Gooch’s Crucible

Walter’s Crucible Holder

Pi-Pump

Graduated Cylinder

Test tube clamp

Clamps

Watch glass

Thermometers

Spatulas

Volumetric pipettes

Evaporation dish

Pestle and mortar

Forceps

Crystallizing Dish as sand bath

Figure 13. Separate varieties of laboratory glassware