UNIT 2: What makes water such a unique chemical? Area of Study 1: How do substances interact with water? Properties of water trends in the melting and boiling points of Group 16 hydrides, with reference to the nature and relative strengths of their intermolecular forces and to account for the exceptional values for water specific heat capacity and latent heat including units and symbols, with reference to hydrogen bonding to account for the relatively high specific heat capacity of liquid water, and significance for organisms and water supplies of the relatively high latent heat of vaporisation of water. Seeing as the world has gone green, the topic of the moment is enviro chem. Will cover global warming, droughts and water saving water treatment – all the hot topics. May go to a water treatment plant…

Water Covers more than 70% Earth Essential for life Unique properties (mainly due to ability to form Hydrogen Bonds, molecular shape)

Through the Water Cycle, water is recycled in the ocean and atmosphere and on the landmass (rivers, lakes, sub-surface water)

Properties of Water Discrete Molecular Compound Polar and have a permanent overall dipole V-shaped Found in all 3 states- solid, liquid, gas Forms Hydrogen Bonds

Trends in group 16 hydrides trends in the melting and boiling points of Group 16 hydrides, with reference to the nature and relative strengths of their intermolecular forces and to account for the exceptional values for water Trends in group 16 hydrides Each of these elements can covalently bond with hydrogen to form compounds known as hydrides. The higher the melting point and boiling point the stronger the intermolecular forces must be. Increasing dispersion forces as you move down the group due to more electrons But despite water being the smallest of the hydrides it has significantly higher melting point and boiling point.

The melting and boiling points for water are quite exceptional. trends in the melting and boiling points of Group 16 hydrides, with reference to the nature and relative strengths of their intermolecular forces and to account for the exceptional values for water The melting and boiling points for water are quite exceptional. The melting and boiling points of water are both significantly higher than those of other group 16 hydrides. The values are also significantly higher than those of other molecular substances of a similar size

Revision…Hydrogen Bonds Strongest intermolecular force Hydrogen bonded to Nitrogen, Oxygen or Fluorine

Relatively High Melting/Boiling Temperature Solid: Enough energy to increase kinetic energy of molecules so they break free of the crystal lattice. Each water molecule forms 4 H-Bonds to 4 other water molecules. Liquid: Enough energy to break/weaken hydrogen bonds Boiling: Enough energy to completely break all hydrogen bonds

Water expands on freezing Expands on freezing – therefore solid water (ice) is less dense than liquid water Molecules arranged in more structured, hexagonal way in ice This is why ice floats on water – even massive bergs

specific heat capacity and latent heat including units and symbols, with reference to hydrogen bonding to account for the relatively high specific heat capacity of liquid water, and significance for organisms and water supplies of the relatively high latent heat of vaporisation of water. High Heat Capacity One particular property of water is that it requires a relatively large quantity of heat energy to raise its temperature by one degree. Water can store large amounts of heat energy. The specific heat capacity of a substance is the amount of energy needed to raise the temperature of 1g of that substance by 1°C. It is a measure of how much energy the substance absorbs as its temperature increases. Higher HC= More energy needed to raise temperature

When the same quantity of heat energy is applied to two substances with heat capacities, they will undergo different temperature changes. The graph below shows when 100g of water and 100g of ethanediol are heated with the same amount of energy. It can be seen that the beaker containing water will increase in temperature by a smaller amount than the ethanediol.

The specific heat capacity of water is: Water = 4.18 J /g / °C specific heat capacity and latent heat including units and symbols, with reference to hydrogen bonding to account for the relatively high specific heat capacity of liquid water, and significance for organisms and water supplies of the relatively high latent heat of vaporisation of water. Specific heat capacity (C) is a measure of the energy required to raise 1g of substance by 1°C. It units are J g-1 °C The specific heat capacity of water is: Water = 4.18 J /g / °C Almost 5x higher than soil and rocks – Excellent Insulator This also means it may take up to 5x longer than land to heat up. The density of liquid water is 1g mL-1 so volumes in mL are equal to the mass of water in grams (g)

Boardworks AS Chemistry Energetics Calculating the amount of energy needed to increase the temperature of a given mass The specific heat capacity of a substance can be used to calculate the heat energy in joules required to increase the temperature of a given mass of a substance by a particular amount. Heat energy is given the symbol q q = mc∆T q = heat energy in joules ∆T = change of temperature in degrees Celsius. Teacher notes Specific heat capacity is the amount of energy needed to increase the temperature of one gram of a substance by one degree Kelvin (or Celsius). If acids and bases are assumed to have the same density as water, 20 cm3 of acid is assumed to have a mass of 20 g because the density of water is 1 g cm-3. m = mass of substance being heated (often water) in grams c = specific heat capacity in joules per degrees Celsius per gram (4.18 J *C-1g-1 for water)

High Latent Heat Values specific heat capacity and latent heat including units and symbols, with reference to hydrogen bonding to account for the relatively high specific heat capacity of liquid water, and significance for organisms and water supplies of the relatively high latent heat of vaporisation of water. High Latent Heat Values The latent heat of fusion is the amount of heat energy required to change one mole of a substance from a solid to a liquid at its melting point. The latent heat of vaporisation is the amount of heat energy required to change 1 mole of a substance from a liquid to a gas at its boiling point.

Latent Heat When a solid is heated at a constant pressure, the temperature of the solid increases until it reaches its melting point. The temperature then remains constant as the solid melts, even though more energy is being absorbed. When a liquid reaches its boiling point, its temperature remains constant until all of the liquid has evaporated. Energy is needed to break Hydrogen Bonds. This causes the temperature to remain constant although the water is being heated. Latent Heat of Fusion: 6.0kJ/ mol Latent Heat of Vaporisation: 44.0kJ/mol Joules are unit. 1kJ= 1000J. Implications of high latent heat – electricity consumed in boiling water, kettles etc

specific heat capacity and latent heat including units and symbols, with reference to hydrogen bonding to account for the relatively high specific heat capacity of liquid water, and significance for organisms and water supplies of the relatively high latent heat of vaporisation of water. Latent Heat Latent heat is the energy required to change a fixed amount of a substance, usually 1 mole, from either a solid to a liquid or a liquid to a gas. Over the period of time that the latent heat is being absorbed, the temperature of the substance will not change. Latent Heat values are the measure of the quantity of heat energy required to melt or boil a given amount of a solid or a liquid. Latent heat values are given the symbol L and have the unit kilojoules per mole, kJ mol-1.

Calculations involving latent heat The amount of heat required to change the state of a given amount of a substance can be calculated if the value of the latent heat for the substance is known. The amount of heat energy required for a change in state is equal to: Amount of substance in mol X latent heat value q= n X L Where q is heat energy (kJ) n is mol changing state and L is the latent heat of fusion or vaporisation (kJ/mol)

Significance of water’s latent heat of vaporization specific heat capacity and latent heat including units and symbols, with reference to hydrogen bonding to account for the relatively high specific heat capacity of liquid water, and significance for organisms and water supplies of the relatively high latent heat of vaporisation of water. Significance of water’s latent heat of vaporization Cooling system for living organisms The high latent heat of vaporization of water means that it takes a relatively large amount of energy to vaporize sweat on your body’s surface. As the sweat evaporates it extracts a high amount of heat from your body, which means that you cool down. If the latent heat of vaporization was less only a little heat would be needed to vaporize the water and you would have to sweat more to extract the heat to cool down the body. Our water supplies Evaporations from our dams is significant but if water’s latent heat of vaporization was lower the amount of evaporation would be much higher.