WaterProperties, Solubility, Precipitates and Ionic equations

What do you think?

What is the chemical structure of water? Draw a diagram and label as many features as possible.

What are the properties of water? List as many as you can think of

Key Knowledge

Unique properties of water: relationship between structure and bonding, properties and uses including solubility

Ways in which substances behave in water: the dissociation of soluble ionic solutes; the separation of non-ionising polar molecules such as ethanol

Calculations including concentration and volume of solutions Maintaining water quality:

solubility, precipitation reactions

Desalination including the principles of distillationWhich of these concepts did we cover last term? Which of these concepts are you still unsure about?

Getting Started: Water Terminology and Properties

Latent heat Melting and boiling temperature Density Specific heat capacity Solubility

Latent HeatLatent heat is the amount of energy required to change the state of a given about of substance. The latent heat of fusion of water (6.02 kJ mol-1) is the amount of energy needed to change a fixed amount of water from a solid to a liquid at 00C. The latent heat of vaporisation of water (40.7 kJ mol1) is the amount of energy needed to change a fixed amount of water from a liquid to a gas at 1000C.

Latent Heat of WaterSubstance Water Hydrogen Oxygen Latent Heat of Fusion (kJ mol-1) 6.0 0.06 0.22 Latent Heat of Vaporisation (kJ mol-1) 40.7 0.45 3.4

The higher the latent heat of a substance, the greater the amount of energy it needs when it changes phases. Water has high latent heat values compared to substances of a similar size. Why do you think this is the case?Due to the stronger intermolecular forces between its molecules in the solid and liquid states.

Melting and boiling pointsWater has a relatively high melting and boiling points

In comparison to a similar molecular weight non-polar molecule more heat is required to enable the molecules to break apart. This is because the molecules need to gain sufficient kinetic energy to break free of the hydrogen bonds between water molecules rather than weaker dispersion forces for a non-polar molecule.

DensityThe density of water varies depending on which physical state it is in. Water expands when it freezes Water expands when frozen. It is less dense as a solid than as a liquid phase. Water expands when frozen due to H-bonding In ice, the hydrogen bonds form a complete network.

Youtube: Ice bomb

Specific Heat CapacitySpecific Heat Capacity is the amount of energy needed to raise the temperature of one gram of the substance by one degree Celsius.The higher the specific heat capacity of a substance, the more energy it must absorb to raise its temperature. Water has a very high specific heat capacity (4.15 KJ g-1) which makes it a good thermal insulator.

High SolubilityWater dissolves many other substances

Waters H-bonds give it a strong attraction to other polar and charged molecules. When substances such as sugar and alcohol dissolve in water they form Hbonds with the water When ionic substances such as salt dissolve in water each ion is surrounded by water molecules.

For each of the pictures below demonstrates a property of water. Identify the property and explain it in terms of the bonding present.

Summary Properties of Water High latent heat High specific heat capacity Less dense as a solid than as a liquid Relatively high melting and boiling point Good ability to dissolve other substances Hydrogen bonding present between molecules.

SolubilityWater is sometimes know as the universal solvent because it dissolves so many different substances.

Solubility measures the extent to which a solute can dissolve in a solvent In general a polar solvent will dissolve a polar solute and a non-polar substance will dissolve in a nonpolar solvent.

SolubilityWhen one substance (solute) dissolves in another (solvent), the following processes occur:The particles of the solute are separated from one another The particles of the solvent are separated from one another The solute and solvent particles attract one another.

A solute will dissolve if the attraction between the solute and solvent particles is strong enough to compete with the solute-solute and solvent-solvent intermolecular forces. Substances that dissolve readily in water have ionic or polar covalent bonding. They fall into the following groups:1. Polar covalent compounds that can form hydrogen bonds with water. 2. Polar covalent compounds that ionise 3. Ionic compounds

Dissolving Process

Solution TerminologySaturated solution contains the maximum amount of solute for the volume of solution at a particular temperature Unsaturated solution contains less than the maximum amount of solute for the volume of solution at a particular temperature. Supersaturated solution can be prepared by slowly cooling a saturated solution so that the solution contains more solute than it normally would at that temperature. Aqueous solutions water samples containing dissolved substances

1. Polar compounds that form H-bonds with H2OSome polar molecules such as sugar and ethanol dissolve easily in water. When a polar substance like ethanol dissolves in water: Hydrogen bonds between water molecules are broken Hydrogen bonds between ethanol molecules are broken Hydrogen bonds form between water and ethanol molecules

Taking it a step further

The more polar a molecule the more likely it is to dissolve in water. For example Vitamin C is soluble in water while Vitamin A is insoluble. What does this mean in termsof nutrition? What types of food will be a good source of Vitamin A? What types of food will be a good source of Vitamin C?Vitamin C

Vitamin A

2. Polar covalent molecular compounds that ionise compounds contain covalent bonds that are so Somehighly polarised they break when the compound is placed bonds break because of the electrostatic These in water. attraction between the solute molecules and water molecules. This hydrolysis process is known as ionisation. For example: Hydrochloric acid inwater

The hydrogen-chlorine bond breaks and both bonding electrons go with the chlorine atom. The H+ forms a covalent bond with a water molecule. Two ions are produced in this process Cl- and the hydronium ion H 3 O+ .The HCl molecules have ionised in water. The chloride ions and hydronium ions are referred to as hydrated ions because they are surrounded by water molecules.

2. SummaryWhen a polar covalent molecular substance ionises in water: Polar covalent bonds within molecules are broken, producing hydrogen ions and anions A hydronium ion forms. Ion-dipole attractions are formed between the newly formed ions and the polar water molecules.

3. Ionic CompoundsWhen an ionic solid dissolves in water: Ionic bonds within the solid are broken Hydrogen bonds between water molecules are broken Ion-dipole attractions between the ions and the polar water molecules are formed

3. Ionic CompoundsIonic compounds are made up of positive and negative ions. When placed in water the negative ions are attracted towards the positive end of the water molecules and the positive ions are attracted towards the negative end of the water molecules.When ions are surrounded by water molecules they are called hydrated ions When an ionic compound dissolves in water, the cations and anions are separated from one another and the process is known as dissociation.1. Dissolving Ionic Compounds Animation 2. Dissolving salt movie

Dissociation EquationThe dissociation of sodium chloride in water can be represented by the equation:

Solubility of Ionic Compounds

Not all ionic substances are soluble. For insoluble ionic substances, the energy required to separate the ions from the lattice is much greater than the energy released when the ions are hydrated.Predicting Solubility Tutorial Use the Flash Version

QuestionsUse the rules for solubility to decide which of the following substances would be soluble in water: (a) potassium nitrite (b) sodium chloride (c) lead sulfate (d) calcium carbonate (e) ammonium hydroxide (f) copper(II) sulfide.

Solubility of Gases in WaterGases can also dissolve in water. Generally the solubility of gases decreases as the temperature increases.

Polar gases will dissolve in water easily, eg. ammonia gas, NH3.Gases such as oxygen and nitrogen are non-polar and do not easily dissolve in water.

Factors influencing the solubility of gases in waterIncrease the pressure on a gas-water mixture and more gas will dissolve in the mixture Eg. Carbonated drinks contain carbon dioxide which is dissolved the water under pressure. When you open the bottle the pressure decreases and the gas starts to come out of solution, hence the bubbles Increase the temperature of a gas-water mixture and this decreases the solubility of a gas

Solubilities of selected gases in water. The units are very small, millimoles (mmol) per

Question

Calculate the percentage of oxygen that would be lost from a pond, saturated with oxygen if the temperature of the water in the pond rose from 0 to 20 degrees.

Mixing Solutions Forming Precipitates

A precipitate is the solid formed when two solutions are mixed together. In a precipitation reaction, ions from the first reactant swap with ions from the second reactant to form new combinations, one of which is insoluble.Full chemical equation Ionic Equation

Writing Ionic EquationsIonic equations are equations that show only the species that are formed or changed in a reaction. Any ions that remain unchanged in a reaction are not included in an ionic equation.Ions that are present in a reaction but do not react are called spectator ions. Rules for writing ionic equations 1. Write the balanced chemical equation. 2. Decide which substances are soluble and which will form precipitates. 3. Expand the equation by dissociating all the soluble compounds into their free ions. 4. Cancel all free ions that are unchanged on both sides of the equation (the spectators). 5. Write the net ionic equation.

Example: Writing Ionic EquationsWhen a salt is dissolved in water it breaks up, or dissociates, into its constituent ions. The symbol (aq) indicates that each ion has become surrounded by water molecules.If a second soluble salt, silver nitrate, is added, it will also dissociate:

When the ions of both solutions come into contact a white precipitate of AgCl is formed.The net ionic equation does not include the spectator ions, Na+ and NO3-; these remain unchanged and dissolved in solution (as if sitting on the sidelines watching).

Online TutorialsPrecipitation Reactions Exercise Exercises for identifying spectator ions and precipitates http://www.files.chem.vt.edu/RVGS/ACT/notes/net_ionic_ rxns/net_ionic_rxns.html

Questions Writing Ionic EquationsFor each of these combinations of reactants predict the products and write full and ionic equations for each reaction. (a) CuSO4 (aq) + NaOH(aq) (b) CaS(aq) + (NH4)3PO4 (aq) (c) MgBr2 (aq) + H2SO4 (aq) (d) KOH(aq) + Ca(NO3)2 (aq) (e) Na2S (aq) + (CH3COO)2Pb (aq) (f) (NH4)3PO4 (aq) + CaCl2 (aq)

Solubility CurvesThe solubility of a substance in water depends on temperature. Most solids are more soluble as temperature increases A solubility curve is a graph of solubility (g per 100 g water) versus temperature.

Solubility curves

Saturated

Supersaturated

Unsaturated

Solubility curvesAny point on a line represents a saturated solution. In a saturated solution, the solvent contains the maximum amount of solute.

Example At 90oC, 40 g of NaCl(s) in 100g H2O(l) represent a saturated solution.

Solubility curves Any point below a line represents an unsaturatedsolution. In an unsaturated solution, the solvent contains less than the maximum amount of solute.

Example At 90oC, 30 g of NaCl(s) in 100g H2O(l) represent an unsaturated solution. 10 g of NaCl(s) have to be added to make the solution saturated.

Solubility curvesAny point above a line represents a supersaturated solution.

In a supersaturated solution, the solvent contains more than the maximum amount of solute. A supersaturated solution is very unstable and the amount in excess can precipitate or crystallize.

Example At 90oC, 50 g of NaCl(s) in 100g H2O(l) represent a supersaturated solution. Eventually, 10 g of NaCl(s) will precipitate.

1. Example for Reading the Graph At 38 C the solubility of copper sulphate, CuSO4, is:

Ans:28g of anhydrous salt per 100g of water.

2. Example for Using the Graph How much potassium nitrate will dissolve in 20g of water at 34 C?

Ans:

At 34 C the solubility of KNO3 is 52g per 100g of water. 52/100 = unknown/20 52/100 * 20 = 10.4 g will dissolve in 20 g of water at 340C

3. Example for Using the Graph At 25 C 6.9g of copper sulphate dissolved in 30g of water, what is its solubility in g/100cm3 of water?

Ans :

6.9/30 = x/100 6.9/30 * 100 = 23 g of CuSO4 will dissolve in 100 cm3 of water Check on graph, just less than 23g/100g water

4. Example for Using the Graph 200 cm3 of saturated copper sulphate solution was prepared at 90C. What mass of copper sulphate crystals form if the solution was cooled to 20C?

Ans :

Solubility of CuSO4 at 90C is 67g/100g and at 20oC 21g/100g. Therefore, mass of crystals formed = 67 - 21 = 46g (for 100 cm3). However, 200 cm3 of solution was prepared, so total mass of copper sulphate crystallised = 2 x 46 = 92g

Activities

Worksheet - Solubility Curve Practice Problems

This might help things stick in your head.

Solubility Song http://www.youtube.com/watch?v=VT mfQUNLlMY

Concentration

What do the terms mean? Concentration Concentrated Dilute Dilution

Common units for ConcentrationConcentration is the amount of a given substance in a given volume.

Examples g/L (gram per litre)

g/mL (gram per millilitre)

g/mL (microgram per millilitre)

Questions on g/LCalculate the concentration (g/L) when 12 g is dissolved into 2.5 L = 12/2.5 = 4.8 g/L 8 g is dissolved in 750 mL = 8/0.75 = 10.7 g/L

What is the mass dissolved in 250 mL to obtain the concentration 20 g/L? mass = 20 g/L x 0.25 L = 5 g What volume of solvent is required when dissolving 15 g to produce a concentration 30 g/L? vol = 15 g / 30 g/L = 0.5 L = 500 mL

Chemistry Units of Concentration

%w/w (percentage by mass) 0.9% w/w means 0.9 g per 100g of solution %v/v (percentage by volume) 12% v/v means 12mL per 100mL of solution %w/v (percentage mass/volume) 2.5% w/v means 2.5 g in 100 mL ppm (parts per million) 12 ppm means 12 g in 106 g of solution

MolarityMolarity is the amount of solute, in moles, dissolved in one liter of a solvent.Molarity is the most important concentration unit used in Chemistry.

C (concentration) = n (mole) M(mol/L) V (volume in L)

Molarity (M) is the units for concentration (NOTE this M IS NOT molar mass as we

Applications of C=n/V Determine the amount of solute required to prepare a solution of known concentration Calculate the concentration of ions in a solution Calculate the concentration of a solution given the concentration of an ion in solution Calculate the concentration of a solution after dilution Using the formulaCan rearrange the formula for concentration; C=n/V n=CxV V=n/C Also need to relate concentration to the other mole formula; n=m/M

Questions on Molarity (M)Calculate the molarity (M) of the following solution when: 5 mol is dissolved in 4 L of water C = n / V = 5 / 4 = 1.25 M 0.25 mol is dissolved in 5 L of water C = n / V = 0.25 / 5 = 0.05 M 1.25 mol is dissolved in 250 mL of water C = n / V = 1.25 / 0.25 = 5.0 M

Calculate the number of moles in the following solutions: 1.2 L of 2.5 M NaCl n = C x V = 2.5 x 1.2 = 3 mol 300 mL of 0.25 M NaOH n = C x V = 0.25 x 0.300 = 0.075 mol 20 mL of 0.50 M KOH n = C x V = 0.50 x 0.020 = 0.010 mol

Questions using massCalculate the molarity (M) of the following solution when: 58.5 g of NaCl is dissolved in 2 L. n = m/M = 58.5/58.5 = 1.0 mol; C = n/V = 1.0/2 = 0.5 M 4.0 g of NaOH is dissolved in 250 mL. n = m/M = 4.0/40.0 = 0.1 mol; C = n/V = 0.1/0.25 = 0.4 M 0.25 g of (NH4)2SO4 is dissolved in 100mL. n = m/M = 0.25/132 = 0.0019 mol; C = n/V = 0.0019/0.10 = 0.019 M Which of the above is more concentrated? The NaCl solution

DilutionJust think about your cordial From the bottle, the cordial is poured into a glass (it is concentrated at this point) and then water is added (ie, it is diluted).glas s Just add water concentrated glass dilute

1

2

What is common about the original solution in the concentrated glass to that in the dilute glass

The amount of cordial is the same for each, (amount of cordial is the mole of cordial.)Glass 1 Glass 2

n1 = C1 x V1 n1 = n 2 C1 x V1 = n1 = n 2

n2 = C2 x V2 = C2 x V2

The dilution ruleC1 x V1 = C2 x V2

Dilution Rule

C1 x V1 = C2 x V2

Questions on DilutionFind the concentration when: 2.0 L of 1.25 M solution is diluted to 5.0 L C1 x V1 = C2 x V2 ; 1.25 x 2.0 = C2 x 5.0 ; C2 = 1.25 x 2.0 / 5.0 = 2.5 / 5.0 = 0.5 M 300 mL of 0.05 M solution is diluted to 500 mL C1 x V1 = C2 x V2 ; 0.05 x 300 = C2 x 500 ; C2 = 0.05 x 300 / 500 = 15 / 500 = 0.03 M

250 mL of 0.30 M solution is diluted to 1.2 L C1 x V1 = C2 x V2 ; 0.25 x 0.30 = C2 x 1.2 ; C2 = 0.25 x 0.30 / 1.2 = 0.075 / 1.2 = 0.0625 M 1.5 L of 2.5 M solution is added to 1.0 L (final vol = 2.5 L); C1 x V1 = C2 x V2 ; 2.5 x 1.5 = C2 x 2.5 ; C2 = 2.5 x 1.5 / 2.5 = 3.75 / 2.5 = 1.5 M 20 ml of 1.2 M solution is added to 200 mL (final vol = 220 mL); C1 x V1 = C2 x V2 ; 1.2 x 20 = C2 x 220 ; C2 = 1.2 x 20 / 220 = 3.75 / 2.5 = 0.11 M Note the difference when saying diluted to and added to