baseline year 11

percentage yield

actual yield/theoretical yield x 100

atom economy

total Mr of the desired product/total Mr of all reactants x 100

what is a theoretical yield

a theoretical yield is the maximum possible mass of a product that can be made in a chemical reaction

how can a theoretical yield be calculated

it can be calculated from: the balanced equation, the mass and relative formula mass of the limiting reactant, and the relative formula mass of the product

what is an actual yield

an actual yield is the mass of a product actually obtained from the reaction. It is usually less than the theoretical yield

why is the actual yield usually less than the theoretical yield

incomplete reactions, in which some of the reactants do not react to form the product, practical losses during the experiment, such as during pouring or filtering, side reactions (unwanted reactions that compete with the desired reaction)

atoms forming by products

no atoms are created or destroyed in a chemical reaction, however, the atoms in the reactants may not become the desired product, they instead end up forming by-products

what is the atom economy

the atom economy of a reaction is a measure of how many reactant atoms form a desired product

calculating percentage atom economy

the atom economy of a reaction is 100% if there are no unwanted by-products in the reaction, usually the atom economy is less than 100%, the more atoms that end up in the by-products, the lower the atom economy

how can the atom economy of a particular reaction be improved

the atom economy of a particular reaction can only be improved by finding a use for the by-product, by making it another desired product, sometimes, the by-product can be sold for additional profit

method for acid-alkali titration practical (1)

use a pipette and pipette filler to add 25 cm3 of dilute sodium hydroxide solution to a clean conical flask, add a few drops of phenolphthalein indicator and put the conical flask on a white tile, fill the burette with dilute hydrochloric acid and note the starting volume

method for acid-alkali titration practical (2)

slowly add the acid from the burette to the conical flask, swirling to mix, stop adding the acid when the end-point is reached (when the colour first permanently changes from pink to colourless), note the final volume reading, repeat steps 1 to 5 until you get concordant titres (see step 1 in the Analysis)

results for acid-alkali titration practical

readings should be recorded to two or three decimal places, ending in 0 or 5 (where the liquid level is between two graduations on the burette), the titre is the volume added (the difference between the end and start readings)

acid-alkali titration practical: explain why a pipette is used to measure the acid, rather than a measuring cylinder

the pipette allows the same volume of acid to be added each time, helping to make the results repeatable

hazards risks and precaution in acid-alkali titration practical (dilute sodium hydroxide solution)

dilute sodium hydroxide solution causes skin irritation and serious eye irritation, wear gloves and eye protection and use a pipette filler

hazards risks and precaution in acid-alkali titration practical (spilling hydrochloric acid while filling the burette)

spilling hydrochloric acid while filling the burette causes eye irritation, fill the burette slowly below eye level using a funnel

formula for concentration in moldm^-3

amount of solute in mol/volume in dm³

typical properties of transition metals

high melting points, high densities, they form coloured compounds and they (and their compounds) can act as catalysts

iron as a transition metal: high melting point and density

the table shows the melting point and density of iron, compared to three non-transition metals, iron, a transition metal, has a higher melting point and a higher density than the non-transition metals

iron as a transition metal: coloured compounds

metals that are not transition metals usually form white or colourless compounds, like other transition metals, iron forms coloured compounds

iron as a transition metal: catalytic activity

catalysts are substances that speed up the rate of reaction without being used up in the reaction, iron is the catalyst used to make ammonia in the Haber process, iron(III) oxide is a catalyst used to make hydrogen by reacting carbon monoxide and steam together

corrosion

metals can oxidise in air. they react with oxygen and form metal oxides e.g sodium is a very reactive metal. when sodium is cut or scratched, its freshly exposed shiny surface rapidly turns dull as a thin layer of sodium oxide forms, other metals may oxidise more slowly. sodium is a very reactive metal. when sodium is cut or scratched, its freshly exposed shiny surface rapidly turns dull as a thin layer of sodium oxide forms

corrosion definition

corrosion happens when a metal continues to oxidise. the metal becomes weaker over time, and eventually all of it may become metal oxide

rusting

rusting occurs when iron or steel reacts with oxygen and water: iron + oxygen + water → hydrated iron(III) oxide, hydrated iron(III) oxide (rust) is the orange-brown substance seen on the surface of rusty objects

rusting experiment

the experiment in the diagram shows that both oxygen and water are needed for rusting to happen, the nail only rusts in the left-hand test tube. It does not rust in the middle test tube, where there was water but no oxygen (because there was no air in the water) and in the right-hand test tube, where there was oxygen (air) but no water

how can rusting be prevented

rusting can be prevented by keeping oxygen or water away from the iron or steel: oxygen can be excluded by storing the metal in an atmosphere of unreactive nitrogen or argon, water can be excluded by storing the metal with a desiccant such as calcium chloride

what is a desiccant

a substance that absorbs water vapour, so it keeps the metal dry

physical barriers to oxygen and water to prevent rust

painting, oiling and greasing and coating with plastic, different methods are used depending on the situation.

preventing corrosion: electroplating

electroplating involves using electrolysis to put a thin layer of a metal on the object: the cathode is the iron or steel object, the anode is the plating metal, the electrolyte contains ions of the plating metal

preventing corrosion: electroplating (example)

e.g, steel cutlery can be electroplated with silver using a silver anode and silver nitrate solution, electroplating improves the corrosion resistance of metal objects, it also improves their appearance and may be used to produce gold-plated jewellery

preventing corrosion: sacrificial protection

Iron can be protected from rusting if it is in contact with a more reactive metal, such as zinc, the more reactive metal oxidises more readily than iron, so it 'sacrifices' itself while the iron does not rust, once the sacrificial metal has corroded away, it can simply be replaced

preventing corrosion: sacrificial protection (worked example) (1)

three nails are left in contact with air and water for a few days, a nail wrapped in magnesium does not rust, a nail alone rusts but a nail wrapped in copper rusts more

preventing corrosion: sacrificial protection (worked example) (2)

magnesium is more reactive than iron, it oxidises more readily than iron so the nail does not rust, iron is more reactive than copper, this means it oxidises more readily than copper, so it rusts faster than the nail alone

preventing corrosion: galvanising

when iron is coated in zinc, the process is called galvanising, the zinc layer stops oxygen and water reaching the iron, zinc is more reactive than iron, so it also acts as a sacrificial metal, this protection works, even if the zinc layer is scratched

alloy

an alloy is a mixture of two or more elements, where at least one element is a metal, many alloys are mixtures of two or more metals.

alloy strength

converting pure metals into alloys often increases the strength of the product e.g, brass is an alloy of copper and zinc. It is stronger than copper or zinc alone:

relative tensile strength for metals

copper: 3, zinc: 2, brass: 5

explaining alloy strength (1)

solid metals have a regular lattice structure, when a force is applied to a metal, layers of atoms can move past each other, the more difficult it is for the layers to move, the more force is needed and the stronger the metal

explaining alloy strength (2)

copper and zinc atoms have different sizes, this distorts the regular lattice structure in brass, so layers of atoms cannot slide over each other so easily, this makes brass stronger than copper or zinc alone

alloy steels

Iron is alloyed with other metals to produce a range of alloy steels. Different steels have different properties, depending on their composition.

different examples of alloy steels

mild steel (other elements include carbon and properties include malleable and ductile), tool steel (other elements include tungsten and properties include hard and resistant to high temperature, stainless steel (other elements include chromium and properties include hard and resistant to rusting)

advantages and disadvantages of different alloy steels

mild steel is useful for making car body parts because it is easily pressed into shape, although mild steel rusts, it can be protected by galvanising and painting, tool steel is useful for making drill bits, these do not easily become damaged by the heating caused by friction during drilling

what is the alloy magnalium comprised of

aluminium and magnesium

uses of metals: aluminium and magnalium (1)

aluminium does not react with water, its surface is protected by a natural layer of aluminium oxide that allows the metal to resist corrosion, aluminium foil is used in the home for wrapping and storing food as it does not react to substances in food, it is malleable, so it is easily folded into shape around the food

uses of metals: aluminium and magnalium (2)

aluminium has a low density, so pieces of aluminium are relatively lightweight, magnalium is stronger than aluminium alone but still has a low density, it is used to make aircraft parts

what is the alloy brass comprised of

copper and zinc

uses of metal: copper and brass

copper and brass resist corrosion and are good electrical conductors, copper is a better conductor than brass, and it is used in electrical wiring, brass is stronger than copper, so it is used for the pins in electrical plugs

what is the alloy jewellery gold comprised of

gold and copper

uses of metals: gold

gold is a very soft and malleable metal. It is also very unreactive, so it resists corrosion and stays shiny, the gold used for jewellery is gold alloyed with other metals, often copper, this makes the jewellery much stronger while keeping its ability to stay shiny

electrolytes

electrolytes are ionic compounds that are: in the molten state (heated so they become liquids), or dissolved in water, under these conditions, the ions in electrolytes are free to move within the liquid or solution

electrolysis

Electrolysis is a process in which electrical energy, from a direct current (dc) supply, decomposes electrolytes, the free moving ions in electrolytes are attracted to the oppositely charged electrodes, which connect to the dc supply

cations in electrolysis of molten salts

positively charged ions are called cations, they move towards the negatively charged electrode, which is called the cathode

anions in electrolysis of molten salts

negatively charged ions are called anions, they move towards the positively charged electrode, which is called the anode

products of electrolysis

when ions reach an electrode, they gain or lose electrons, as a result, they form atoms or molecules of elements: cations gain electrons from the negatively charged cathode and anions lose electrons at the positively charged anode

use molten lead bromide as an example to describe products of electrolysis produced

Pb2+ ions gain electrons at the cathode and become Pb atoms, Br- ions lose electrons at the anode and become Br atoms, which pair up to form Br2 molecules, so lead forms at the negative electrode and bromine forms at the positive electrode

electrolysis of solutions: ions in water (1)

pure water can conduct electricity because a small proportion of its molecules dissociate into ions, the two ions formed in water are, hydrogen ions and hydroxide ions

electrolysis of solutions: ions in water (2)

during the electrolysis of water hydrogen ions are attracted to the cathode, gain electrons and form hydrogen gas and hydroxide ions are attracted to the anode, lose electrons and form oxygen gas, the volume of hydrogen given off is twice the volume of oxygen given off.

electrolysis of dissolved ionic compounds

an electrolyte formed by dissolving an ionic compound contains two pairs of negative and positive ions, positive hydrogen ions from the water, and positive ions from the compound, negative hydroxide ions from the water, and negative ions from the compound, the ions compete at each electrode to gain or lose electrons

electrolysis of dissolved ionic compounds at the cathode

whether hydrogen or a metal is produced at the cathode depends on the position of the metal in the metal reactivity series: the metal is produced at the cathode if it is less reactive than hydrogen and hydrogen is produced at the cathode if the metal is more reactive than hydrogen

electrolysis of dissolved ionic compounds at the anode

either oxygen or a non-metal from the electrolyte can be produced at the anode: for the most common compounds oxygen is produced (from the hydroxide ions), if halide ions (chloride, bromide or iodide ions) are present, then the negatively charged halide ions lose electrons to form the corresponding non-metal halogen (chlorine, bromine or iodine)

rank the reactivity series of metals from most reactive to least reactive

potassium, sodium, lithium, calcium, magnesium, aluminium, carbon, zinc, iron, hydrogen, copper, silver, gold

method to investigate the electrolysis of copper sulfate solution (1)

pour some copper sulfate solution into a beaker, place two graphite rods into the copper sulfate solution, attach one electrode to the negative terminal of a dc supply, and the other electrode to the positive terminal and completely fill two small test tubes with copper sulfate solution and position a test tube over each electrode as shown in the diagram

method to investigate the electrolysis of copper sulfate solution (2)

turn on the power supply and observe what happens at each electrode, test any gas produced with a glowing splint and a burning splint, record your observations and the results of your tests

investigating the electrolysis of copper sulfate solution (results)

observations at the negative electrode is a brown/pink solid forms, observations at the positive electrode include bubbles of a colourless gas form, gas test at the positive electrode is the gas relights a glowing split

analysis to investigate the electrolysis of copper sulfate solution

use your observations and results to draw conclusions on the changes that occur at each electrode: copper metal is formed at the negative electrode and oxygen gas is formed at the positive electrode

method to investigate electrolysis of copper sulfate solution using copper electrodes (1)

pour some copper sulfate solution into a beaker, measure and record the mass of a piece of copper foil, attach it to the negative terminal of a dc supply, and dip the copper foil into the copper sulfate solution, repeat step 2 with another piece of copper foil, but this time attach it to the positive terminal

method to investigate electrolysis of copper sulfate solution using copper electrodes (2)

make sure the electrodes do not touch each other, then turn on the power supply, adjust the power supply to achieve a constant current as directed by your teacher, after 20 minutes, turn off the dc supply

method to investigate electrolysis of copper sulfate solution using copper electrodes (3)

carefully remove one of the electrodes, gently wash it with distilled water, then dip it into propanone, lift the electrode out and allow all the liquid to evaporate, do not wipe the electrodes clean, measure and record the mass of the electrode, repeat step 6 with the other electrode, make sure you know which is which, repeat the experiment with fresh electrodes and different currents

analysis to investigate electrolysis of copper sulfate solutions using copper electrodes (1)

calculate the change in mass of each electrode, plot a graph to show: change in mass of the negative electrode on the vertical axis and current on the horizontal axis, make sure you choose suitable scales so that at least 50% of the graph area includes plotted points, draw a line of best fit through these points, as the current is increased the change in mass of the electrodes becomes greater.

analysis to investigate electrolysis of copper sulfate solutions using copper electrodes (2)

the gain in mass by the negative electrode is the same as the loss in mass by the positive electrode, so the copper deposited on the negative electrode must be the same copper ions that are lost from the positive electrode

hazards, risk and precautions when investigating electrolysis of copper sulfate solution using copper electrodes (copper sulfate solution)

copper sulfate solution causes skin and serious eye irritation, wear gloves and eye protection

hazards, risk and precautions when investigating electrolysis of copper sulfate solution using copper electrodes (propanone)

propanone is highly flammable liquid and vapour may cause drowsiness or dizziness, keep away from naked flames - use it in a fume cupboard

purifying copper by electrolysis

copper is purified by electrolysis, electricity is passed through solutions containing copper compounds, such as copper sulfate, the anode (positive electrode) is made from impure copper and the cathode (negative electrode) is made from pure copper

how does purifying copper by electrolysis work

a beaker with pure and impure copper rods dipped into copper sulfate solution, the pure copper rod is connected to the negative terminal of a battery and the impure rod is connected to the positive terminal, the pure copper rod has increased in size while the impure rod has deteriorated, leaving a pool of anode sludge at the bottom of the beaker

describe how pure copper forms at the cathode

a beaker with pure and impure copper rods dipped into copper sulfate solution, the pure copper rod is connected to the negative terminal of a battery and the impure rod is connected to the positive terminal, the pure copper rod has increased in size while the impure rod has deteriorated, leaving a pool of anode sludge at the bottom of the beaker

describe what happens at the purification of copper by electrolysis

four Cu ions are attached to the rod on the right, and four Cu²+ ions are floating in the space between the rods, a battery is connected between the rods and the Cu ions are pulled towards the left rod, there are now four Cu ions attached to the left rod, with four Cu²+ ions floating in the middle

what is an ion

an atom or group of atoms with a positive or negative charge. Ions form when atoms lose or gain electrons to obtain a full outer shell: metal atoms lose electrons to form positively charged ions and non-metal atoms gain electrons to form negatively charged ions

forming positive ions

metal atoms lose electrons from their outer shell when they form ions: the ions formed are positive, with more protons than electrons and the ions formed have full outer shells

forming negative ions

the outer shell of non-metal atoms gains electrons when they form ions: the ions formed are negative, because they have more electrons than protons and the ions formed have full outer shells

cations and ions

positively charged ions are called cations, and negatively charged ions are called anions, these ions can form when a metal reacts with a non-metal, by transferring electrons, the oppositely charged ions are strongly attracted to each other, forming ionic bonds

dot and cross diagrams

can model the transfer of electrons from metal atoms to non-metal atoms, the electrons from one atom are shown as dots, and the electrons from the other atom are shown as crosses

regular arrangement of ions in an ionic lattice

the ions in a solid ionic compound are not randomly arranged. Instead, they have a regular, repeating arrangement called an ionic lattice, the lattice is formed because the ions attract each other and form a regular pattern with oppositely charged ions next to each other

ionic bonds

the ionic lattice is held together by ionic bonds, in three-dimensional models, ionic bonds are shown as straight lines between ions, this is to keep things simple because ionic bonds can act in any direction, ionic bonds are strong electrostatic forces between oppositely charged ions

physical properties of ionic compound: high melting and boiling points (1)

ionic compounds are solids at room temperature, melting and boiling are state changes, energy has to be transferred to a substance in order to melt or boil it, this energy is needed to break the bonds between particles in the substance

physical properties of ionic compound: high melting and boiling points (2)

some bonds are overcome during melting, all remaining bonds are overcome during boiling, the more energy needed, the higher the melting point or boiling point

explain high melting and boiling points in ionic compounds

ionic compounds are held together by many strong electrostatic forces between the oppositely charged ions, these forces are usually referred to as ionic bonds, as the ionic lattice contains such a large number of ions, a lot of energy is needed to overcome these ionic bonds so ionic compounds have high melting and boiling points

physical properties of ionic compounds: conduction of electricity

a substance can conduct electricity if: it contains charged particles, and these particles are free to move from place to place

explain conduction of electricity in ionic compounds

ionic compounds conduct electricity when molten to form a liquid or dissolved in water to form an aqueous solution as both processes make their ions free to move from place to place, ionic compounds cannot conduct electricity when solid, as their ions are held in fixed positions and cannot move

polyatomic ions (cations)

ammonium (NH₄⁺), calcium (Ca²⁺), sodium (Na⁺), lead (Pb²⁺)

polyatomic ions (anions)

hydroxide (OH⁻), nitrate (NO₃⁻), carbonate (CO₃²⁻), sulfate (SO₄²⁻)

naming ionic compounds with ide and ate

the name of an ionic compound ends in: -ide if it contains just two elements and -ate if it contains three or more elements, one of which is oxygen

covalent bonds

a covalent bond is formed when a pair of electrons is shared between two atoms, usually non-metals, these shared electrons are found in the outer shells of the atoms. In general, each atom contributes one electron to the shared pair of electrons

molecules

consists of a group of two or more atoms joined together by covalent bonds, molecules of the same element or compound will have a set size - in other words, they will always contain the same number of atoms of each element

typical size of a molecule

0.1nm, individual atoms and molecules are too small to see even with the strongest light microscope, some electron microscopes can produce images of atoms and simple molecules

how are dot and cross diagrams drawn

a dot and cross diagram can model the bonding in a simple molecule: the outer shell of each atom is drawn as a circle, circles overlap where there is a covalent bond and electrons from one atom are drawn as dots, and electrons from another atom as crosses

how are molecular drawings drawn

a simple molecule can be modelled by drawing its structure, in these structures: show each atom by its element symbol and show each covalent bond as a straight line

properties of simple molecular substances: low melting and boiling points

simple molecular substances generally have low melting points and boiling points and are often liquids or gases at room temperature.

explain low melting and boiling points in simple molecular substances (1)

there are intermolecular forces between simple molecules, these intermolecular forces are much weaker than the strong covalent bonds in molecules, when simple molecular substances melt or boil, it is these weak intermolecular forces that are overcome

explain low melting and boiling points in simple molecular substances (2)

the covalent bonds are not broken, very little energy is needed to overcome the intermolecular forces, so simple molecular substances usually have low melting and boiling points

properties of simple molecular substances: conduction of electricity (1)

a substance can conduct electricity if: it contains charged particles, and these particles are free to move from place to place