The Chemistry of H2O


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  • examples: oxygen diffusing into the capillaries of the lungs, water moving into a living cell
  • Remember: a solvent is the substance into which a solute is evenly distributed in a solution
  • The cohesion of water molecules to each other is exploited by plants and animals.
  • HCl can have a pH of 1 to -2, depending on the concentration NaOh can have a pH around 13 (depending on concentration)
  • The Chemistry of H2O

    1. 1. The Chemistry of H2O Science is Organized Knowledge
    2. 2. Diffusion• Mixing due to molecular movement is called diffusion. – All particles of matter are constantly moving. • gas molecules move in straight lines until colliding with other molecules • liquid molecules slide in one direction, but are "pulled" by the bonds they have with the molecules around them • even in solids, atoms move by vibrating in place – those movements allow other molecules to enter the space around them Diffusion Animation Equilibrium is reached when the two substances are evenly mixed.
    3. 3. Diffusion Rates• The rate of diffusion measures how fast molecules disperse. – A slow process, because it depends on the random movement of molecules.• Rate of diffusion is affected by: – concentration gradient: the difference in concentration of a substance across space • molecules always move from an area of higher concentration to an area of lower concentration – temperature & pressure • higher temperature = faster moving molecules • higher pressure = more frequent collisions • The size of molecules also affects diffusion rates –
    4. 4. Osmosis• Osmosis is the diffusion of water across a semi-permeable membrane.• A membrane is like a net. – will not let bigger molecules through – some small molecules will collide and bounce back
    5. 5. Interactions of Water • Nonpolar substances are hydrophobic – literally means "water-fearing" – will repel water – disperse into fats (oily or greasy substances) – i.e. oil & water do not mix
    6. 6. Hydrogen Bonds in Water• The polar nature of water results in hydrogen bonding – weak attractive forces between the negative Oxygen end of the molecule and the positive Hydrogen end – gives water many unique physical and chemical properties
    7. 7. Water as a Solvent• Water is an excellent solvent because of its strongly polar molecular arrangement.• Hydrophilic (water-loving, polar) substances readily dissolve in water – because the polar ends of the molecule are attracted to (and pulled apart by) the charged ends of the H2O molecule – i.e. salt (NaCl) dissolves easily in water to become Na+ and Cl-
    8. 8. Surface Tension• Surface tension is caused by cohesion – an attraction between H2O molecules due to hydrogen bonding
    9. 9. H-Bonding and Changing States Water is unlike most substances, which get denser when going from liquid to solid.• Hydrogen bonds cause water to become less dense as it freezes. • Water in its solid state (ice) floats. • Important for aquatic species in cold climates.
    10. 10. Insulating Properties of Water• Water resists temperature change. • It absorbs a huge amount of heat as it changes into vapor. • H2O releases a lot of heat when freezing into ice.
    11. 11. Week 4 Lab Acids & BasesLab Coat Diffusion
    12. 12. The pH Scale• pH describes how acidic or basic a solution is – a solution is any mixture with one or more solutes distributed evenly in a solvent• Substances with pH less than 7 are acids – i.e. battery acid, hydrochloric acid (HCl)• A pH of 7 is neutral – i.e. water, blood• Substances with pH higher than 7 are bases – i.e. bleach, drain cleaner
    13. 13. pH• pH stands for potential of Hydrogen ions – when a substance dissolves in a solution, the molecules will break apart – in acids and bases, the broken-apart molecules form either OH- or H+ – the more H+, the lower the number on the pH scale and the more acidic it is – Log scale means 10X change per unit! • Water has a pH of 7 (its neutral) – equal # of hydrogen & hydroxide ions • Substances with pH less than 7 are acids – molecules separate to form hydrogen ions (H+) – more H+ than OH- – the farther from 7, the stronger the acid • Substances with pH higher than 7 are bases – molecules separate to form hydroxide ions (OH-) – more OH- than H+ – the closer to 14, the stronger the base
    14. 14. Acids• A pH below 7 is an acid – acids form hydrogen ions (H+) in water• In strong acids, the H+ will dissociate (separate) from most or all molecules – i.e.: lemon juice, hydrochloric acid (HCl), battery acid• With weak acids, only a few of the molecules will dissociate into hydrogen ions – i.e.: tomatoes, milk, vinegar (CH3COOH or acetic acid)
    15. 15. Bases• A pH above 7 is a base – also described as alkaline – bases form hydroxide ions (OH-) in water• In bases, hydroxide ions (OH-) will form in two ways – by the dissociation of the base – by stripping an H+ from water• Examples of strong bases – bleach, oven cleaner, drain opener – sodium hydroxide (NaOH)• Examples of weak bases – baking soda, eggs, ammonia, dish soap
    16. 16. ReviewThe basic building blocks of matter are atomsThe Atomic Number of an atom equals the number of protons in the nucleus (also the number of electrons)The Atomic Mass of an atom equals the number of Protons + Neutrons in the nucleus (average of all isotopes)Elements are different than compounds because compounds have 2 or more different elements bonded togetherElectrons are found where in an atom? circling the nucleus in shells (also called orbitals or energy levels).Each energy shell can only hold a set number of electrons.*(octet rule)The small (subscript) 2 in H2O tells the number of Hydrogen atoms in each molecule of water.What do we get if one neutron is added to the nucleus of a carbon atom? an isotope - Carbon-13 (13C)What do we get if one electron is added to the outer orbital of chlorine? an ion is formed, creating a more stable atom due to the filled energy levelElectrons are shared by two or more atoms in covalent bonds.
    17. 17. ReviewA polar molecule is one in which electrons are not equally shared, so there is a slight + and - charge at certain locations.What do we get if one proton is added to the nucleus of a boron atom? a carbon ion - (C+) + charges = 6 - charges = 5 6 - 5 = +1
    18. 18. Review• The polarity of water allows it to... dissolve other polar molecules (especially ionic compounds).• Non-polar compounds are hydrophobic and thus not easily dissolved in water.• Many properties of water are due to hydrogen bonding.• How might a pine tree take advantage of the cohesion of water ? If water molecules “stick” to each other because of hydrogen bonds, it is easier to draw the water up to the leaves.• How does water resist temperature changes? by absorbing (and releasing) lots of heat• Why does ice float in water? Hydrogen bonds cause water to become less dense as it freezes solid• What is the term that describes molecules moving from an area of higher concentration to an area of lower concentration? diffusion• What is osmosis? the diffusion of water across a membrane