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
difference between homogenous and heterogenous catalysts
homogenous catalysts are in the same phase as reactants
heterogenous catalysts are in different phase to reactants
4 benefits of using catalysts
- allows for less extreme conditions which saves money
- also saves energy, resulting in fewer CO2 emissions from burning fossil fuels
- enables different mechanisms to be used with better atom economy + reduced waste + less use of hazardous reactants
- many can be enzymes, which operate effectively close to room temperature and pressure
example of a reaction with a homogenous catalyst
CH3OH(aq) + CH3COOH(aq)»_space; CH3COOCH3(aq) + H2O(l)
catalyst = conc H2SO4(aq)
example of a reaction with a heterogenous catalyst
haber process
H2(g) + 3H2(g)»_space; 2NH3(g)
catalyst = iron (s)
dynamic equilibrium definition
this occurs when a reversible reaction takes place in a closed system (or reactions in solution), when the forwards and backwards reactions are occurring at the same rate, so product and reactant concentrations remain constant
products and reactants constantly reacting together, so dynamic
equilibrium position definition
refers to the relative amount of product and reactant in a reaction mixture
what is le chatilier’s principle
if a change is made to a system in dynamic equilibrium, the position of equilibrium will move to counteract this change
what 3 factors influence the position of equilibrium
- pressure
- concentration
- temperature
how does concentration affect equilibrium position
if the concentration of reactants/products increases, the position of equilibrium will move to favour the other side
if the concentration of reactants/products decreases, the position of equilibrium will move to favour that side
how does pressure affect equilibrium position
when pressure increases, position of equilibrium shifts to the side with less gaseous moles
when pressure decreases, position of equilibrium moves to favour the side with more gaseous moles
how does temperature affect equilibrium position
when temperature increases, position of equilibrium moves to favour the endothermic reaction
when temperature decreases, position of equilibrium moves to favour exothermic reaction
what is the effect of catalysts on dynamic equilibrium
catalytsts increase the rates of both the forwards and backwards reactions equally, so they have no effect on equilibrium position, although they can cause equilibrium to be reached faster
describe the process by which equilibrium can be formed for a reaction in a closed system
- at the start, reactant concentration is high so the position of equilibrium moves to the right, favouring forward reaction
- as the reaction progresses, products concentration increases and products start reacting to reform reactants
- the rate of reaction of reactants decreases as reactant conc decreases, and the rate of reaction of the products increases as concentration increases
- eventually the rates of the forwards and backwards reaction will be equal, and the concentrations of reactants and products will be constant
what conditions are used in the haber process and why
pressure = 200atm
high pressures move equilibrium position to side with less moles, increasing yield of ammonia, very high pressures are expensive/energy intensive to produce- compromise
temperature = 400C
forward reaction is exothermic, favoured by equilibrium at low temps, but this decreases reaction rate, so equilibrium wouldn’t be reached- compromise
catalyst = iron catalyst speeds up reaction rate
what 2 other features are used to improve yields, costs and sustainability in the haber process
- a heat exchanger warms incoming gas mixture to maintain an optimal temp so reaction rate is high, can also remove excess heat to help keep NH3 yield high
- excess/reproduced H2 and N2 can be re-reacted to make more NH3, so high yield of ammonia
substances in what states should be included in the kc expression
(g) + (l) + (aq) are included
solids (s) should be ignored
what does it mean if kc»1
equilibrium lies to the RHS so reaction mixture is mostly products
what does it mean if kc«1
equilibrium lies to the LHS so reaction mixture is mostly reactants
what does it mean if kc is close to 1
there is a similar concentration of both reactants and products in the reaction mixture, equilibrium is somewhere in the middle
