AQA GCSE Chemistry - Paper 2

What is the rate of a chemical reaction?

What is the rate of a chemical reaction?

How fast the reactants are changed into products

Examples of slow chemical reactions

Rusting of iron

Chemical weathering e.g. acid rain

Example of moderate chemical reactions

Reaction of magnesium with an acid to produce a gentle stream of bubbles

Examples of fast chemical reactions

Burning

Explosions - over in a fraction of a second

Rate of Reaction Graphs

Steeper lines = faster rate - the quickest reactions have the steepest lines & become flat in the least time

Shallower lines = slower rate - the slowest reactions have the shallowest lines & become flat in the most time

What is collision theory?

It affects the rate of a chemical reaction

1) The collision frequency of reacting particles - the more collisions there are, the faster the reaction is

2) The energy transferred during a collision - particles have to collide with enough energy for the collision to be successful

What does the rate of reaction depend on?

1) Temperature

2) Concentration of a solution or the pressure of gas

3) Surface area - this changes depending on the size of the lumps of a solid

4) The presence of a catalyst

Rate of Reaction - Increasing the Temperature

1) Causes the particles to move faster - if they're moving faster, they're going to collide more frequently

2) The faster the particles move, the more energy they have, so more of the collisions will have enough energy to make the reaction happen

Rate of Reaction - Increasing the Concentration/Pressure

1) A more concentrated solution will contain more particles in the same volume of water

2) If the pressure of a gas is increased, there will be more particles occupying a smaller space

3) This causes collisions between the reactant particles to be more frequent

Rate of Reaction - Increasing the Surface Area

Breaking a solid up into smaller pieces will increase its surface area to volume ratio - for the same volume of the solid, the particles around it will have more area to work on, thus there will be more frequent collisions

Rate of Reaction - Using a Catalyst

Not part of the overall reaction equation - doesn't get used up in the reaction itself

They all decrease the activation energy needed for the reaction to occur - they do this by providing an alternative reaction pathway with a lower activation energy

Equation for Rate of Reaction

Rate of reaction = Amount of reactant used or amount of product formed / Time

Measuring the Rate of Reaction - Precipitation & Colour Change

1) Recording visual changes in a reaction if the initial solution is transparent and the product is a precipitate which clouds the solution

2) Observing a mark through the solution & measure how long it takes for it to disappear - the faster it disappears, the faster the rate of reaction

3) If the reactants are coloured & the products are colourless, time how long it takes for the solution to lose (or gain) its colour

4) Results can be subjective & you can't plot a rate of reaction graph from the results

Measuring the Rate of Reaction - Change in Mass

1) Measuring the speed of a reaction that produces a gas can be carried out using a mass balance

2) As the gas is released, the mass disappearing is quickly measured on the balance - the quicker the reading on the balance drops, the faster the reaction

3) Measurements can be taken at regular intervals for a rate of reaction graph to easily find the rate

4) Most accurate but the gas is released straight into the room

Measuring the Rate of Reaction - The Volume of Gas Given Off

1) Uses a gas syringe to measure the volume of gas given off

2) The more gas given off during a given time interval, the faster the reaction

3) Gas syringes usually give volumes accurate to the nearest cm³, so they're quite accurate

4) Measurements can be taken at regular intervals for a rate of reaction graph to be plotted

5) If the reaction is too vigorous, the plunger could easily be blown out of the end of the syringe

How do you find the mean rate for the whole reaction?

Work out the overall change in the y-value (amount of product formed or amount of reactant used) and divide it by the total time taken for the reaction

How do you find the rate of reaction at a particular point?

Finding the gradient of the curve at that point - do this by drawing a tangent to the curve and work out the gradient of the tangent

Reversible reactions

1) As the reactants react, their concentrations fall, so the forward reaction will slow down - but, as more and more products are made and their concentrations rise, the backward reaction will speed up

2) After a while the forward reaction will be going at exactly the same rate as the backward one - the system is at equilibrium

What happens at equilibrium during a reversible reaction?

Both reactions are still happening, but there's no overall effect - the concentrations of reactants & products have reached a balance & won't change

When will the reaction reach equilibrium?

When the reaction takes place in a closed system - none of the reactants or products can escape & nothing else can get in

What happens if the equilibrium lies to the left?

The concentration of reactants is greater than that of the products

What happens if the equilibrium lies to the right?

The concentration of products is greater than that of the reactants

What does the position of equilibrium depend on?

1) The temperature

2) The pressure (only affects the equilibria involving gases)

3) The concentration of the reactants & products

Are reversible reactions endothermic or exothermic?

They can be both - if the reaction is endothermic in one direction, it will be exothermic in the other

The energy transferred from the surroundings by the endothermic reaction is equal to the energy transferred to the surroundings during the exothermic reaction

Le Chatelier's Principle

The idea that if you change the conditions of a reversible reaction at equilibrium, the system will try to counteract that change

It's used to predict the effect of any changes you make to a reaction system

Changes to Temperature

1) If you decrease the temperature, the equilibrium will move in the exothermic direction to produce more heat, therefore you'll get more products for the exothermic reaction and fewer products for the endothermic reaction

2) If you increase the temperature, the equilibrium will move in the endothermic direction to try & decrease it, therefore, you'll get more products for the endothermic reaction and fewer products for the exothermic reaction

Changes to Pressure

Only affects an equilibrium involving gases

Increasing the pressure will cause the equilibrium to try & reduce it, thus making it move in the direction where there are fewer gas molecules

Decreasing the pressure will cause the equilibrium to try & increase it, thus making it move in the direction where there are more gas molecules

Changes to Concentration

Changing the concentration will cause the system to bring itself back to equilibrium

Increasing the concentration causes the system to decrease it by making more products

Decreasing the concentration causes the system to increase it by reducing the amount of reactants

Reaction of Magnesium & Hydrochloric Acid to produce Hydrogen gas

1) Add a set volume of dilute HCl to a conical flask & carefully place on a mass balance

2) Add some magnesium ribbon to the acid & quickly plug the flask with cotton wool

3) Start the stopwatch & record the mass on the balance - take readings at regular intervals

4) Plot the results in a table & work out the mass lost for each reading. Plot a graph

5) Repeat with more concentrated acid solutions, but keep variables like the amount of magnesium ribbon and the volume of acid the same

Reaction of Sodium Thiosulfate & Hydrochloric Acid to produce a Cloudy Precipitate

Both the solutions are colourless - they react together to form a yellow precipitate of sulfur

1) Add a set volume of dilute Na2O3S2 to a conical flask

2) Place the flask on a piece of paper with a black cross drawn on it - add some dilute HCl & start the stopwatch

3) Watch the black cross disappear through the cloudy sulfur & time how long it takes

4) Repeat the reaction with solutions of either reactant at different concentrations - only change one concentration at a time

How can you find the speed of a reaction?

Recording the amount of product formed or the amount of reactant used over time

What is a successful collision?

A collision that ends in the particles reacting to form products

What do the particles need the activation energy for?

To break the bonds in the reactants & start the reaction

What factors will increase the rate of reaction?

Factors that increase the number of collisions or the amount of energy particles collide with

Thermal Decomposition of Hydrated Copper Sulfate - endothermic

If you heat blue hydrated copper(II) sulfate crystals, it drives the water off & leaves white anhydrous copper(II) sulfate powder

Thermal Decomposition of Hydrated Copper Sulfate - exothermic

If you add a couple of drops of water to the white powder, you get the blue crystals back

Are all reactions endothermic in one direction & exothermic in the other?

Yes

What is a hydrocarbon?

Any compound that is formed from carbon and hydrogen atoms only

What are alkanes?

The simplest type of hydrocarbons

They have single bonds between all the carbon atoms

They are a homologous series

Are alkanes saturated or unsaturated?

They are saturated - each carbon atoms forms four single covalent bonds

Viscosity of hydrocarbons

The shorter the carbon chain, the less viscous it is

Volatility of hydrocarbons

The shorter the carbon chain, the more volatile it is - the lower the boiling point

Flammability of hydrocarbons

The shorter the carbon chain, the more flammable it is

Complete combustion of hydrocarbons formula

Hydrocarbon + oxygen --> carbon dioxide + water (+ energy)

What happens during the complete combustion of a hydrocarbon?

Both the carbon and hydrogen atoms are oxidised

What is crude oil?

A mixture of lots of different hydrocarbons, most of which are alkanes

How can the different compounds in crude oil be separated?

Fractional distillation

How many carbons make petrol?

Eight carbons

What are the products from crude oil known as?

Organic compounds - compounds containing carbon atoms

How are alkenes produced?

Cracking alkane molecules produced from fractional distillation

What is cracking?

A thermal decomposition reaction which involves breaking molecules down by heating them

Catalytic cracking method

1) Heat long chain hydrocarbons to vaporise them

2) Pass the vapour over a hot powdered aluminium oxide catalyst

3) The long-chain hydrocarbon molecules split apart on the surface of the specks of catalyst

Steam cracking method

Vaporise hydrocarbons, mix them with steam and then heat them to a very high temperature

What are alkenes?

Hydrocarbons which have one double bond between two of the carbon atoms in their chain

They have two fewer hydrogen atoms than alkanes with the same number of carbon atoms

Are alkenes saturated or unsaturated?

They are unsaturated

Are alkanes or alkenes more reactive?

Alkenes - their double bond can open up to allow the carbon atoms to form more bonds

Formula for alkanes

CnH2n+2

Formula for alkenes

CnH2n

What is complete combustion?

When there's a large amount of oxygen in the air and something combusts to produce only water and carbon dioxide

Incomplete combustion of alkenes formula

Alkene + oxygen --> carbon monoxide + carbon dioxide + water

What is a functional group?

A group of atoms in a molecule that determine how that molecule typically reacts

What is the functional group of an alkene?

C=C

What happens when hydrogen reacts with the double-bonded carbons in an alkene?

The double bond will open up and form the equivalent, saturated alkane

What happens when alkenes react with steam?

Water is added across the double bond and an alcohol is formed

E.g. ethene --> ethanol

How can the alcohol be purified from the alkene mixture?

Fractional distillation

What are polymers?

Long molecules formed when lots of monomers join together

What is the reaction called when polymers are created & what conditions does it require?

Polymerisation - requires high pressure & a catalyst

What are addition polymers?

They're made up of many unsaturated monomers with a double covalent bond

What happens during addition polymerisation?

Lots of unsaturated monomer molecules (alkenes) open up their double bonds and join together to form polymer chains

Formula for alcohol

CnH2n+1OH

Complete combustion of alcohol

Alcohol + oxygen --> carbon dioxide + water

Are alcohols flammable?

Yes

Which alcohols are soluble in water?

Methanol, ethanol, propanol and butanol

Fermentation equation

Sugar --> ethanol + carbon dioxide

What are carboxylic acids?

A homologous series of compounds that all have '-COOH' as a functional group

Their names end in '-anoic acid'

What happens when carboxylic acids react with carbonates?

They form a salt, water and carbon dioxide

The salts usually end in '-anoate'

What happens when carboxylic acids dissolve in water?

They ionise and release positive hydrogen ions - however, they don't ionise completely so only form weak acidic solutions

What are esters?

They are formed from an alcohol and a carboxylic acid - acid catalysts are used in the reaction

They have the functional group '-COO-'

Ester equation

Alcohol + carboxylic acid --> ester + water

What happens during condensation polymerisation?

Monomers containing different functional groups react together and form bonds between them, creating polymer chains

For each new bond that forms, a small molecule (e.g. water) is lost

Addition polymerisation

Only one monomer type

Only one product formed

C=C bond in monomer

Condensation polymerisation

Two monomer types each containing two of the same functional groups OR one monomer type with two different functional groups

Two types of product - the polymer and a small molecule

Two reactive groups on each monomer

What does an amino acid contain?

Two different functional groups - a basic amino acid (NH2) and an acidic carboxyl group (COOH)

E.g. glycine - the smallest and simplest amino acid

What polymers can amino acids form and how?

Polypeptides via condensation polymerisation

What are proteins made from?

One or more long-chains of polypeptides

What are DNA molecules made from?

Nucleotide polymers - two polymer chains of monomers called 'nucleotides'

What do sugars contain?

Carbon, oxygen and hydrogen

E.g. glucose = C6H12O6

What happens to sugars during polymerisation?

They react together to form larger carbohydrate polymers

E.g. starch and cellulose

What is a homologous series?

A group of organic compounds that all react in a similar way

What are short chain hydrocarbons used for?

Bottled gases - stored under pressure as liquids in bottles

This is because they have lower boiling points

Fractional distillation - process

1) Crude oil is heated to a gas - this then enters a fractionating column

2) The gas is heated and the different-length hydrocarbons condense back into liquids at different points, depending on their boiling point

3) When they condense, they then drain out of the column

What does the petrochemical industry use hydrocarbons for?

Feedstock to make new compounds for use in polymers, solvents, lubricants & detergents

What is hydrogenation?

Hydrogen is added to an alkene - it reacts with the double-bonded carbon to open up the double bond and form the equivalent, saturated alkane

The alkene is reacted with hydrogen in the presence of a catalyst

How do alkenes react with halogens?

Addition reactions

Molecules formed are saturated with the C=C carbons each becoming bonded to a halogen atom

What happens when bromine & ethene react together?

They form dibromoethene

Testing for alkenes - adding bromine

1) When orange bromine water is added to a saturated compound (alkane), no reaction happens, thus it stays bright orange

2) When orange bromine water is added to an alkene, the bromine will add across the double bond, creating a colourless dibromo-compound - the bromine water is decolourised

What happens when the first four alcohols react with sodium?

One of the products is hydrogen

Why are alcohols like methanol & ethanol used as solvents in industry?

They can dissolve most things water can dissolve & dissolve substances that water can't dissolve e.g. hydrocarbons, oils & fats

Fermentation

Uses an enzyme in yeast to convert sugars into ethanol

Happens fastest at 37°C in slightly acidic solution & under anaerobic conditions

How do amino acids form polymer chains?

The amino group of an amino acid reacts with the acid group of another and so on