Module 3 (chapter 9 and 10) - enthalpy and catalysts Flashcards
enthalpy
(H) a measure of heat energy stored in a system
-a chemical system refers to the atoms, molecules or ions making up the chemicals
enthalpy changes
(delta H) the difference between the enthalpy of the reactants and the enthalpy of the products
-enthalpy of the products minus the enthalpy of reactants
heat
the process whereby thermal energy (J) is transferred from a hotter object to a cooler one
temperature
the direction of energy transferred is determined by the temperature of the objects (K or degrees)
system and surrounding
- system is the chemicals (reactants and products)
- surroundings are the apparatus and everything that isn’t in the chemical system
- the universe is everything it includes both the system and the surroundings
exothermic reactions
- where the enthalpy of the products is smaller than the enthalpy of the reactants due to heat loss to the surroundings
- reactants higher than products
- the energy lost by the system is balanced by the same energy gain from the surroundings
- temperature goes up
endothermic reactions
where the enthalpy of the products is greater than the enthalpy of the reactants de to heat being taken in from the surroundings
- temperature goes down
- any energy gain by the chemical system is balanced by the same energy loss by the surroundings
activation energy
the minimum amount of energy required for a reaction to take place by breaking chemical bonds in the reactants
-they determine how reactions happen, if there was no activation energy, every reaction would happen
average bond enthalpy
the average enthalpy change that takes place when breaking by homolytic fission 1 mol of a given type of bond in the molecules of a gaseous species
why is the true value for the energy change slightly different from there value calculate using bond enthalpies
we use average bond enthalpies and these vary in different environments
standard state
refers to the enthalpy change under standard conditions
-standard state refers to the physical state of a substance under standard conditions
standard conditions
- standard pressure (100kPa, 1atm)
- standard temperature (298K, 25 degrees)
- standard concentration (1moldm-3)
standard enthalpy change of reaction
the enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard contains with all reactants and products in their standard states
delta r H sigma
enthalpy change of formation
the enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions with all reactants and products in their standard states
-elements have an enthalpy change of formation of 0 as it is the formation of one mole of an element from its element (no change)
delta f H sigma
enthalpy change of combustion
the enthalpy change of combustion is the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions with all reactants and products in standard states.
delta c H sigma
-when a substance reacts completely with oxygen the products are the oxides of the elements in their substance
enthalpy change of neutralisation
the enthalpy change that accompanies the reaction of an acid by a base to form one mole of H20 (l) under standard conditions with all reactants and products in their standard states.
-involves the reaction of H+ with OH- to form one mole of H20 and so the value is the same for all neutralisation reactions
how do you calculate energy change
Q = m x c x delta T
- (m = mass of the surroundings (g) and is used to identify the materials that are changing temperature)
- (c = specific heat capacity changes for each substance but for water is 4.18 Jg-1K-1)
- (delta T = change in temperature by thermometer (final - initial)
specific heat capacity
the energy required to raise the temperature of 1g of a substance by 1K
how accurate is the experimental enthalpy change value?
- heat loss to the surroundings other than the water including the beaker but mainly the air surrounding and flame
- incomplete reaction (e.g. combustion of methanol to form carbon monoxide rather than dioxide)
- evaporation of methanol from the wick (burner must be weighed as soon as possible after extinguishing the flame, there’s usually a cover on the spirit burner to reduce this)
- non standard conditions (the conditions for this experiment are unlikely to be identical the the standard conditions)
how to measure enthalpy using spirit burners?
- measure out a set volume of water and pour into beaker, measure initial temp of this
- add methanol to spirit burner and weigh it
- light it, place the spirit burner under the water and stir with the thermometer
- after three minutes extinguish flame and record temp of water
- reweigh the spirit burner (assume the wick hasn’t been burnt)
what is a way of mitigating heat loss
use a plastic cup made from polystyrene, these are cheap, waterproof and light weight offering some insulation
what are the surroundings for enthalpy
- the solution itself is the immediate surroundings
- the chemical particles within the solution may react when they collide and any energy transfer is between the chemical particles and water molecules in the solution
- a thermometer will record any temperature change
averaged bond enthalpy
the energy requited to break one mole of a specified type of bond in gaseous molecule
- bond enthalpies are always endothermic and so have a positive energy value
- this is because energy is always required to break bonds
limitations of average bond enthalpies
- average bond enthalpy can very on the chemical environment of the bond.
- you will therefore often be given an average bond enthalpy rather than the actual bond enthalpy of an individual bond. this may lead to slight variations
enthalpy of bond making and bond breaking
bond breaking is endothermic (positive)
bond making is exothermic (negative)
how do you calculate enthalpy changes from average bond enthalpies
sum of bond enthalpies in reactants - sum of bond enthalpies in products
how to measure enthalpy changes
- using a calorimeter
- themometer
- bon enthalpies
Hess’s law
if a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same delta H (route A) = delta H (route B) - delta H (route C) -when doing it if it stays as it is, things cancel (e.g. oxygen in excess you can ignore them) -allows the determination if enthalpy changes indirectly, coming from the idea of conservation of energy with there being two routes to covert reactants into products
problems with hess’s law
- high activation energy
- a slow reaction rate
- more than one reaction taking place
indirect routes - enthalpy change of formation
delta reaction H = (sum of the enthalpy change of products) - (sum of enthalpy change of formation reactants)
indirect routes - enthalpy change of combustion
delta reaction H = (sum of the enthalpy change of reactants) - (sum of the enthalpy change of products)
rate of chemical reaction
how fast a reactant is being used up or how fast a product is being formed
-rate of reaction is the change in concentration of a reaction of product in a given time
(moldm-3s-1)
how does rate of reaction change
- fastest at the start of a reaction as each reactant is at its highest concentration
- the rate of reaction slows down ad the reaction proceeds because the reactants are being used up and their concentration decrease
- once one of the reactants has been used up, the concentrations stop changing and the rate of reaction is completely zero.
why are some collisions effective whilst others are not?
- the particles must collide with the correct orientation
- they must have sufficient energy to overcome the activation energy barrier of the reaction
how does increasing the concentration effect the reaction?
increasing the concentration increases the number of particles in the same volume
- the particles are closer together and therefore collide more frequently
- in a given period of time there will therefore be more effective collisions and an increased rate of reaction
how doe increasing the pressure of a gas affect the rate of reaction?
- when a gas is compressed into a smaller volume, the concentration of the gas molecules increases as the same number of gas molecules are in a smaller volume
- the gas molecules are closer together and collide more frequently leading to more effective collisions in the same time.
how do you monitor the progress of a reaction?
- monitoring the removal (decrease in concentration) of a reactant
- following the formation (increase in concentration) of a product
- this will depend on the properties and physical states of the reactants and products in the reaction
how do you measure reactions that produce gases?
- monitoring the volume of gas produced a regular time intervals using gas collection
- monitoring the loss of mass of reactants using a balance
- both are proportional to the change in concentration of reactants and products and so can give a measure of the rate of reaction
how do you monitor reactions using a balance
- the mass of the flash and contents is initially recorded and at regular time intervals
- the reaction is complete when no more gas is produced so no more mass is then lost
catalysts
- substance that changes the rate of reaction without undergoing any permanent change itself
- not used up during the reaction
- may react with a reactant to form an intermediate or may provide a surface on which the reaction can take place
- at the end the catalyst is regenerated
how does a catalyst work
-increases the rate of reaction by providing an alternate reaction pathway of lower activation energy
energy profile diagrams
- endothermic = products higher than reactants
- exothermic = reactants higher than products
homogeneous catalysts
- has the same physical state as the reactants
- the catalysts reacts to form an intermediate which then breaks down to give the products and regenerates the catalyst
examples of homogeneous catalysts
- making ester with sulphuric acid as a catalyst
- the reactants (ethanol and ethnic acid) and the catalyst are all liquids
- ozone depletion with chlorine radicals as catalysts
- the reactant (O3) and the catalyst (Cl.) are both gases
heterogeneous catalysts
- has a different physical state from the reactants
- usually solids in contact with gaseous reactants or reactants in solution
- reactant molecules are absorbed onto the surface of the catalyst where the reaction takes place. after reaction, the product molecules leave the surface of the catalyst by desorption
examples of heterogenous catalysts
- iron is a solid in the hater process (reactants are gases)
- nickel is a solid in the hydrogenation of alkene (reactants are gases)
catalysts -sustainability and economic importance
- reduces the temperature needed for the process and the energy requirements
- if a chemical process requires less energy, then less electricity of fossil fuel is used
- making the product faster and using less energy can cut costs and increase profitability
- the economic advantages outweigh any costs associated with developing a catalytic process
- the modern focus on sustainability requires industry to operate processes with high atom economies and fewer pollutants
- using less fossil fuels will cut carbon dioxide emissions, mitigating against global warming
particles energy
- some molecules more slowly with low energy, other faster with higher energy
- the spread of molecules energy is known as the Bolzmann distribution
features of a Boltzmann distribution
- no molecules have zero energy (the curve starts at the origin)
- the area under the curve is equal to the total number of molecules
- there is no maximum energy for a molecule (the curve doesn’t meet the x axis at high energy, it would need to reach infinite energy to do s0)
- y axis is number of molecules with a given energy
- x axis is energy
Boltzmann distribution and higher temperatures
- more molecules have an energy greater than or equal to the activation energy
- therefore a greater proportion of collisions will lead to a reaction, increasing the rate of reaction
- collisions will also be more frequent as the molecules are moving faster, but the increased energy of the molecules is much more important than the increased frequency of collisions
- the peak is lower and shifted to the right meaning a greater proportion of molecules can overcome the activation energy
the Bolzmann distribution and catalysts
-activation energy shifts left meaning a greater proportion of molecules exceed the new lower activation energy
reversible reactions
- reactions that take place in both the forwards and reverse directions
- e.g. haber process
dynamic equilibrium
- the rate of the forwards reaction is equal to the rate of the reverse reaction
- the concentrations of the reactants and products do not change
- must be a closed system, isolated from the surroundings meaning components aren’t effected from outside influences
le Chatelier’s principal
- the position of the equilibrium indicates the extent of the reaction
- states that when a system in equilibrium is subjected to an external change the system readjusts itself to minimise the effect of the change
how does concentration effect equilibrium?
- if there are more products formed, the position of the equilibrium will shift right
- if there are more reactants formed the position of the equilibrium will shift left
investigating changes in equilibrium concentration
chromate ions are yellow and dichromate are orange
- add yellow potassium chromate to a beaker
- add dilute sulphuric acid dropwise until there is no fitter change (solution turns orange)
- add aq sodium hydroxide until there is no further change in colour (solution goes back to yellow)
how does the concentration investigation work?
- when you add sulphuric acid you are increasing the concentration of H+ ions. This increases the rate of the forwards reaction and so causes the position of equilibrium to shift right to minimising the H+ concentration
- the equation shifts right making more products turning solution orange
- when sodium hydroxide added, OH- ions react with H+ ions decreasing concentration of H+ ions
- this shift increases the concentration of the reactant that has been removed, H+
- this decreases the rate the forwards reaction causing the equilibrium to shift left, turning it yellow
effect of pressure on equilibrium
- only if there are more gases molecules on one side of the equation than on the other
- pressure is proportional to concentration
- increasing pressure will shift the equilibrium to the side with the fewer molecules, reducing the pressure of the system
- decreasing pressure shifts the position in the opposite direction
effect of catalyst on equilibrium
- does not change the posit of equilibrium
- it speeds up both reactions equally
- it will increase the rate at which an equilibrium is established
the equilibrium law
-used to calculate the exact position of equilibrium
-equilibrium constant = Kc
Kc = [C]c [D]d / [A]a [B]b
(products/reactants)
-square brackets are short for concentration of
-abcd are balancing numbers in the overall equation
-[A] are the equilibrium concentrations of the reactants and products of this equilibrium
what does Kc tell us?
- a Kc value of 1 indicates a position of equilibrium that is halfway between reactants and products
- a Kc value > 1 indicates a position of equilibrium that is towards the products
- a Kc value < 1 indicates a position of equilibrium that is towards the reactants
- larger Kc the further right the equilibrium
effect of temperature of equilibrium
- increasing temperature shifts the equilibrium positions in the endothermic direction
- decreasing temperature shifts it in the exothermic direction
how do you investigate the effects of temperature on equilibrium?
- dissolve cobalt chloride in water in a boiling tube. add a small quantity of HCl and place in iced water.
- solution is pink at this point
- set up water bath and place the boiling tube into the boiling water and the solution turns blue
- transfer boiling tube back into iced water and solution turns pink
how does the investigation of temperature of equilibrium work?
cobalt chloride (blue) dissolves in water to form a pink solution and the forwards reaction
- [Co(H20)6]2+ + 4Cl- –> [CoCl4]2- + 6H20
- forwards reaction is endothermic so the position shifts to the right to take heat energy in and minimise the increase in temperature
- this turns the solution blue
- decreasing temperature shifts the position of equilibrium ledt to give out energy