OCR CHEMISTRY GCSE PAPER 2

Physical and chemical properties of group 1

• Physical - soft, low densities, shiny when freshly cut but tarnish in air

• Chemical - low melting and boiling point compared to other metals, highly reactive especially with water and oxygen

• Trends - reactivity increases as you go down and melting and boiling points decrease, density increases,

Physical and chemical properties of group 7

• Physical - brittle when solid, poor electrical conductors, low melting points

• Chemical - form diatomic molecules, react with metals to produce salts, react with group 1 metals vigorously

• Trends - melting and boiling points increase as you go down, reactivity decreases as you go down

Explain the trends in group 1 that occur as you go down and why they happen

• Reactivity increases - more electron shells meaning the outer electron is farther from the nucleus so it is less strongly attracted making it easier to lose the outer electron

• Melting and boiling point decreases - atomic size increases meaning distance between the metals nucleus and the delocalised electrons are larger so the electrostatic forces holding the structure become weaker and take less energy to break

• Density - increase in mass is greater than increase in size

Explains the trends in group 7 that occur as you go down and why they happen

• Reactivity decreases - atomic radius increases so nucleus is further from outer shell reducing the strength of electrostatic attraction between nucleus and incoming electron, making it harder to gain an electron

• Melting and boiling point increases - size of molecules increase, so intermolecular forces between molecules become stronger and take more energy to break

• Density increases - increase in mass is greater than increase in size

Explains the trends in group 0 that occur as you go down and why they happen

• Reactivity stays the same - all of them have full outer shells

• Boiling point increases - size of molecules increase, so intermolecular forces between molecules become stronger and take more energy to break

• Density increases - increase in mass is greater than increase in size

Noble gases

• Very unreactive due to full outer shells, gas at room temperature

• Monatomic

• Weak intermolecular forces between them causing low melting and boiling point

• Boiling point increases as you go down the group

Transition metals

• High melting and boiling points

• Good electrical and thermal conductors

• High densities

• Less reactive than group 1 metals

• Can form coloured compounds

• Can form ions with different charges

Why are transition metals good catalysts

due to their ability to form ions with different charges

What determines the reactivity of metals with water or dilute acids

• The tendency of the metal to form its positive ion

• SO the more easily a metal loses electrons, the more reactive it is

Why can metals conduct electricity

they contain delocalised electrons which can move around and carry the charge

What is a song for the reactivity series

• Please Stop Calling Me A Careless Zebra, Instead Try Learning How Copper Saves Gold

• Potassium, sodium, calcium, magnesium , aluminium, carbon, zinc, iron, tin, lead, hydrogen, copper, silver, gold

How can you detect oxygen?

if oxygen is present a glowing splint should be relit near the mouth of a container.

How can you detect hydrogen?

place a lit splint at the mouth of a container and if hydrogen is present there should be a squeaky pop heard.

How can you detect carbon dioxide?

limewater (calcium hydroxide) turns cloudy in the presence of CO2 due to a white precipitate (calcium carbonate) being formed.

How can you detect chlorine?

if you place damp blue litmus paper in the presence of chlorine it will become red and then colourless.

How do you perform a flame test

• Get a loop of nichrome wire and coat in hydrochloric acid

• Then coat the nichrome wire in the metal you want to test

• Place the nichrome wire into a blue flame and see what colour the flame turns

Colour metals turn during a flame test

• Lithium - red/pink

• Sodium - yellow

• Potassium - lilac

• Calcium (II) - orange/red

• Copper (II) - blue green

Hydroxide precipitate test

• Add a few drops of sodium hydroxide solution to a solution containing the metal ions and note the colour of the solution formed

Colour metals turn during a hydroxide precipitate test

• Iron (II) - green

• Iron (III) - orange brown

• Copper (II) - blue

• Calcium - white

• Zinc - white

• NOTE: zinc dissolves in excess sodium hydroxide but calcium does not so if it is white add more sodium hydroxide

How do you detect sulphate (SO4 2-) ions?

• Add a few drops of dilute hydrochloric or nitric acid to the solution

• Add a few drops of barium chloride to the solution.

• If sulphate ions are present, a white precipitate is formed.

• This is because barium ions react with sulphate ions producing white insoluble barium sulphate.

How do you detect carbonate (CO3 2-) ions?

• Add hydrochloric acid to the solution and if carbonate ions are present, CO2 should form which we can test for with limewater.

• This is because : carbonate + acid -> salt + carbon dioxide + water

How do you detect halide (chloride, bromide, iodide) ions?

• Add dilute nitric acid then add silver nitrate (AgNO3)

• If a halide ion is present you will get a precipitate. The colour depends on the ion.

Colour halide ions turn

• Chloride - white

• Bromide - cream

• Iodide - yellow

What are the advantages of instrumental methods?

sensitivity, accuracy, and speed.

How do you interpret a mass spectrum

• The peaks show the main isotopes - for example copper has two peaks showing us that there are two main isotopes

• Y Axis shows the relative abundance of the isotopes

• X axis is the relative mass of the ions.

How to find the element based of mass spectra

look at the rightmost peak, that is the relative mass of the element

Concentration (mol

dm3) / the number of moles of the solute divided by the volume of the solution

Titration

experiment used to find the concentration of an unknown acid or alkali

What equipment is needed in a titration

• A pipette to accurately measure a certain volume of acid or alkali

• Conical flask to contain liquid from pipette

• Burette to add alkali or acid to conical flask

• White tile to place conical flask on

How do you do a titration

• Add 25cm3 of standard solution alkali to a clean conical flask

• Add a few drops of indicator (phenol phthalein) and put conical flask on white tile

• Fill burette with acid and note starting volume

• Slowly add acid from burette, swirling to mix it

• Stop when the end point is reached (when the solution has changed colour)

• Note final volume and calculate how much acid has been added

• Repeat until you get concordant results

Why do we swirl the conical flask and use a white tile

• Swirl - to evenly distribute the added acid

• White tile - to spot the colour change earlier

Difference between accuracy and precision

• Accuracy - how close a measurement is to the true value

• Precision - how close measurements of the same item are to each other

How can you increase the accuracy of a titration

• Repeat the titration until you get three concordant results

• Using a while tile

• Using a pH probe - indicators are subjective pH probe states exact point the solution is neutralised

• Rinse the apparatus with the solutions they contain before the experiment

What is the relationship between the volume of gas in dm3 and the amount of moles at room temperature and pressure

moles x 24 = the volume of gas in dm3

What must be considered when choosing a reaction

• Usefulness of byproducts

• Rate of reaction

• Cost of raw materials

• Energy requirements

• Atom economy

What is atom economy and how do you calculate it

• Atom economy is the percentage of the reactants that form desired products.

• Atom economy = (Mr of desired products divided by Mr of all reactants) x 100.

Why is it important to consider atom economy

• Raw materials are expensive so if they are being made into waste products, the process is less profitable.

• Less sustainable using large quantities of reactants to make small quantities of products

• Waste products are expensive to dispose of.

Solutions to low atom economy

• Use a more efficient reaction

• Find a use for the waste products

Other important factors that determine how profitable a reaction is

• Percentage yield

• Cost of raw materials

• Position of equilibrium

• Rate of reaction

• Cost of maintaining the right conditions

What is percentage yield and how do you calculate it

• Percentage yield is the comparison between theoretical yield and actual yield.

• Percentage yield = actual yield / theoretical yield.

What are some reasons for a reaction not having 100% percentage yield

• Reactants might not all react, so some of the mixture is still reactants at the end. Could be a reversible reaction or the rate of reaction might be too slow.

• Side reactions - reactants react to produce a different product.

• Some product is lost during the process. Gaseous products can float off for example.

Rate of reaction

the speed at which the reactants get turned into products.

How can we measure the rate of reaction

• Quantity of reactants used / time taken

• Quantity of products formed / time taken

How does the rate of reaction change over time

during the start of the reaction, the rate is very fast as there are lots of reactants that can react with each other. Over time, the rate would slow down as the reactants get used up.

Factors that affect the rate of chemical reactions

• Temperature - particles gain more energy, they move faster, they collide more frequently and with more energy, more successful collisions, higher rate of reaction.

• Pressure and concentration - higher pressure and concentration means more particles per unit of volume, makes collisions more frequent, increases rate of reaction

• Surface area - more area for particles to collide, more frequent collisions, higher rate of reaction.

• Presence of a catalyst - catalysts provide a different reaction pathway with lower activation energy, less energy required for particles to have a successful collision, more successful collisions, higher rate of reaction.

What is collision theory

for particles to react with each other, they need to collide with each other with sufficient energy (activation energy).

What does a reaction depend upon

• The amount of energy the particles have - this is so the particles are more likely to surpass the activation energy.

• The frequency of the collisions - more successful collisions the more often they collide.

What is a catalyst

• A catalyst is a substance that increases the rate of reaction without being used up.

• They do this by providing a different reaction pathway with a lower activation energy

• Catalysts are most commonly transition metals as they can form multiple ions

Where is energy and time plotted on a reaction profile

energy is on the y-axis and time is on the x-axis

What happens to the rate of reaction if a solid is crushed up

• Crushing up the solid increases the surface area to volume ratio

• This means there is more area for particles to collide with

• This leads to more frequent collisions increasing the rate of reaction

What is a reversible reaction

a reaction where the reactants form products, and the products can also form reactants under the same conditions

Dynamic equilibrium

• where the rate of reaction in the forward reaction and backward reaction is the same.

• The reaction is still happening, but they cancel each other out.

• The concentration of reactants and products will not change.

• Occurs in a closed system

How do you determine the position of equilibrium

• When there are more products, the equilibrium lies to the left.

• When there are more reactants, the equilibrium lies to the right.

What happens when equilibrium position shifts to the right

means the forward reaction is favored and more products are formed.

What happens when equilibrium position shifts to the left

the backward reaction is favored and more reactants are formed.

La Chatelier's principle

if you change the conditions of a reversible reaction, the position of equilibrium will shift to try and counteract that change.

How does temperature affect the position of equilibrium

• If the reaction is exothermic in the forward direction, if you decrease the temperature the equilibrium position will move in the exothermic direction to release heat energy and counteract the change. Equilibrium moves to the right

• If you increase the temperature, the equilibrium position will move in the endothermic direction to absorb the heat. Equilibrium moves to the left.

How does pressure affect the position of equilibrium

• If we increase the pressure, the equilibrium will move to the side with the least number of moles.

• If we decrease the pressure, the equilibrium will move to the side with more moles.

How does changing the concentration affect the position of equilibrium

if we increase the concentration of the reactants, equilibrium position will move to the side of the reactants. And vice versa.

If equilibrium moves to the right or to the left what does it mean

• To the right - favors the forward reaction so more products are formed

• To the left - favors the reverse reaction so more reactants are formed

How is carbon used to extract metals

• Most metals react with oxygen in the air to form metal oxides, EXCEPT unreactive metals like gold

• Chemical reactions are required to extract the metal

• If a metal is less reactive than carbon, the metal can be extracted from its compound through reduction with carbon

How does carbon reduce metal oxides

• If a metal oxide is less reactive than carbon, the carbon reduces (takes the oxygen) the metal to form carbon dioxide

• This leaves behind a pure metal

• Metal oxide + carbon --> metal + carbon dioxide

Advantages and disadvantages of using carbon to extract metals

• Advantage - cheap, only requires carbon

• Disadvantage - can only be used for elements less reactive than carbon

Why is electrolysis used to extract some metals

it is used to extract metals that are more reactive than carbon and cannot be extracted by reduction with carbon

How do we use electrolysis to extract metals

• The metal ore is either melted or dissolved in water so ions can move freely

• An electric current is then passed through the solution

• Positive metal ions move towards the negative cathode and gain electrons, forming pure metal

• Negative non-metal ions move towards positive anode and lose electrons, released as gases

What are some downsides of using electrolysis to extract metals from their ores

it is very expensive due to high energy costs

Extraction of aluminium from its ore

• Aluminium oxide is extracted from bauxite

• Aluminium oxide's melting point is too high (2000) so it is dissolved in molten cryolite (900)

• Negative cathode lines bottom of the cell and positive anode are large graphite rods hanging in the solution

• Cathode - positive aluminium ions gain electrons and form molten aluminium, collects at bottom and is tapped off

• Anode - oxygen reacts with carbon anodes and forms CO2, anodes burn away and must be replaced

Ore

naturally occurring rocks that contain enough metal to make it economical to extract the metal

Phytoextraction

• Plants are planted in metal-rich soil

• Plants then absorb metal ions in the soil through their roots and concentrate the metal in their biomass

• Plants are then harvested, dried and burned leaving high grade metal

When would phytoextraction be used

• Ores are low quality or scarce

• Metal are present in the soil at low concentrations

• Areas where traditional mining is not environmentally or economically viable

Advantages and disadvantages of phytoextraction

• Advantages - reduces soil contamination, carbon neutral, cheap, rehabilitates contaminated land, produces little waste

• Disadvantages - slow process, limited to certain metals, climate must be suitable to grow plants

Bioleaching

• The use of bacteria to break down metal ores.

• The bacteria feed on the minerals and release metal ions into a solution called leachate

• Leachate is then recovered and processed leaving the metal

When would bioleaching be used

• In a mine, where only low grade ores remain

• Areas where mining generates significant waste

Advantages and disadvantages of bioleaching

• Advantages - has less impact on environment and is cheaper than traditional mining,

• Disadvantages - very slow, toxic substances and sulfuric acid can be produced which could potentially damage the environment

Haber process

• Exothermic, reversible reaction

• Industrial production of ammonia (NH3) from nitrogen and hydrogen

• Nitrogen - acquired from fractional distillation of air

• Hydrogen - made from hydrocarbons

• 450C, 200atm, iron catalyst

How does the Haber process work

• Hydrogen and nitrogen fed into reaction vessel, where they mix together

• Reaction vessel is kept at 450C and 200atm, and contains an iron catalyst

• Under these conditions, nitrogen and hydrogen react to form ammonia BUT reaction vessel still contains nitrogen and hydrogen

• Solution in reaction vessel then passes through condenser

• Since ammonia has low boiling point, it is condensed into liquid ammonia AND gaseous nitrogen and hydrogen stay gaseous

• Leftover gaseous hydrogen and nitrogen is then recycled back into the reaction vessel

Temperature of the Haber process, and why it is chosen

450C

• Exothermic reaction so low temperature favours forward reaction, achieves higher % yield

• HOWEVER, high temperature needed for higher rate of reaction

• 450C chosen as compromise as it gives us a lower yield but higher rate of reaction, and the temperature is not too costly

Pressure of the Haber process

200atm

• Fewer moles of products than reactants so high pressure favours forward reaction

• High pressure achieves high rate of reaction as particles collide more frequently

• Cost and safety limit how high the pressure can be so 200atm is chosen

Why is the Haber process important

• The ammonia produced is used to make nitrogen based fertilisers

• Prevent soul depletion

• These allow us to grow food more effectively

Contact process

industrial method of making sulfuric acid

How does the contact process work

• Sulfur is combusted with oxygen to make sulfur dioxide

• Sulfur dioxide is further oxidised with vanadium oxide catalyst to form sulfur trioxide

• Sulfur trioxide reacted with water to form sulfuric acid (H2SO4)

Optimal conditions of contact process

• Temperature - high temperature increase RoR but decreases yield as forward reaction is exothermic. 450C chosen as compromise

• Pressure - higher pressure increases RoR and yield but is expensive, 2atm used

• Catalyst - vanadium oxide catalyst used to increase RoR

Compare industrial methods of making fertilisers with laboratory methods

• Industrial production is optimised for high efficiency, cost effectiveness and large output

• Laboratory methods are simpler, safer, and easier to replicate but produce less

How do you carry out a life cycle assessment

life cycle assessments assess the environmental impact of products in different stages

• Extracting and processing raw materials

• Manufacturing and packaging

• Use and operation during lifetime

• Disposal

Advantages and disadvantages of recycling

• Advantages - preserves materials, requires less energy than obtaining new materials

• Disadvantages - energy costs of transporting materials to recycling centre, must be sorted into different types, saves different amount of energy for different materials

Composition of steel and its uses

• Iron and carbon

• High strength and hardness, resistant to corrosion, durable

• Used for cars, construction, surgical scalpels

Composition of brass and its uses

• Copper and zinc

• Malleable, ductile, electrical conductive

• Used for musical instruments, plumbing, ornaments

Composition and uses of bronze

• Copper and tin

• Hard, brittle, resistant to rough usage

• Used for utensils, propellers, coins, medals

Composition and uses of solder

• Lead and tin

• Low melting point, good electrical conductivity

• Used for joining wires in electronic circuits due to low melting point

Composition and uses of duralumin

• Aluminium and copper

• Lightweight, high strength to weight ratio, ductile, resistant to corrosion

• Used in aircraft, tools, scientific instruments

Corrosion

refers to the process by which metals are slowly broken down by reacting with substances in their environment. ONLY THE SURFACE OF THE METAL CORRODES

Rusting

where iron reacts with water and oxygen in the environment to form hydrated iron oxide, known as rust. Redox reaction

Conditions required for rusting to take place

water AND oxygen in the environment must be present.

How can we prevent iron from rusting

• Barrier methods - prevent oxygen and water from touching the iron

• Sacrificial methods

Types of barrier methods

• Painting - works well for big pieces of iron

• Oil or grease it

• Electroplating - expensive

Sacrificial method

• Adding a more reactive metal to the iron

• If the object is exposed to oxygen, the more reactive metal will be oxidised instead of the iron.

Galvanising

coating an iron object in a layer of zinc to prevent rusting, barrier and sacrificial method

Why does aluminium not break down when it corrodes like iron

the aluminium oxide forms a protective layer, preventing further oxidation

Properties and uses of glass ceramics

• Transparent, hard, brittle, poor heat and electrical conductors, high compressive strength, low tensile strength

• Uses - windows, bottles

Properties and uses of clay ceramics

• Opaque, hard, brittle, poor heat and electrical conductors, high compressive and low tensile strength

• Uses - brick and porcelain