Developing Fuels Flashcards
Define enthalpy change of reaction.
The amount of energy given out/taken in during a reaction
What are exothermic reactions?
Reactions that give out energy to the surroundings, meaning they heat up.
ΔH is negative- energy to form bonds is greater than energy needed to break the bonds
What are endothermic reactions?
Reactions that take in energy from the surroundings, making them cool down.
ΔH is positive- energy to form bonds is less than energy needed to break the bonds
How do you work out ΔH?
ΔH=H(products)-H(reactants)
Define the system of a reaction.
The products and the reactants that chemists are interested in
What are a reaction’s surroundings?
The rest of the world, like the test tube, air etc
What are standard conditions?
298K (25°C), 1 atm, 1 mol dm^-3, standard states (solid, liquid, or gas)
Define standard enthalpy change for a reaction.
The enthalpy change of a reaction when molar quantities of reactants react together under standard conditions in standard states
Define standard enthalpy of combustion
The enthalpy change that occurs when one mole of reactant completely burns in oxygen in standard states under standard conditions
Define standard enthalpy of formation
The enthalpy change when one mole of a compound is formed from its elements under standard conditions in standard states
Define standard enthalpy of neutralisation
The enthalpy change one one mole of H+ ions react with one mole of OH- ions to form one mole of water
Energy transferred=
Specific heat capacity x mass x temperature change
What is Hess’s law?
The enthalpy change for any chemical reaction is independent of the intermediate stages, so long as the initial and final conditions are the same
What is the overall formula to determine the enthalpy change?
ΔH1 = ΔH2 -ΔH3
What is fractional distillation?
The method of separating hydrocarbons according to chain lengths
State the general formula of alkanes
C(n)H(2n+2)
How do you identify aromatic molecules?
Molecules with one or more benzene rings
What are aliphatic molecules?
Molecules that don’t have any benzene rings
Hydrocarbons with no double/triple bonds are…
saturated
Hydrocarbons with double bonds are…
unsaturated
How do you name hydrocarbons?
1) Name the longest carbon chain
2) Identify any side chains and name them
3) Identify the location of the side chains. Use the lowest number possible
4) List the side chains alphabetically
Define energy density
How much energy you can get per kilogram of fuel
What is the formula for energy density?
100 x (standard enthalpy/molecular mass)
Why is it difficult to measure exact bond enthalpies?
The exact value depends on the particular compound in which the bond is found
Why is bond breaking endothermic?
It requires energy
Why is bond making exothermic?
It releases energy
Give two reasons why theoretical calculations are different to experimental data
1) Bond enthalpies found in tables are averages from several compounds
2) Products/reactants aren’t in their standard states
What is cracking?
Any reaction where a larger molecule is made into smaller molecules. In terms of hydrocarbons, normally long chain alkane –> shorter chain alkane + alkene
What are catalysts?
Chemicals that speed up reactions by providing an alternative pathway with a lower activation enthalpy, and are chemically unchanged at the end of the reaction
Why is catalyst poisoning an issue?
The catalyst becomes inactive because a substance stops it from functioning properly, leading to reduced reactivity
Give the mechanism of heterogeneous catalysis
Reactants adsorb onto the catalyst surface.
Intramolecular bonds weaken and break within the reactants.
New bonds form, leading to the product forming.
Product diffuses away from the catalyst.
What is the general formula of alkenes?
C(n)H(2n)
Define the term ‘electrophile’
A positive ion or a molecule with a partial positive charge that accepts a pair of electrons to form a covalent bond
Give the mechanism for electrophilic addition with bromine
The double bond in the alkene polarises the bromine molecule- bromine atom closest to alkene is δ+, other is δ-
The δ+ bromine acts as an electrophile and the double bond attacks it, forming a carbocation, leaving a bromide ion
A pair of electrons on the bromide ion moves to the carbocation, to form another carbon-bromine bond
Why is bromine used to test to see if a hydrocarbon is saturated or unsaturated?
The addition of bromine to alkenes causes it to lose its red-brown colour to become colourless
When are sigma (σ) bonds formed?
Single bonds, when 2 orbitals overlap. It gives the highest possible electron density possible, making sigma bonds very strong
What are double bonds made up of?
One σ bond and one pi (π) bond
When are π bonds formed?
When two p-orbitals overlap. They are weaker than σ bonds, which explains why double bonds aren’t twice as strong as single bonds
Define the term ‘polymer’
A long molecule made up of smaller monomers
What is polymerisation and what are the two types?
Polymerisation is when monomers join together to form long chain polymers
Two types: addition, condensation
What is the atom economy of addition polymerisation?
100%
How many molecules of a substance are there in a mole?
6.02 x10^23
