10.5 Water

Summary

This video covers the structure and properties of water, including its bent molecular shape with polar ends, leading to hydrogen bonding between molecules. It explains how ice forms a hexagonal structure, making it less dense than water, and why ice floats. The video also explores water's high molar enthalpies of fusion and vaporization, emphasizing its role in cooling. A practical example is provided, demonstrating how to calculate the energy absorbed when 47 grams of ice melts, showcasing the real-world application of these thermodynamic principles.

Takeaways

• 😀 Water has a bent molecular structure with two hydrogen atoms and one oxygen atom, with oxygen being more electronegative than hydrogen.
• 😀 The polarity of water molecules leads to a negative charge on the oxygen end and positive charges on the hydrogen ends, enabling hydrogen bonding.
• 😀 Water forms hexagonal structures in its solid state (ice) due to hydrogen bonding, resulting in less density and causing ice to float on liquid water.
• 😀 Ice has a density of approximately 0.92 grams per cubic centimeter, while liquid water has a density close to 1 gram per cubic centimeter.
• 😀 As water freezes, it expands to accommodate the empty space in its hexagonal structure, which makes ice less dense than water.
• 😀 Water reaches its maximum density at 4°C before boiling, as the molecules fill in the empty space in the ice structure.
• 😀 Water's high molar enthalpy of fusion (6 kJ/mol) and molar enthalpy of vaporization (40.79 kJ/mol) are due to the strong hydrogen bonds in water.
• 😀 The strong hydrogen bonds in water are the reason for water's high specific heat and its ability to absorb large amounts of heat energy.
• 😀 Water's high heat absorption properties, due to its molar enthalpies, make it an effective coolant.
• 😀 In a sample problem, to calculate the energy absorbed when 47 grams of ice melt, you need to first find the number of moles of water, then multiply by the molar enthalpy of fusion, which results in 15.7 kJ of energy released when 47 grams of water freeze.

Q & A

What is the basic structure of a water molecule?

A water molecule has a bent shape with two hydrogen atoms and one oxygen atom. Oxygen is more electronegative, leading to an uneven distribution of electrons.

Why does water have a bent structure?

The bent structure of water is due to the angle formed between the two hydrogen atoms (104.5°), which is a result of the electron pairs around the oxygen atom, causing repulsion between them.

How does water's polarity affect its behavior?

Water’s polarity, with a negative oxygen end and positive hydrogen ends, allows it to form hydrogen bonds with other water molecules. This gives water unique properties such as high surface tension and the ability to dissolve many substances.

What causes ice to float on water?

Ice floats on water because it is less dense than liquid water. The hexagonal structure of ice molecules creates empty spaces, reducing its overall density compared to liquid water.

Why does water expand when it freezes?

When water freezes, it forms a crystalline structure with hydrogen bonds that create open spaces between molecules, causing the water to expand. This is why ice takes up more space than the same amount of liquid water.

At what temperature is water most dense?

Water is most dense at around 4°C, just before it freezes. At this temperature, the water molecules are closest together, and there is no crystal structure as seen in ice.

What are the molar enthalpies of fusion and vaporization of water?

The molar enthalpy of fusion for water is about 6 kJ/mol, which is the energy required to melt 1 mole of ice. The molar enthalpy of vaporization is about 40.79 kJ/mol, which is the energy required to vaporize 1 mole of water.

How do the high enthalpies of fusion and vaporization benefit water in cooling applications?

The high enthalpies of fusion and vaporization mean that water can absorb a significant amount of energy before changing states. This makes water effective for cooling, as it can absorb large amounts of heat without a significant increase in temperature.

How is the density of ice calculated, and how does it compare to liquid water?

The density of ice is about 0.92 g/cm³, which is less than the density of liquid water (1 g/cm³). This is due to the open hexagonal structure of ice, which creates empty spaces between molecules.

What is a practical example of how to use the molar enthalpy of fusion for calculations?

For example, to find the energy absorbed when 47 grams of ice melt, you first convert the mass of ice to moles (2.61 moles), then multiply by the molar enthalpy of fusion (6 kJ/mol), resulting in about 15.7 kJ of energy absorbed.