* Developing fuels

Question 1

What volume does gas take up

Answer 1

• if temperature and pressure are constant
• one mole of any gas always has the same volume
• at RTP - volume of 1 mole = 24.0dm³
• RTP - 298K = 25^oC
• and 100kPa

Question 2

What is the ideal gas equation?

Answer 2

• lets you find the number of moles in a certain volume at any temperature and pressure

Question 3

How is the volume of gas measured in an experiment?

Answer 3

• attaching a gas syringe to the opening of a reaction vessel lets you measure how much gas is produced in a reaction
• it will just show the total volume of gas produced
  
  • if more than one gas is produced, wont show the volume of each one
• can tell the reaction has completed when no further change in gas volume
• really vigorous reactions can blow the plunger out - so need to be careful

Question 4

What is an enthalpy change of a chemical reaction?

Answer 4

• when chemical reactions happen, some bonds are broken and some bonds are made
• this will cause a change in energy (enthalpy)

Question 5

How can you find enthalpy changes?

Answer 5

• either by experiment or in data books
• enthalpy changes you find in data books are usually standard enthalpy conditions - 298K and 100kPa
• you write ΔH^ϴ​ to show measurements of enthalpy change were made under standard conditions and that elements were in standard states

Question 6

What is an exothermic reaction?

Answer 6

• exothermic reactions give out energy, because more bonds are made than broken. ΔH is negative
• In exothermic reactions the temperature often increases
• Oxidation = usually an exothermic reaction

Question 7

What is an endothermic reaction?

Answer 7

• endothermic reaction absorbs energy, more bonds are broken than made, ΔH is positive
• temperature in endothermic reactions often decrease
• thermal decomposition of calcium carbonate is endothermic
• CaCO_3(s) –> CaO_(s) + CO_2(g)

Question 8

How can enthalpy changes be calculated?

Answer 8

• you need energy to break bonds, so breaking is an endothermic process (ΔH is positive)
• energy is released when bonds are formed, so bond making is an exothermic process (ΔH is negative)
• enthalpy change for a reaction = overall effect of both these changes
  
  • if you need more energy more energy to break bonds than when bonds are made ΔH is positive, if you need less than ΔH is negative
• bond enthalpy = energy needed to break a bond, or the energy given out when a bond forms - stronger bonds have higher bond enthalpies
• average bond enthalpies in data books can be used to calculate enthalpy changes of reactions

Question 9

Why are average bond enthalpies not accurate?

Answer 9

• average bond enthalpies = not exact
• water (H₂O) has two O-H bonds - but doesnt take the same amount of energy to break each one
• average bond enthalpy is the energy needed to break one mole of bonds in the gas phase, averaged over many different compounds

Question 10

What is bond enthalpy related to?

Answer 10

• in covalent molecules, the position of nuclei are attracted to the shared electrons
• but there isnt just an attraction between the nuclei and the shared electrons
  
  • the two positively charged nuclei also repel each other, as they do the electrons
• the distance between the two nuclei is the distance where the attractive and repulsive forces balance each other. the distance = bond length
• stronger the attraction between two atoms, the higher the bond enthalpy and the shorter the bond length (more attraction = nuclei pulled closer together)

Question 11

What are the different types of ΔH?

Answer 11

• standard enthalpy change of reaction Δ_rH^ϴ - enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation under standard conditions
• standard enthalpy change of formation Δ_fH^ϴ - enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
• standard enthalpy change of combustion Δ_cH^ϴ - enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions

Question 12

What is Hess’ law?

Answer 12

• the total enthalpy change of a reaction is always the same, no matter which route is taken

Question 13

What do enthalpy level diagrams show

Answer 13

• an enthalpy level diagram shows you how the enthalpy changes during a reaction
• the activation energy, Ea is the minimum amount of energy needed to begin breaking reactant bonds and start a chemical reaction
• the less enthalpy a substance has, the more stable it is

Question 14

How can you find out enthalpy changes using calorimetry?

Answer 14

• in calorimetry - you find out how much heat is given out by a reaction by measuring temperature change of some water
• to find enthalpy of combustion of a flammable liquid, you burn it
• as the fuel burns, it heats the water - you can work out the heat absorbed by the water if you know the mass of water, the temperature change of the water (ΔT) and the specific heat capacity of water

Question 15

What are the issues in practical of calorimetry?

Answer 15

• ideally all heat given out by the fuel as it burns would be absorbed by water - allowing you to be able to work out the enthalpy of combustion
• but heat always lost to apparatus and surroundings

Question 16

How is calorimetry used to calculate the enthalpy change for a reaction that happens in solution

Answer 16

• calorimetry can be used to calculate the enthalpy change for a reaction that happens in solution, such as neutralisation or displacement
• Standard enthalpy change of neutralisation Δ_neutH^ϴ - enthalpy change when an acid and an alkali react together under standard conditions to form 1 mole of water
• to find enthalpy change in a neutralisation reaction, add a known volume of acid to an insulated container and measure the temperature
• then add a known volume of alkali and record the temperature rise (stir to make sure evenly heated)
• you can work out the heat needed to raise the temperature of the solution formed using q = mcΔT - this gives you heat given out by reaction
• you can assume that all solutions (reactants and products) have the same density as wate
  
  • this means you can use volume instead of mass in calculations
  • you can also usually assume that the specific heat capacity of the solution formed is the same as that for water

Question 17

What is the equation for calculating enthalpy

Answer 17

Question 18

how do catalysts increase the rate of a reaction?

Answer 18

• a catalyst increases the rate of a reaction by providing an alternative reaction pathway with a lower activation enthalpy
• the catalyst takes part in the reaction but is chemically unchanged at the end of a reaction
• many are specific and will only work on a single reaction

Question 19

define catalysis

Answer 19

• catalysis means speeding up a chemical reaction by using a catalyst

Question 20

How are catalysts used in the cracking process?

Answer 20

• short chain hydrocarbons tend to be more liquid than longer chain ones
• long chain alkanes can be cracked (broken up) into smaller hydrocarbons (including alkenes)
• the products of cracking are random and the same molecule can give different cracking products
• without a catalyst, the cracking process requires extremely high temperatures and pressures, which makes it expensive
• by passing hydrocarbon vapour over a heated solid catalyst, cracking can take place at much lower temperatures (around 450^oC) and pressures, saving a lot of money

Question 21

Is iron in the haber process a heterogeneous or homogeneous

Answer 21

• heterogeneous (solid)
• reactants = gas

Question 22

Is platinum in catalytic converters homogeneous or heterogeneous catalyst?

Answer 22

• heterogeneous
• catalytic converters in a car exhaust stop pollutants from coming out
• without a catalytic converter - a lot of bad stuff would be released in the atmosphere - carbon monoxide (which is poisonous), oxides of nitrogen and unburt hydrocarbons
• when the sun shines on nitrogen oxides and hydrocarbons, low level (ground level) ozone is produced
• ground level ozone can combine with other pollutants to form smog (irritates respiratory system)
• catalytic converters change them into harmless gases, like water vapour, and nitrogen, or less harmful gases like carbon dioxide

Question 23

how do reactions happen with a heterogeneous catalyst?

Answer 23

• solid heterogeneous catalysts can provide a surface for a reaction to take place on
• reactant molecules arrive at the surface and bond with the solid catalyst. This is called adsorption
• the bonds between the reactants atoms are weakened and break up
• this forms radicals - atoms or molecules with unpaired electrons, these radicals then get together and make new molecules
• new molecules then detached from the catalyst - desorption

Question 24

why must adsorption not be too strong in a reaction with a heterogeneous catalyst

Answer 24

• adsorption cant be too strong or the catalyst wont let go of the atoms after the reactions
• but strong enough to weaken bonds between reactant molecules so new molecules can form

Question 25

How do catalyst become poisoned?

Answer 25

• catalysts can be poisoned so they dont work anymore, for example
  
  • carbon monoxide, CO, poisons the solid iron catalyst used in the haber process
  • lead poisons the catalyst in catalytic converters
  • was a problem when lead was in petrol - now all petrol = unleaded petrol
• heterogeneous often get poisoned because the poison clings on to the cataylsts surface more strongly than the reactant does
  
  • so catalyst is prevented from getting involved in the reaction its meant to be speeding up

Question 26

What is the general formula of an alkane?

Answer 26

• C_nH_2n+2
• hydrocarbon - contain only hydrogen and carbon
• every carbon atom has 4 single bonds with other atoms
• impossible for carbon to make more than 4 bonds - so alkanes are saturated

Question 27

what is the general formula for a cycloalkane?

Answer 27

• C_nH_2n
• two hydrogens attached to each carbon
• saturated

Question 28

What is the general formula for an alkene?

Answer 28

• C_nH_2n
• hydrocarbon - made of only hydrogen and carbon atoms
• have at least one double C=C bond
• are unsaturated because they can bond to extra atoms in addition reactions

Question 29

Describe benzene

Answer 29

• Benzene = C₆H₆
• cyclic alkene
• more stable (less reactive) than expected, because double bond electrons are delocalised around the carbon ring. this is why symbol has a circle in it

Question 30

What is an aromatic compound?

Answer 30

• compounds with benzene structures
• called arenes or aromatic compounds

Question 31

what are aliphatic compounds?

Answer 31

• compounds without benzene ring structures
• like alkanes and alkenes

Question 32

What is the suffix for arenes

Answer 32

• -benzene

Question 33

what is the suffix for a cycloalkane

Answer 33

• cyclo- -ane

Question 34

explain the cause of shapes of molecules

Answer 34

• depends on electrons in the outer shells of the atoms involved
• electron pairs repel each other
• so molecules take the shape that allows all the pairs of electrons to get as far from each other as possible

Question 35

describe a tetrahedral shape

Answer 35

• when a carbon atom makes four single bonds
• it will have a tetrahedral shape
• angles between any two of the bonds = 109.5
• the electron pairs are as far apart as possible

Question 36

Describe the shape of a molecule around a double bond?

Answer 36

• the C=C double bond and the atoms bonded to these carbons are planar
• each double bonded carbon and the atoms attached to it are said to be trigonal planar
• the bond angles are all 120^o

Question 37

What are the two types of bonds

Answer 37

• sigma σ bonds
• and pi π bonds

Question 38

Describe where sigma bonds are formed?

Answer 38

• single bonds in organic molecules are sigma bonds
• sigma bond formed when two orbitals overlap, in a straight line, in the space between two atoms
• this gives the highest possible electron density between two postive nuclei so sigma bonds are usually very strong (single covalent bond)

Question 39

Describe where a pi bond is found

Answer 39

• double bond made up of sigma bond and pi bond
• pi bond = formed when two p orbitals overlap sideways
• pi bond = weaker than sigma bond
  
  • double bond is not double the strength as a single bond

Question 40

What are the types of formula?

Answer 40

Question 41

What are the structural isomers?

Answer 41

• functional group in a different place
• different functional group - alcohols and ethers
• different carbon skeleton (branched)

Question 42

Why do we get E/Z isomerism?

Answer 42

• because atoms cannot rotate around C=C bonds
• double bond = fairly rigid

Question 43

What is E/Z isomerism an example of

Answer 43

• stereoisomerism
• have the same shortened structural formula but a different arrangement in space
• because of lack of rotation around the double C=C bond - some alkenes can have stereoisomerism
• if each carbon on the end of a C=C bond has 1 hydrogen attached to it - then you can get a Z-isomer and an E-isomer
• Z(zusammen -together (on the same side))
• Z- cis
• E- trans
• (E/z specifically when you have H’s - cis and trans when you dont)

Question 44

How are addition polymers formed

Answer 44

• double bond in alkenes can open up and join together to make long chains called polymers
• individual small alkenes = monomers
• called addition polymerisation

Question 45

How can an alkane be produce from an alkene

Answer 45

• ethene will react with hydrogen to produce ethane
• needs a catalyst either (nickel and temp of 150 and high pressure)
  
  • or platinum catalyst at room temp and pressure
• other alkenes react in the same way - the C=C double bond opens up and then hydrogen atoms join on to give an alkane
• this is an addition reaction because atoms being added across a double bond to make one product

Question 46

How is bromine water used to test for a C=C double bond

Answer 46

• when you shake an alkene with orange bromine water - the solution quickly decolourises
• bromine is being added across the double bond to form a colourless dibromoalkane
• adding bromine is used as a test to test for unsaturation (presence of double/triple carbon to carbon bonds)
• saturated compounds like alkanes dont react - solution stays orange
• bromine water reacting is an example of electrophilic addition