3.2 Physical Chemistry Flashcards
Define enthalpy change of reaction
Energy change associated with a given reaction
Define enthalpy change of formation.
The energy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard state under standard conditions.
Define enthalpy change of combustion.
The energy change that takes place when 1 mole of substance is completely combusted.
Define enthalpy change of neutralisation.
The energy change associated with the formation of 1 mole of water from a neutralisation reaction
What are standard conditions?
100kPa
298K
All substances in standard states
What is the standard enthalpy change of the formation of an element?
0kJ/mol
What is calorimetry?
The quantitative study of energy in a chemical reaction.
What is specific heat capacity?
The energy required to raise the temperature of 1g of a substance by 1K.
What is q in the equation for calorimetry?
Heat exchanged with the surroundings (J)
What is m in the equation for calorimetry?
Mass of the substance heated or cooled (g)
What is c in the equation for calorimetry?
The specific heat capacity of the substance that is heated or cooled (J/g/K)
How do you reduce heat lost to surroundings in calorimetry?
Insulating equipment with draught shields.
What is average bond enthalpy?
The mean energy needed for 1 mole of a given type of gaseous Bonds to undergo homolytic fission
Examples of exothermic reactions
Combustion
respiration
Examples of endothermic reactions
Photosynthesis
Thermal decomposition
Use ideas about the enthalpy changes that take place during bond breaking and bond
making to explain why some reactions are exothermic
Bond breaking absorbs energy
AND bond making releases energy
More energy released than absorbed
Suggest three reasons why standard enthalpy changes of combustion determined
experimentally are less exothermic than the calculated theoretical values.
Heat released to the surroundings
Incomplete combustion OR incomplete reaction
OR not everything burns
Non-standard conditions
Name the catalyst used in C2H4(g) + H2O(g) C2H5OH
acid
State le Chatelier’s principle.
When conditions on a system in equilibrium are changed
The equilibrium moves to minimise the effects of the change
What is important when calculating kJmol-1?
Remember + or - sign
It is very difficult to determine the standard enthalpy change of formation of
hexane directly. Suggest a reason why
many different hydrocarbons would form
OR activation energy too high
OR reaction too slow
OR they don’t react together
Choose a reaction that you have studied that is catalysed by chlorine radicals
Cl + O3 → ClO + O2 ClO + O → Cl + O2 overall: O3 + O → 2O2 OR Cl + CH4 → CH3 + HCl CH3 + Cl2 → CH3Cl + Cl overall: CH4 + Cl2 → CH3Cl + HCl
What is a catalyst?
A substance that increases rate of reaction without being used up during the process.
What is a homogenous catalyst?
A catalyst that is in the same phase as the reactants (eg liquid catalyst that is mixed with liquid reactants)
Give an example of a homogenous catalyst
Enzymes in blood and saliva
What is a heterogeneous catalyst?
A catalyst that is in a different phase from the reactants.
What is an example of a heterogeneous catalyst?
A catalytic converter in a car.
What catalyst breaks down the triple bond in nitrogen in the Haber process?
Iron
What is the Boltzmann distribution?
The distribution of energies of molecules at a particular temperature, often shown as a graph.
What does the area under the curve of a Boltzmann distribution graph represent?
The number of molecules
Why does the Boltzmann curve start at the origin?
There are no molecules with 0 energy.
Why does the Boltzman curve not touch the energy axis?
There is no maximum energy of a molecule
When is a chemical system in dynamic equilibrium?
the concentrations of the reactants and the products remain constant
the rate of forward reaction is the same as the rate of the reverse reaction
How can the position of equilibrium be altered?
By changing:
concentrations of reactants and products
pressure in reactions involving gas
temperature
What does a catalyst do to equilibrium?
Speeds up the rate of the forward and reverse reactions equally
Establish it faster
Examples of important equilibrium systems?
Haber process
the conversion of sulphur dioxide into sulphur trioxide in the Contact process.
Where do nitrogen and hydrogen come from in the Haber process?
Nitrogen is obtained from air by fractional distillation
Hydrogen is prepared by reacting methane and water
What are the optimum conditions for the Haber process?
High pressure and low temperature
Forward reaction produces less molecules
The forward reaction is exothermic
What are the drawbacks of the optimum conditions of the Haber process?
Low temperature means low rate
High pressure very expensive high concentration bad safety
What are the actual conditions used on a modern ammonia plant?
450 degrees
200atm
Iron catalyst
Why do bond enthalpies have positive values?
Breaking bonds is endothermic and energy must be put in
Define the term bond enthalpy
The energy needed to break 1 mile of bonds in the substance in the gaseous state
How do catalysts improve sustainability?
Not used up in reaction Lower temperatures Lower Ea Less fuel Less CO2 Different reactions can be used Better atom economy Less hazardous chemicals Generate specific products
