topic 2 - aqueous solutions and acidity

Question 1

distinguish between solute, solvent, solution and concentration

Answer 1

• solution = mixture where one substance dissolves and mixes fully with another
• solute is the substance being dissolved into the other substance (gas, liquid or solid)
• solvent is the substance the solute is dissolving in (liquid)
• concentration is a measure of how much solute is dissolved in how much solvent (e.g. mg/litre)

Question 2

describe the structure of water

Answer 2

• an oxygen atom bonded covalently with two hydrogen atoms
• oxygen has greater electronegativity, so it pulls the electrons closer to it (away from hydrogen), causing oxygen to be slightly more negatively charged (δ-) and hydrogen is slightly positive (δ+)
• this leads to water being a polar moelcule
• also structured in a bent shape

Question 3

why does the water molecule have a bent shape?

Answer 3

• because there are two lone pairs of electrons, which repel each other (negative and negative repel - electrostatic repulsion), so they push against the two covalently bonded electrons, which makes the structure become a slightly bent shape

Question 4

what force is it that connects water molecules? describe how it works

Answer 4

hydrogen bonding
* type of intermolecular force; not a chemical reaction
* the slightly negative charge of oxygen makes it attracted to the positively charged hydrogens of other water molecules (and vice versa: slighlty positive hydrogen is attracted to negatiev oxygen)
* so they are attracted together by forces, but do not chemically react
* numerous hydrogen bonds in water make it a very stable structure

Question 5

what is a precipitate? what is a precipitate reaction? describe.

Answer 5

• precipitate is an insoluble solid formed in a precipitate reaction of aqueous solutions
• a precipitate reaction = reaction of aqueous solutions to form an insoluble precipitate

Question 6

how do you predict whether a precipitate will form?

Answer 6

1. write names of reactants
2. switch the non-metals of the reactants
3. determine whether the new products are insoluble - if they are, a precipitate is formed, if they are both soluble, no precipitate.

Question 7

what are units of concentration?

Answer 7

• molarity - mol L^-1
• grams per unit - g L^-1 (for if you don’t know the molarity)
• parts per million - ppm/mgL^-1 (for very very low concentration)

Question 8

formula for concentration?

Answer 8

C = m / v
or
m = C x v

C=concentration, m=moles/mass(g/mmg), v=volume(L)

Question 9

define solubility

Answer 9

the amount of solute that can dissolve in a solvent at a given temperature

Question 10

what affects solubility?

Answer 10

changes in temperature!
* for solid and liquid solutes, increase in temp causes increase in solubility
* for gases, vice versa (hotter = less soluble)

Question 11

define saturated, unstaurated and supersaturated

Answer 11

1. saturated: has max amount of solute dissolved at any given temp
2. unsaturated: can still dissolve more solute at any given temp
3. supersaturated: has more solute dissolved than would dissolve at any given temp (temp has been carefully increased to dissolve more)

Question 12

what is a hydride?

Answer 12

group 16 elements bonded with hydrogen (e.g. H2O, H2S, H2Se, H2Te)

Question 13

why does oxygen have a higher melting/boiling point than other hydrides?

Answer 13

H2O has stronger hydrogen bonds/intermolecular forces, so a significant amount of energy is needed to overcome that/break the forces and change states of matter

Question 14

explain why ice is less dense than liquid water

Answer 14

because in ice/solid water, the molecules form hydrogen bonds with 4 other molecules and create a rigid, widely-spaced arrangement with space between molecules - whereas in liquid water, the molecules aren’t in this rigid arrangement and can move freely and be closer, and therefore, it is more dense

Question 15

describe water’s surface tension

(explain surface tension, then why water’s is how it is)

Answer 15

surface tension = measure of the resistance of a liquid to separating its molecules
* water has relatively high surface tension because it has relatively strong hydrogen bonds
* (e.g. insects can walk on it)

Question 16

explain why water is a good solvent

Answer 16

• for a solute to dissove in a solvent, the forces attracting the solute molecules (to each other) must be overcome by the solvent’s attraction (polarity)
• if the attraction between the solvent and solute particles is strong enough, a solution forms – therefore, a good solvent needs strong electrostatic attraction/high polarity
• WATER is a good solvent because it’s very POLAR, and thus has strong intermolecular forces - therefore is strong enough to overcome solute’s attraction → it’s a good solvent

Question 17

finish the sentence:

polar solvents dissolve…
and non-polar solvents dissolve…

Answer 17

1. polar solutes
2. non-polar solutes

Question 18

briefly:

explain the process of salt dissolving in water

Answer 18

• describe water: H2O is two hydrogen bonded covalently to oxygen; bent shape due to two lone pairs of electrons being repelled; this makes the molecule polar, with the hydrogen end being slightly positive and oxygen slightly negative
• describe NaCl: an ionic substance with an anion (chloride) and cation (sodium); each ion has a whole charge, so they’re electrostatically attracted to each other
• describe attraction between water and NaCl: the attractions within NaCl are very strong because of the whole charges of the ions, and comparatively, in hydrogen bonding, the attraction is weak; this means that there is resistance for the solvent to overcome the solute’s bonds, BUT if there is multiple water molecules to single salt molecules, then, the attraction of the water molecules’ partial charges is strong enough to overcome the NaCl bonds – meaning that the sodium cation (positive) is attracted to water’s oxygen end (slightly negative), and chloride’s anion (negative) is attracted to water’s hydrogen end (slightly positive) – when water has surrounded each ion, a hydrogen shell is formed

Question 19

difference between concentrated and dilute solution?

Answer 19

• concentrated solution = many particles per unit volume of solution
• dilute = opposite (few particles/unit volume of solution)

Question 20

how can concentration be represented? why are there so many?

list the ways, what are each’s purpose?

Answer 20

1. number of moles of solute perlitre of solution (mol L-1) (most common)
2. mass of the solute per litre of solution (g L-1) (for if you don’t know the molar mass)
3. parts per million (ppm) / (mg L-1) (for if concentration is very low)

Question 21

what affects a solute’s ability to dissolve in water?

(not temperature, dumbo)

Answer 21

• attraction between solute and polar water particles (intermolecular bonding)
• attractive forces within the colute holding it together

Question 22

explain how ionic substances dissolve in water

Answer 22

• ionic substances have whole charges (cations and anions), making it strongly bonded, while water has partial charges (polar)
• multiple partial charges hydrate each ion, interacting with the full charge and breaking the intermolecular bond of the ionic substance, causing it to dissociate
• anion is attracted to hydrogen end, and cation is attracted to oxygen end

Question 23

explain the solubility of molecular substances (covalent substances)

Answer 23

• can dissolve in water if they, too, are polar (like water) - then, dipoles attract
• often, though, molecular substances aren’t polar and only have dispersion forces, which make a temporary dipole, but attraction is not to the same extent
• non-polar molecular substances are MUCH LESS soluble than ionic substances in water