enthalpy, reaction rates and equilibria Flashcards
enthalpy definition
the total internal energy inside a chemical system
- this includes thermal and chemical energy
chemical system definition
all the atoms, ions and molecules that make up all the chemicals in a space
why does the enthalpy of a chemical system change during reactions
when a reaction takes place, energy is transferred between the system and its environment, causing enthalpy changes
exothermic definition
when products have less energy than reactants, therefore energy is given off in the form of heat (this can be measured with a thermometer)
- overall enthalpy decrease
endothermic definition
when products have more energy than reactants so energy is taken into system in the form of heat
- overall enthalpy increase
what are standard conditions
-temp
-pressure
-concentration
-state
temp - 298K / 25 C
pressure - 100 kPa
concentration - 1 moldm-3
state - the physical state of a substance under standard conditions
enthalpy change of formation definition
the enthalpy change that occurs when 1 mole of a substance is formed from its raw elements under standard conditions and states
remember the enthalpy change of formation of a raw element is always 0
enthalpy change of combustion definition
the enthalpy change that occurs when 1 mole of a substance burns completely in excess oxygen under standard conditions
enthalpy change of neutralisation definition
the enthalpy change that occurs when 1 mole of water is formed in a reaction between an acid and a base under standard conditions
what are the units for enthalpy change
kJmol-1
activation energy definition
the minimum amount of energy needed for a reaction to take place
enthalpy change formula
= H(products) - H(reactants)
bond enthalpy definition
(also known as bond energy/bond dissociation energy)
the amount of energy needed to break and separate 1 mole of a specific bond in gaseous molecules so that the resulting gaseous particles exert no force upon each other, under standard conditions
what is the difference between bond enthalpy and average bond enthalpy
bond energies are affected by other atoms in the molecule
average bond enthalpy is an average for many bonds taken from a wide range of compounds containing the bond
e.g. an O-H bond in water will have a different bond enthalpy to an O-H bond in methanol
why might listed average bond enthalpy for a specific bond differ from the average measured bond enthalpy of a specific bond in a specific molecule
the listed figure is an average taken from many compounds with that specific bond
the measured value is specific to the bond within a specific molecule, although this is also an average of breaking many of this type of bond
how do you determine bond enthalpies
bond enthalpies cannot be determined directly so enthalpy cycles are used to calculate the average
bond breaking = exothermic (-) as energy is needed to break bonds
bond forming = endothermic (+) as energy is released when they are made so
enthalpy change of a reaction = (+ enthalpy of bonds broken) + (- enthalpy of bonds formed)
why might calculations for the same bond enthalpy from 2 different reactions produce different answers
the other bond enthalpies are averages so they will differ from the actual enthalpies involved in the reactions
why does enthalpy change of combustion of alkanes become more exothermic as chain length increases
as chain length increases there are more atoms and bonds burned so more energy is released, so more exothermic
what is hess’ law
the idea that products can be formed directly from raw elements or indirectly from raw elements
outline collision theory
collision theory states that for a chemical reaction to take place, the particles need to collide with each other in the correct orientation AND with sufficient energy
- ineffective collisions occur when particles collide in the wrong orientation or without enough energy, bouncing off each other without causing a chemical reaction
collision frequency definition
the number of collisions per unit of time
what is the relationship between collision frequency and reaction rate
as collision frequency increases, the number of particles with energy greater than the Ea increases, so reaction rate increases
3 factors that affect the rate of reaction and how
concentration
high conc = large no. of particles = high collision freq, etc
temperature
high temps = lots of movement = high collision freq, etc
pressure
high pressure = less space between molecules = high collision freq, etc
catalyst definition
a substance which increases the rate of a reaction by facilitating an alternative mechanism with a lower activation energy, without being used up in the process of the reaction