Liquids and Solids H2O (g) H2O (s) H2O ()

Heating and cooling curve for water heated at a constant rates. A-B = Solid ice, temperature is increasing. Particles gain kinetic energy, vibration of particles increases. Ice

B-C = Solid starts to change state from solid to liquid B-C = Solid starts to change state from solid to liquid. Temperature remains constant as energy is used to break inter-molecular bonds. H2O (s)  H2O () energy required  6 kJ/mol 0ºC

C-D = temperature starts to rise once all the solid has melted C-D = temperature starts to rise once all the solid has melted. Particles gain kinetic energy. Liquid water

D-E = Liquid starts to vaporize, turning from liquid to gas D-E = Liquid starts to vaporize, turning from liquid to gas. The temperature remains constant as energy is used to break inter-molecular forces. H2O ()  H2O (g) energy required  41 kJ/mol 100ºC

E-F = temperature starts to rise once all liquid is vaporized E-F = temperature starts to rise once all liquid is vaporized. Gas particles gain kinetic energy. steam

Energy Requirements for changing state: In ice the water molecules are held together by strong intermolecular forces. The energy required to melt 1 gram of a substance is called the heat of fusion ( H fus) For ice it is 335J/g The energy required to change 1 gram of a liquid to its vapor is called the heat of vaporization (Hvap ) For water it is 2260 J/g H (delta H) is the change in energy or heat content.

What is specific heat capacity? The amount of energy required to change the temperature of one gram of a substance by 1C . 10 C 11 C Another name for specific heat is a calorie (1 calorie = 4.184 Joules) Specific heat capacity of liquid water (H2O (L) ) is 4.18 J /gC. Water (s) = 2.03 J/ g C Water (g) = 2.0 J/ g C

Calculating Energy Requirements using the equation: Q = m x T x Cp Q = energy (heat) required Cp = specific heat capacity m = mass of the sample T = change in temperature in C EXAMPLE: How much energy does it take to heat 10g of water from 50 to 100 C ? Q = m  T x Cp Q = (10g)  (4.184 J /g C )  (50 C) = 2.1  10 3 J Specific heat capacity of water = 4.184 J/ gC

Problem How much energy is required to heat 25 g of liquid water from 25C to 100C and change it to steam?

Step 1: Calculate the energy needed to heat the water from 25C to 100C Q = m  Cp  T Q = 25g  4.184 J/ g C  75 C = 7.8  10 3 J

Step 2: Vaporization: Use the vap H to calculate the energy required to vaporize 25g of water at 100C Q=m x  H vap .25g  1mol H2O / 18g mol-1 H2O = 1.4 mol H2O vap H (H2O) = 1.4 mol H2O  40.6kJ/mol = 57 kJ

 H vap (H2O) = 2260 J/g Q = 25g  2260J/g= 57000J

Total energy change is: 7800J + 57000J = 64800 J

Other properties of Liquids: Many of their properties are due to the forces between the particles. Why when you pour a liquid onto a surface does it form droplets? Why do some liquids exhibit capillary action? Hg H2O Why are some liquids more viscous than others?

Surface tension This allows insects to walk on water! The inward force or pull which tends to minimize the surface area of any liquid is surface tension.

The smallest surface area a liquid can form is a sphere. Surface tension is caused by hydrogen bonding between water molecules. The more polar a liquid the stronger its surface tension. Hg pure H2O H2O with detergent Surfactants are chemicals that decrease the surface tension of water, detergents and soaps are examples. The smallest surface area a liquid can form is a sphere.

Viscosity is the resistance to motion of a liquid. Maple syrup is more viscous than water. But water is much more viscous than gasoline or alcohol. The stronger the attraction between molecules of a liquid, the greater its resistance to flow and so the more viscous it is.

Capillary action is the spontaneous rising of a liquid in a narrow tube. Two forces are responsible for this action: Cohesive forces,the intermolecular forces between molecules of the liquid Adhesive forces, between the liquid molecules and their container If the container is made of a substance that has polar bonds then a polar liquid will be attracted to the container. This is why water forms a concave meniscus while mercury forms convex meniscus Hg H2O

The fact that water has both strong cohesive (intermolecular) forces and strong adhesive forces explains why water pull itself up a glass capillary tube. These forces are what allow water to be drawn up high into trees like giant redwoods.