OCR A - A level Chemistry Module 2

Acids, Bases and Salts

Acids are proton donors. When they are mixed with water, the acids release hydrogen ions, H+.Hydrogen atoms only contain one proton and one electron so as an ion, hydrogen ions only contain one proton as it has lost its only electron. When hydrogen is mixed with water, the water and hydrogen ions combine to form hydroxonium ions.Bases, on the other hand, are the opposite of acids, they are proton acceptors and want to have the H+ ions. Basically, Acids produce H+ ions in an aqueous solution, they are proton donors Bases remove H+ions from an aqueous solution, they're proton acceptors Bases that are soluble in water are known as alkalis and they release OH- ions in a solution Common names for acids: Hydrochloric acid - HCL Sulphuric acid - H2SO4 Nitric acid - HNO3 Ethanoic acid - CH3COOH Common names for bases: Sodium hydroxide - NaOH Potassium hydroxide - KOH Ammonia - NH3 Ammonia is more difficult to write equations with. It is an alkali and readily accepts a proton from an acid to form an ammonium ion. This then forms an ammonium salt. Ammonia doesn't directly produce hydroxide ions though, it has to accept a hydrogen ion from a water molecule before the hydroxide ion can be produced.

The reaction between an acid and water, or base and water is reversible. At any moment the forwards and backwards reactions will be happening. Acids and bases can be classed as strong or weak, depending on whether the forwards or backwards reaction is more prevalent and to what extent the acid or base is ionised in the solution. For strong acids, very little of the reverse reaction happens, so nearly all of the acid will dissociate in water (all of the H+ ions released) For strong bases, the forwards reaction is favoured again and all of the base (almost) dissociates in water and lots of the OH- ions are released For weak acids, the backwards reaction is favoured, only a small amount of the acid will dissociate in water and few H+ ions are released. For weak bases, the backwards reaction is more prevalent and so few OH- ions are released. It will only ionise slightly

Acid molecules release their hydrogen ions, so other ions can hop into their places. You get a salt if the hydrogen ions are replaces by metal ions or ammonium ions. When acids react with bases, they neutralise each other and you get a salt and water. The water formed is the combination of the H+ ions released by the acid and the OH- ions released by the alkali. The general equation is:Acid + Base = Salt + WaterSalts are also produced when acids react together with metals. However, this time hydrogen is produced instead of water. The general equation:Metal + Acid = Salt + HydrogenMetal oxides react with acids according to the general equation:Metal Oxide + Acid = Salt + WaterMetal hydroxides are usually alkalis. Sodium hydroxide is an example of this. The general equation for a metal hydroxide reacting with an acid is:Metal Hydroxide + Acid = Salt and WaterThe last general equation when reacting metal carbonates with acid is:Metal Carbonate + Acid = Carbon Dioxide + Salt + Water

Anhydrous and Hydrated SaltsSolid salts consist of a lattice of positive and negative ions. Sometimes, water molecules are incorporated into this lattice as well. The water in the lattice is known as the water of crystallisation . A solid salt containing water of crystallisation is hydrated. A salt is anhydrous if it doesn't contain water of crystallisation.Many hydrated salts loose their water of crystallisation when heated, to become anhydrous. If you known the mass of the salt when hydrated and anhydrous, you can work its formula out: Find the mass of water lost by subtracting mass of anhydrous salt away from the mass of the hydrated salt. Find number of moles of water lost Find number of moles of anhydrous salt that's produced Work out ratio of moles of anhydrous salt to water Scale ratio You can use the same method even if you are given percentage mas

Yield and Atom Economy

The theoretical yield is the mass of a product that should be formed in a chemical reaction. You can use the masses of the reactants and a balanced equation to calculate the theoretical yield : Work out how many moles of the limiting reactant you have Use the equation to work out how many moles of product you would expect that much reactant to make Calculate the mass of that many moles of product For any reaction, the actual mass of product obtained (the actual yield) is always less than the theoretical yield. Reasons for this are: Not all the reactants react fully Some chemicals are lost eg solution in containers Once the theoretical yield and the actual yield have been found you can work out the percentage yield . The formula is:PERCENTAGE YIELD = (ACTUAL YIELD / THEORETICAL YIELD) x 100

The efficiency of a reaction is often measured by the percentage yield. This is based on how much of the product is lost but it doesn't measure how wasteful a reaction is itself. A reaction that has 100% yield could still be very wasteful if a lot of the atoms from the reactants wind up being by -products rather than the desired product. Atom economy is a measure of the proportion of reactant atoms that become part of the desired product in the balanced equation.Atom economy is calculated using this formula:% ATOM ECONOMY = (MOLECULAR MASS OF DESIRED PRODUCT / SUM OF MOLECULAR MASSES OF ALL PRODUCTS) x 100In an addition reaction, the reactants combine to form a single product. The atom economy for addition reactions is always 100% since no atoms are wasted.A substitution reaction on the other hand is one where some atoms from one reactant are swapped with atoms from another reactant. This type of reaction always results in at least 2 products so the atom economy is always less than 100%.Companies in the chemical industry will often use reactions with high atom economies. High atom economy has environmental and economic benefits: Reactions with low atom economies are less sustainable. Many raw materials are in limited supply, so it makes sense to use them efficiently so they last as long as possible. Also, waste has to go somewhere so its better for the environment is less is produced. A low atom economy means there is lots of waste produced. It costs money to dispose of this waste and to separate out the waste from the desired products. Companies will usually pay good money to buy the reactant chemicals and its a waste of money if a high proportion of them end up as a by product. However, reactions with low atom economies may still be useful if the by product can be sold and used for something else.

The Atom

The table below shows the relative charges and masses of protons, neutrons and electrons

IonsWhen an atom gains a charge, its called an ion. Negative ions have more electrons than protons, they are an anion. Positive ions have more protons than electrons, they are cations. Isotopes of an element are atoms with the same number of protons but a different number of neutrons. As the number of the electrons are the same, they have the same chemical properties as it is the number and arrangement of electrons that determines this. The physical properties differ slightly as they are more dependent on the mass of the atom.Isotopes with more neutrons have: Higher mass Higher density Higher melting and boiling points Slower rates of reaction The relative atomic mass calculation is:Ar = ((mass of each isotope x abundance of that isotope) + (the next isotope and abundance) + etc) / total abundanceSometimes total abundance is a percentage, other times its relative abundance. The relative atomic mass is the weighted mean mass of an element, compared to 1/12th of the mass of an atom of carbon-12.The relative isotopic mass is the mass of an atom of an isotope compared to 1/12th the mass of an atom of carbon-12Brief History of the Model of the Atom A Greek philosopher Democritus gave us the first model of the atom around 460BC.John Dalton received the idea that an atom was the smallest piece of matter. He developed the billiard ball theory. That they were solid spheres that differed slightly. JJ Thomson declared that atoms were not solid. He measured change and mass, discovering that they had chargesRutherford conducted the gold foil experiment and developed the nuclear model. This stated that there was a tiny positive nucleus at the centre of the atom, surrounded by a cloud of electrons. The issue with this model was that the electrons cant be in a cloud otherwise they will be attracted to the nucleus. Bohr finally developed a model stating : Electrons exist in fixed orbits Each shell has fixed energy Electrons that move between shells, em radiation is emitted/absorbed The energy of the shells decrease with increased distance

Electronic Structure

Electrons can be found in regions around the nucleus in shells. The further away the nucleus, the higher its energy. Therefore, electrons have fixed amounts of energy associated with them.The PQN is known as the Principle Quantum Number and its the number given to each shell. The further away the shell, the higher the number.Shells are made up of subshells and each shell has a different number of subshells. There are 4 different subshells; s, p, d, f.The further away the nucleus, the more energy subshells have. The first shell has 1 subshell - 1s The second shell has 2 subshells - 2s and 2p The third shell has 3 subshells - 3s, 3p, 3d and so on Sub shells are further made up of orbitals. This is a region of space in an atom which can hold up to 2 electrons that have opposite spins. The s subshell has 1 orbital The p subshells have 3 orbitals The d subshells have 5 orbitals The f subshells have 7 orbitals Shells are made up of subshells and subshells are made up of orbitalsEach orbital has its own shape associated with it in which the electron moves around:S orbital - spherical in shape, holds only 2 electronsp orbital - Dumbbell shape, 3 associated with each sub shape, each p orbital has same energy, their orientation differs in space.There are 2 rules within electron configuration: Aufbau Principle - electrons fill the sublevels in order of increasing energy. The order is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p for the elements hydrogen to krypton. The 3d sub level is higher in energy than the 4 sublevel, even though it is in a lower principle energy level. Hund's Rule - Within a sublevel, the orbitals are first occupied singly by unpaired electrons. The electrons only pair up when there are no empty orbitals left in the sub level. Electrons are represented by up and down arrows. Chromium and copper are the only exceptions and they ake an electron from their 4s subshells and puts it in their 3d sub level which makes its more stable.Sub Shell Notation:

Ionic Bonding

Cations Atoms lose electrons More protons than electrons Atom becomes positively charged ion, charge is equal to electrons lost Anions Atoms gain electrons More electrons than protons Atom becomes negatively charged ion. Charge is equal to electrons gained Ionic compound formulae dependent on the ion charges involves. The charges must balance each other out to be zero.Common molecular ions you need to know the charges of: Nitrate = 3- Carbonate = 2- Hydrogencarbonate = 1- Sulphate = 2- Phosphate = 3- Hydroxide = 1- Ammonium = 1+ Properties of ionic compounds; Electrical conductivity - conduct electricity when molten or in solution. This is because the ions in a liquid are free to move and carry a charge whereas in a solid position, they are fixed in position by strong ionic bonds Melting/boiling points - high because the giant ionic lattices held together by strong electrostatic forces. It takes loads of energy to overcome these forces Solubility - Tend to dissolve in water because water molecules are polar and these molecules pull the ions away fro the lattice and cause it to dissolve.

Ions form when atoms lose or gain electrons. Electrostatic attraction holds positive and negative ions together and this very strong. The ionic bond is the electrostatic attraction between oppositely charged ions. When oppositely charged ions form an ionic bond, you get an ionic compound. Dot and cross diagrams are used to show how ionic bonding works. They show the arrangement of electrons in the shells:

Ionic crystals are giant lattices of ion. A lattice is just a regular structure. This structure forms because each ion is electrostatically attracted in all directions to ions of the opposite charge.

Covalent Bonding

Molecules are formed when two or more atoms bond together. Molecules are held together by strong covalent bonds. A covalent bond is the strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms in order for it to become more stable.Dot and cross diagrams can be used again here to represent covalent bonding. In a single covalent bond, the atoms share one pair of electrons however more atoms can be covalently bonded and there can be double or triple covalent bonds too.

Dative covalent bonds - both electrons in the bond come from the same atom. It is normally represented by an arrow. An example is the formation of NH4+

To measure the strength of covalent bonds, we use bond enthalpies. The bond enthalpy will change when the molecules differ even if the same bond occurs. This is due to how much the outer atomic orbitals of the bonded atoms overlap and how strongly the atomic nuclei are attracted to the shared electrons. The stronger a bond is, the greater the bong enthalpy

There are some exceptions when it comes to covalent bonds as normally they are used in order for the molecules to attain full outer shells, but in some cases, this is not what happens.Boron triflouride - boron only has 6 electrons in its outer shellSulphur Hexafluoride - sulphur only has 12 electrons in its other shell

Properties of Giant Covalent CompoundsGiant covalent lattices are also known as intermolecular structures. Compounds in which every atom is bonded covalently to another making the structure infinite in every direction, forming 3D shapes They're not molecules, the number of atoms joined together are completely variable High melting/boiling points because every single bond requires lots of energy as they're strong so they have to be broken before melting occurs Generally don't conduct electricity because there are no free electrons/ions Insoluble in water and organic solvents as there are no attractions which could occur between the solvent molecules and atoms in the lattice than can overcome covalent bonds Non polar Properties of simple Covalent Compounds Strong covalent bonds between atoms in molecule but weak forces between molecules Don't conduct electricity - no free charged particles Polar covalent compounds are soluble low melting/boiliong points due to the weak forces holding the molecules together so they don't require lots of energy

Intermolecular Forces

Electronegativity - the ability of an atom to attract a shared pair of electrons. Generally, electronegativity increases if you go across a period and decreases as you go down a group. Fluorine is the most electronegative element. The bigger the difference between electronegativity, the more attracted the electron is to the most electronegative element. For this reason, you get a very small charge around the element and this is represented as delta minus (-). The other element that is less electronegative gets a small positive charge of delta plus (+). There are 3 types of intermolecular forces: Hydrogen Bonding Induced Dipole Dipole Permanent Dipole Dipole

Hydrogen BondingHydrogen bonding only occurs when hydrogen is bonded to either oxygen, fluorine and nitrogen. These three elements have a higher electronegativity than the hydrogen and so the pair of electrons is attracted more to that element rather than to the fluorine, giving in it a slight negative charge. That means there is a slight positive charge on the hydrogen which can go on to form bonds between the other molecules.

Permanent Dipole DipoleThese are generally the strongest out of the intermolecular forces. It is similar to hydrogen bonding but it doesn't involved the OH, NH or FH groups. Instead the slight positive and negative charges can happen between all of the elements involved in bonding as long as they have a different electronegative. The stronger the charge, the more electronegative the element is and the stronger the bond.

Induced Dipole DipoleThis is generally the weakest and it occurs when a molecules forms a temporary dipole dipole. When molecules are made up of the same element, they have the same electronegativities but they are surrounded by electrons which constantly move around, creating an electron cloud. Due to their random movement, every now and then, electrons gather on one side of the cloud, temporarily forming a negative charge to one side of that molecule. This then induces a charge on the other molecule and an induced dipole dipole is formed. All elements can form induced dipole dipoles.

Valency

To form bonds, atoms must lose or gain or share electrons with other atoms. The number of electrons involved in this process is known as valence. It doesn't tell us if electrons are gained, lost or shared. Valence can also be given to groups of atoms. For example: Sodium atoms lose one electron - they have a valence of one Oxygen atoms gain 2 electrons - they have a valence of two

Valence and Formula (cross over technique) Write each symbol (atom/group) Write valence of each Cross over the valence's Simplify to lowest ratio Example: SiO Si4O2 Si2O4 SiO2

VSEPR theoryValence Shell Electron Pair Repulsion TheoryThis theory is based on the repulsions between the electron pairs in the valence shell of the atoms. The shape of moelcules is determined by both the total number of electron pairs (bonding and non-bonding) around the molecules central atom and the orientation of these electron pairs in the space around the central atom. Electron pairs around the molecules central atom can be bonding pairs or can be lone pairs. In order to minimise the repulsion forces between them, electron pairs around the molecules central atom, tend to stay as far away from each other as possible. Lone pairs of electrons repel more than bonding pairs of electrons.

Shapes of Molecules

Molecules and molecular ions come in loads of different shapes. The shape depends on the number of pairs of electrons in the outer shell of the central atom. Pairs of electrons can be shared in a covalent bond or can be unshared. The shared pairs are the bonding pairs while the unshared pairs are the lone pairs. Due to electron negativity, the electron pairs will repel each other as much as they can. Lone pairs of electrons repel more than bonding pairs of electrons. This means that the greatest angles are between lone pairs of electrons and the bonding pairs of electrons get pushed together. This is known as the electron pair repulsion theory.

Drawing the shapes of molecules can be a bit tricky so we use different types of lines to represent which way the bonds are pointing if it were to be a 3D molecule. A broken line (slashes) shows a bond pointing AWAY from you General lines show bonds that are neutral - they are front on, neither facing toward or away Wedges show a bond pointing TOWARDS you

To work out the shape of a molecule you need to know how many bonding pairs and how many lone pairs of electrons it has. To do this: Find the central atom (the one which the other atoms are bonded to) Use the periodic table to work out how many electrons are in the outer shell for the central atom Add one electron for each bond being formed Add any electrons for a negative charge or subtract ant electrons for a positive charge Divide the total number of electrons by 2 This calculation gives you the number of electron pairs formed and from here you can identify the lone pairs and the bonding pairs by looking at the bonds formed from the molecule

The Different Types of Shapes LINEAR - molecules with two bonding pairs that have a bond angle of 180 degrees. For example, carbon dioxide (CO2) TRIGONAL PLANAR - Molecules that have three bonding pairs. The repulsion is the same so the bond angel is 120 degrees. For example, Boron Triflouride (BF3) NON LINEAR - two bonding pairs of electrons and one lone pair - bond angle is 104.5 degrees. Example: H2O TETRAHEDRAL - Four pairs of bonding electrons and no lone pairs. The bond angles are all 109.5 degrees. Example: methane, CH4 TRIGONAL PYRAMIDAL - Three bonding pairs of electrons and one lone pair. The bond angle is 107 degrees. Example: Ammonia, NH3 TRIGONAL BYPYRAMIDAL - Five bonding pairs of electrons. There is another shape within this one which is trigonal planar so there are two bond angles. The planar shape is 120 degrees while the other two bonding pairs will be at 90 degrees to them. Example: Phosphorous pentachloride. PCl3 OCTAHEDRAL - there are six bonding pairs and they all have bond angle, 90 degrees. Example: Sulphur Hexaflouride, SF6

Take note: in the shapes above, the values with pm added to them are not needed

Oxidation Numbers

The oxidation number or state of an atom in a compound is the change that atom would have if the compound consisted of only separate numbers.The rules for assigning oxidation states: Elements by themselves have an oxidation state of 0 Monoatomic ions have an oxidation state equal to their charge In neutral compounds, the sum of oxidation states equals 0 In compound ions, the sum of the oxidation states is equal to their charge Groups 1,2 and 3 have oxidation states +1,+2 and +3 respectively Oxygen is -2 unless its a peroxide (-1) or bonded to F, Cl or Br etc (+1) Hydrogen is +1 or if its bonded to a metal, -1 The systematic name of a compounds is written as words and sometimes they contain roman numerals. The roman numeral donates the oxidation state of the element that precedes it. The Roman numeral charge is always plus and you use the crossover technique to find out the formula