(103) the importance of water Unit 3

In a certain lake badly damaged by acid rain, the pH is measured to be 6. What is the [H+]?

1x10^-6 M

What is the hydroxide ion concentration [OH-] of the lake described in the question above?

1x10^-8 M

Water is most dense at which temperature? A) 212°C B) 32°C C) 100°C D) 0°C E) 4°C

4°C

You are a scientist analyzing the effects of pollution on the Hudson River. One day, you measure a hydroxide ion concentration of 1 X10-8 M. What is the pH of the river on this day?

6

A 1L solution contains 0.0000001 moles of hydroxyl ions [OH-]. Which statement describes this solution?

This solution is neutral.

What is a substance that resists pH changes when added to a solution?

buffer

Buffers resist changes in pH by

donating protons to a solution when they have been depleted. accepting protons when the are in excess.

) A buffer maintains the pH of a solution by absorbing or donating protons or hydroxide ions.

f

A strong acid or base is one that does not dissociate completely in solution.

f

Hydrogen bonds occur between the slightly positive regions of oxygen of one water molecule with the hydrogens of another.

f

Water floats because the ionic bonds in water help to make it less dense than the liquid state.

f

In a solution, the two adjacent water molecules are bound to each other by

hydrogen bonds

Which bonds are broken in order for water to evaporate?

hydrogen bonds

Hydrophobic substances such as vegetable oil are

nonpolar substances that repel water molecules.

In a single molecule of water, the two hydrogen atoms are bonded to a single oxygen atom by

polar covalent bonds

A pH buffer

resists pH changes in a solution.

As water evaporates the temperature usually increases due to the heat released from the breaking of multiple hydrogen bonds.

t

As you increase the pH of a solution, there is a corresponding decrease in protons.

t

Hydrophilic substances dissolve in water because they are polar or charged and have an attraction for water molecules.

t

The pH and pOH of a solution will add up to 14.

t

Van der Waals forces occur with there are pockets of negative or positive charges that can attract or repel molecules.

t

Water is considered a polar molecule due to the unequal sharing of electrons.

t

An example of a hydrogen bond is the bond between

the H of one water molecule and the O of another water molecule.

After you take the measurement above (previous question), you look back at your notes and realize that the river is now one full pH unit lower than it was last year. true or false: The concentration of H+ ions is now ten times higher.

true

true or false: NH3 is a weak base (alkali), and HCl is a strong acid.

true

Water is able to form hydrogen bonds because

water molecule is a polar molecule.

Which of the following solutions has the smallest concentration of hydroxide ions [OH-]? A) seawater at pH 8 B) lemon juice at pH 2 C) urine at pH 6 D) vinegar at pH 3

B) lemon juice at pH 2

Which of the following solutions has the smallest concentration of hydrogen ions [H+]? A) vinegar at pH 3 B) seawater at pH 8 C) urine at pH 6 D) lemon juice at pH 2

B) seawater at pH 8

Which of the following solutions has the greatest concentration of hydroxide ions [OH-]? A) lemon juice at pH 2 B) vinegar at pH 3 C) seawater at pH 8 D) urine at pH 6

C) seawater at pH 8

What is energy? How does this relate to electrons and what happens when they jump between orbital levels? In general, what can be said about a high-energy state versus a lower energy state (hint: think in terms of stability)?

Energy is defined as the ability to do work. There is an inherent amount of energy that is held in the electrons and this energy will differ depending on the orbital they reside in. In order for an electron to "jump up" to a bigger orbital, energy must be absorbed. In order for an electron to "jump down" to a smaller or lower orbital, energy is released. In general, high- energy states are considered to be unstable (and thus highly reactive) while low energy states are considered to be stable (and thus less reactive).

When water molecules are in solution there are two reactions that occur. Describe each reaction and why it is important.

In solution, one reaction that occurs is the dissociation of water molecules (H2O) into OH- and H+ ions. This leads to the pH effect, where the pH of a solution is dependent upon the concentration of hydrogen ions. The higher the concentration of H+ ions, the lower the pH will be. If only water is in the solution, the [H+] will equal the [OH-], which will be neutral (i.e. pH = 7). The second reaction that occurs is when the H+ ions bind to H2O molecules to form hydronium (H3O+) ions. This is important because these ions are highly reactive and establish the acid-base reactions when in solution.

What is a buffer and how does it work. Provide a specific example.

A buffer is a substance that is used to resist changes in pH. It does this by absorbing protons (which will lower the [H+] in solution), which will cause a slight increase in pH. A buffer can also donate protons (which will increase the [H+] in solution), which will cause a slight decrease in pH. By absorbing or donating protons, a buffer can keep the pH within a certain range when something acidic or basic is added to the solution. An example of a buffer is carbonic acid (H2CO3), which can donate protons when it breaks down into H+ and HCO3-. If a basic solution is added, there will be an excess of OH- and an initial increase in pH. The buffer will release H+ ions, which will decrease the pH of the solution to stay within the buffer range. If an acid is added, there is a build up of protons, which will initially lower the pH. In this scenario, the HCO3- ions will bind to the excess H+ in solution, which will increase the pH and keep it within the buffer range.

in solution, what is the difference between a strong acid and strong base? What is the difference between a weak acid and weak base? Give an example for each.

A strong acid or base will completely break down in solution. A strong acid (i.e. HCl) will completely dissociates into H+ and Cl- ions, the increase in H+ ions in solution will act to lower the pH of the solution. A strong acid (i.e. NaOH) will break down into Na+ and OH- ions in solution, which will increase the OH- ions in solution. This will lead to a low pOH value and higher pH. A weak acid/base will not completely break down in solution. A weak acid (i.e. H2CO3) will only release one or a few protons to form H+ and HCO3-. This will only slightly lower the pH of the solution. A weak base (i.e. NH3) will gain a proton to form NH4+. This will lead to a slight increase in pH (as the [H+] ions decreases as they are used to form NH4+).

Which of the following solutions has the greatest concentration of hydrogen ions [H+]? A) lemon juice at pH 2 B) urine at pH 6 C) vinegar at pH 3 D) seawater at pH 8

A) lemon juice at pH 2

Which of the following does not ionize completely in solution and is considered to be a weak acid? A) NH3 B) NaCl C) H2CO3 D) HCl E) NaOH

C) H2CO3

List five unique properties of water. Give an example that illustrates each property.

Cohesion - the ability for water molecules to stick to each other. An example would be how water beads together on wax paper because the water molecules prefer to stick to each other rather than the wax paper itself. Adhesion - the ability for water molecules to stick to another substance. An example is how water will stick to the sides of glass and create a meniscus in a graduated cylinder. Water can moderate temperature. An example of this is how water can provide insulation due to its high specific heat content or can cool you off when you sweat due to evaporative cooling. Water is considered a polar solvent due to its slightly negative (O) and slightly positive (H) regions so it can dissolve other polar substances as well as ionic substances. An example of this is when you place salt (ionic) and sodium methanol (polar) in water it will dissolve or break down. Water also creates the pH effect when water molecules dissociate, they break down to form H+ and OH-. An example of this is when you use a pH litmus paper to test for pH, it changes color based on the H+ concentration in the solution.

Which of the following ionizes completely in solution and is considered to be a strong acid? A) NaCl B) NH3 C) NaOH D) H2CO3 E) HCl

E) HCl

Which of the following ionizes completely in solution and is considered to be a strong base? A) H2CO3 B) NH3 C) HCl D) NaCl E) NaOH

E) NaOH

mass

Mass is the amount of matter. An example is atomic mass of an atom, which is made up from the protons and neutrons present in the atom.

matter

Matter is defined as anything that takes up space. The pen used to write this answer is considered matter.

What is the difference between something that is hydrophilic versus something that is hydrophobic. Give an example for each type of compound and how it would react with water.

Something that is termed hydrophilic refers to something that "loves" water and readily dissolves in solution such as other polar substances. Something that is hydrophobic refers to something that is water "hating" that does not dissolve in water, such as non-polar substances. An example of a hydrophilic substance would be sulfuric acid or H2SO4 that easily dissolves in water. When mixed with water, the H+ ions readily dissociate which can be noticed in the increase in pH of the solution (if it was measured). An example of a non-polar substance would be vegetable oil. When mixed with water, it does not dissolve but rather forms a distinct layer on top of the water's surface.

TRUE/False

TRUE/FALSE

Define pH and pOH. What is the pH scale and how is it used? How are these related to the pH scale?

The pH of solution is defined as -log[H+], while pOH is -log[OH-]. The pH scale runs from 0 to 14 and is used to describe the concentration of hydrogen ions in solution. A solution that is acidic has a higher concentration of H+ present and a low pH value (<7). A solution that is basic has a low concentration of H+ ions and a higher pH value (>7). The pH and pOH relate to each other, as in an aqueous solution, the [H+] will be balanced out by [OH-] such that the higher the [H+] ions in solution, the lower the [OH-] will be and vice versa. As a result, when there is a low pH (i.e. a high [H+]) there will be a high pOH value (i.e. a low [OH-]). In general, the pH + pOH = 14. Thus, if you know the pH of a solution, you know the pOH, which can be used to calculate the [H+] and [OH-], respectively.

Carbonic acid (H2CO3) is an important buffers in the blood that is responsible for pH stability. When placed in solution, carbonic acid dissociates according to the following equation: H2CO3 ↔ HCO3- + H+ If someone were to experience an increase in blood pH, which of the following is true?

There would be a decrease in the concentration of HCO3- and an increase in the concentration of both H2CO3.

weight

Weight is the force of gravity acting on matter. Weight will change based upon the force of gravity. An example of this is how a person's weight will change between the earth and the moon since the force of gravity isn't as strong on the moon, this will lead to a smaller weight value on the moon.

Water is considered a universal solvent because it can dissolve a variety of compounds. Describe how water molecules would react with molecule of salt (i.e. NaCl) and a polar molecule (i.e. glucose).

When an ionic compound like salt is placed in water, it is easily dissolved. When this occurs, the water molecules begin to surround the salt particles and split them into their ionic compounds. In NaCl, the slightly negative O regions of a water molecule would surround the Na+ ions, while the slightly positive H+ regions would surround the Cl- ions. When a polar molecule like glucose is placed in water, the slightly negative (or oxygen containing) regions on glucose are surrounded by the slightly positive H regions of water molecules, while the slightly positive regions on glucose are surrounded by the slightly negative O regions of water molecules. In both instances, hydration shells are formed around the ions or polar molecules, which helps the compounds dissolve.

Oxygen is considered to be a highly electronegative atom. When is it found in a molecule bonded to different atoms, what can you usually infer about that molecule (i.e. is it polar or non-polar)? What happens when it is bonded to itself (in the form of O2)? Is this molecule considered polar or non-polar?

When oxygen is in a molecule it usually indicates there is a polar region. Since the oxygen atom is highly electronegative, this usually results in a slightly negative region around the oxygen atom and a slightly positive region around the other atom that is donating its electrons to the bond. Therefore, when oxygen is present you can usually infer that there is a polar covalent bond. When is it bonded to itself, and non-polar covalent bond occurs because the oxygen atoms have the same electronegativity value, which results in the electrons being equally shared between the oxygen atoms.

A solution is neutral and has a pH of 7. This means that

the [H+] equals the [OH- ].