Physical Chemistry
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Definition of bond enthalpy
Enthalpy change required to break 1 mole of a covalent bond (in gaseous phase)
Why do bond enthalpies have positive values
They are an endothermic reaction (bond breaking)
Suggest 2 reasons why a standard enthalphy change of combustion of methanaol will be different from the calculated one ?
• it is not done at the standard condition which is 250 degrees and 1 atp • does not take into account the specific atoms that surround the bond in question
Enthalpy of combustion definition ?
Enthalphy change when 1 mole of a substance is burnt completely in excess oxygen
Enthalpy of formation ?
Enthalphy change when 1 mole ofc compound is formed from its element in their standard states eg C(s) + 1/2O 2 (g) —> CO(g)
Enthalpy of neutralisation ?
Enthalphy change when 1 mole of water is for,ed from an acid reacting with a base
Enthalpy of reaction
Enthalphy change when reactants react in their molar ratio according to the equation eg 2C (s) + O2 (g) —> 2 CO(g)
Average bond enthalphy calculation
Change in enthalpy = sum of bond breaking - sum of bond forming
How do we calculate the energy released ?
• Q = M X C X change temperature • Work out moles of fuel = mass change / mr of fuel • (-Q/1000) ———— MOLES - KJ/degrees Celsius
During determination of enthalphy changes of combustion - possible errors which would give less exothermic change compared to theoretical values ?
• incomplete combustion - less efficient • some heat is transferred to copper pot/surrounding • energy used to evaporate water instead of increasing temperature • evaporation of fuel from the wick
What does a mass spectrum help identify ?
Structure of the molecule
What does fragmentation lead to ?
Fragmentation always results in the formation of a new ion and a neutral species
Write an equation for the formation of the fragment shown at m/z=29 on the mass spectrum of pentane
C5H12+ —-> C2H5+ + C3H7 (parent ion,fragment ion and the neutral would all be needed in the equation)
Define entropy
Measure of the disorder of a system - or measure of dispersal of energy in a system
Feasible reactions ?
*change H (negative) change S (positive) - change G would be always negative and reaction would be feasible *change H(positive) change S (negative) - change G always positive and reaction never feasible *change H(positive) change S (positive) - change G negative at high temp and feasible at high temp *change H(negative) change s (negative) - change G is negative at low temp and reaction is feasible at low temp
What are the limitation OF predictions of feasibility made using change G
*reaction may have high activation energy *rate of reaction may be very slow
For a reaction to occur spontaneously change G must be positive or negative?
Negative
Explain 2 ways increasing temp increases rate of reaction
*particek have more energy ,move faster , mine frequent and more collisions *more particles have at least the activation energy so more frequent successful collision
When is a reversible reaction in dynamic equlibrimum
*rate of forward reaction + rate of backward reaction *it is at constant temperature and pressure *closed system - nothing gets in or out
What is le chateliers principle
Whenever a dynamic equilibrium is disturbed ,it moves to oppose the disturbance (and equilibrium is restored)
Concentration - (increasing concentration explanation)