11CHEM Solubility and Precipitation

Solubility

. the ability of a substance (solute), to dissolve in another substance (solvent) to form a solution

. represents the maximum concentration/amount of a solute that can be dissolved in a certain amount of solvent at a specific temperature and pressure to create a saturated solution

How to write solubility

(…) grams of salt (NaCl) in 100g of water at (…)°C

EG: 200g of sugar per 100g of water at 18°C

Relationship between temperature and solubility

. temp affects how much of the solute can be dissolved by solvent

—> solids: more heat = more energy to break up the solid = more soluble

—> Gases: solubility depends not on breaking them up, but trying to trap them in solvent —> more energy = easier to escape solvent = less soluble

Describe Le Chatelier’s principle 

. if a system at equilibrium is disturbed by a change in conditions (like temperature, pressure, or concentration), the system will shift in a way that counteracts the change to re-establish equilibrium

. chemical systems respond to changes to stay balanced 

. if dissolving gas creates heat (exothermic due to slowing down gas molecules), and even more heat is added, the system restores balance by reversing the process (letting gas escape) 

. dissolving a solid is endothermic as it requires energy to break the bonds in the solid 

—> heating water allows for more energy to break bonds 

Solubility graphs and how to read them

. used to show how the solubility of a substance changes with temp

. the relationship between solubility and temp is shown on the solubility curve

To read: 

1. Find the line for the substance

2. The amount of that substance that dissolves at a given temp is on the y-axis 

. any point on a curve represents a saturated solution, below is unsaturated solution, above is supersaturated solution

Relate IMF to dissolving/solubility

. like polarities dissolve like

—> IMFs are overcome as the substances form stronger bonds with each other than the bonds within themselves

Saturated solution

. contains max amount of dissolved solute at a specific temp

. points on the line

. undissolved solute will remain at the bottom (no more solute can be dissolved at that temp)

. is in a state of equilibrium with the solid solute

. made by adding solute until no more will dissolve, or by cooling a supersaturated solution

. contains what it should hold

Supersaturated solution

. contains more dissolved solute than it can normally hold at a given temp

. points above the line

. contains more solute than a saturated solution 

. is unstable; adding a seed crystal or disturbing it will cause excess solute to crystallise out

. can be forced to precipitate the excess solute 

. made by dissolving a large amount of solute at a high temp and then cooling the solution slowly without disturbance 

. contains more than it should hold 

Unsaturated solution

. contains less solute than the max amount that can be dissolved at a specific temp

. points below the line

. contains less solute than a saturated solution and than it could hold

. all solute is completely dissolved

. can dissolve more solute

. made by simply dissolving solute in a solvent

Temperature and solubility relationship for solids

. positive relationship

. for most solids, the solubility increases as temperature increases

Temperature and solubility relationship for gases

. inverse relationship

. for gases, as temp increases, solubility decreases

. at low temps, gases are very soluble

. most gases are far less soluble than many ionic solids

How is pressure used to aid gas solubility

. because most gases don’t dissolve very well, solutions are put under high pressures to increase the gas solubility

. EG: CO2 from a high pressure tank must be forced into a soda container just before it’s sealed

—> this high pressure is quickly decreased when container is opened

—> the longer the beverage is open to the air, the more CO2 escapes (becoming flat)

Identify the factors that affect solubility (3)

1. Pressure

2. Nature of solvent and solute

3. Temperature

Factors affecting gas solubility: Pressure

. increased pressure = increased solubility (positive relationship)

—> more gas particles forced into solvent, increasing the chance they’ll interact with solvent molecules and stay dissolved

Factors affecting gas solubility: Nature (polarity) of solvent and gas

. like dissolves like

. if gas and solvent have similar polarity, stronger IMFs (dipole-dipole or H-bonding) can form between them, increasing solubility

. if gas and solvent different in polarity, only weak IMFs (dispersion forces) can form between them, lowering solubility

Factors affecting gas solubility: Temperature

. as temp increases, gas solubility decreases

. as temp increases, gas particles gain kinetic energy and are more likely to escape from the solvent (liquid) into gas phase

. the IMFs between the gas and solvent are weak so they’re easily overcome at higher temps

What is the affect of increased ocean temperatures

. oxygen dissolved in water is necessary for aquatic wildlife to survive

. if temp of water increases: less O2 dissolves in water as the increased kinetic energy of O2 particles are more likely to escape from water into the air

—> warmer water holds less O2, which can lead to O2 shortages for aquatic life

. as temp increases, cold-blooded animals (eg: fish) metabolism increases (require more oxygen for respiration)

—> meaning there’s an increased O2 demand, but decreased O2 supply

How do cold blooded aquatic life cope with warmer waters

. adapted in various ways:

—> reduce activity levels to conserve O2

—> seek cooler, deeper water where O2 levels higher

—> some species are more tolerant to low O2 conditions but for sensitive species this can cause stress, suffocation and death

Examples of causes of increased water temperatures

. summer heat, power plant cooling water

What is the affect of supersaturated water

. there can be too much O2, as when O2 dissolves, so does N2 (both in air)

. gas bubble trauma: bubbles form in the blood + tissues of fish

—> the O2 can be used during metabolism

—> N2 bubbles block capillaries resulting in the death of fish

How does supersaturation of water occur

. supersaturation (110% to 124%) of O2 and N2 happens at the base of dams

—> released water forms froth, trapping large quantities of air

Precipitation reaction

. a reaction in which two solutions are mixed and an insoluble solid (precipitate) forms

Why will a precipitate form

. happens when ions from the dissolved reactants combine to form a compound that is insoluble in the solvent, causing it to clump together and settle out of the solution, often appearing as a cloudiness or a coloured solid

—> the insoluble compound formed = precipitate

. solubility happens when a solvent can break a solute’s (eg: a salt) internal bonds and form new bonds with the ions (ion-dipole)

. if the solute’s internal bonds are stronger than the solvent’s pull, it stays solid

. the precipitate forms when 2 ions form stronger internal bonds with each other than what the solvent is able to overcome

Explain an example of a precipitation reaction in the environment

. a precipitation reaction occurs naturally in undersea hydrothermal vents 

—> the vents release superheated solutions containing sulfides, which then combine with metal ions to form precipitates of mineral sulfides, creating chimney-like structures 

—> the areas around these vents are biologically rich, often hosting complex communities fuelled by the chemicals dissolved in the vent fluids 

Ionisation vs dissociation

Ionisation:

. the formation of ions from neutral molecules, typically covalent compounds

. creates new ions that weren't present before

. Starting compound: covalent molecule

. Ions present before: no

. Cause: water reacts with molecule to form ions

. Example: HCl —> H+ + Cl-

Dissociation:

. the separation of pre-existing ions from an ionic compound

. releases ions that were already part of the compound's structure

. Starting compound: ionic compound

. Ions present before: yes

. Cause: water breaks apart crystal

. Example: NaCl —> Na+ + Cl-

Electrolytes

. a substance that conducts electricity when dissolved in a solvent like water, or when melted, because it dissociates into positively and negatively charged ions

. aqueous solutes that produce ions when dissolved in water

Strong electrolyte

. ionises completely

. ionic compounds + strong acids/bases

Weak electrolyte

. ionises partially

. weak acids/bases

Non-electrolyte

. doesn’t ionise

What is the strength of an acid/base

. refers to the substance’s capacity to dissociate

Rule for precipitation reactions

AB+CD—>AD+BC

EG: potassium iodide + silver nitrate —> potassium nitrate + silver iodide

Full molecular vs net ionic equation

. full molecular equation shows all the species in a reaction mixture, regardless of whether they react or not

. a net ionic equation only shows the species which are reacting (only the species which have changed state are included, no spectator ions)

How do you write a net ionic equation for a precipitation reaction

Method 1:

1. write full molecular equation

2. Write full ionic equation

3. Remove all spectator ions in the full ionic equation

4. Write the net ionic equation

Method 2:

1. determine the precipitate

2. Write the precipitate as the only product

3. Write only the reactants which are the ions required to form the product

What happens to the number of moles, volume and pH of a solution when it’s diluted 

. number of moles in the solution stays the same

. volume of water increases so the concentration decreases

. when an acid diluted, pH value increases as results in a decrease in concentration of H+ ions

Concentration

. the quantity of solute dissolved in a quantity of solution

. represented by:

1. the number of moles of the solute per litre of solution (mol/L)

2. the mass of the solute per litre of solution (g/L)

3. Parts per million (ppm)

Molarity

. molar concentration

. the number of moles of solute per litre of solution

. expressed in moles per litre (mol/L)

. most useful for concentration measurements, as chemists use ratios for reactions

Concentration of ions

. ionic substances and some covalent substances, such as strong acids, produce almost 100% ions when dissolved in water

. EG: NaCl(s)—> Na+(aq) + Cl-(aq)

—> knowing the concentration of the solute + its formula, the concentration of ions formed in solution can be calculated

—> (number of moles of ion in balanced equation) x [concentration of solution]

Limiting reagent

. the reactant that’s completely consumed first, thereby limiting the amount of product that can be formed

—> determines the maximum amount of product that can be formed

—> Once the limiting reagent is used up, the reaction stops, and any other reactants that were not fully used are called excess reagents

Why do limiting reagents occur

. reactants aren’t always mixed in the correct stoichiometric ratios

. 1 reactant may then run out before the other one is used up

Steps to find the mass of product formed and the amount of excess reagent left unused

1. calculate the moles of each reactant

2. Use the mole ratio in the balanced equation to deduce which reactant is fully used up (limiting reagent) and which reactant is left over (excess reagent)

3. Use the amount of limiting reagent to work out the amount of product formed, or the amount of excess reagent left over