Module 3 Section 2: Physical Chemistry Flashcards
What is enthalpy change
Enthalpy change ΔH, is the heat energy transferred in a reaction at constant pressure
The units of ΔH are kJ mol-1
What does writing ΔH mean
Shows that the measurements were made under standard conditions and that the elements were in their standard states (physical states under standard conditions)
What are the standard conditions
100kPa (about 1atm) pressure
298K (25°C)
What happens in Exothermic reactions
Reaction gives out energy
ΔH is negative
Examples are combustions of fuels and oxidation of carbohydrates
What happens in endothermic reactions
Reaction absorbs energy
ΔH is positive
Examples are thermal decompositions and photosynthesis
What do enthalpy profile diagrams show
Shows how the enthalpy (energy) changes during reactions
What is the activation energy
Symbol is Ea
It’s the minimum amount of energy needed to begin breaking reactant bonds and start a chemical reaction
What do enthalpy profiles looks like for exothermic and endothermic reactions
Endothermic reaction:
Reactants are on a lower enthalpy than products
Exothermic reaction:
Reactants are on a higher enthalpy than products
Why must you know the specific conditions for enthalpy changes
You can’t directly measure the actual enthalpy of a system
You only need to know the enthalpy change
This can be found by experiment or in data books
Enthalpy changes in data books are usually standard enthalpy changes, done under standard conditions
Enthalpy changes are affected by temperature and pressure
What are the different enthalpy changes
Standard enthalpy change of reaction, ΔrH
Standard enthalpy change of formation, ΔfH
Standard enthalpy change of combustion, ΔcH
Standard enthalpy change of neutralisation, ΔneutH
How does enthalpy change for making and breaking bonds
Energy is needed to break bonds so this is endothermic (ΔH is positive)
Energy is released when bonds are formed so this is exothermic (ΔH is negative)
Enthalpy change for a reaction is the overall effect of these two changes
If more energy is needed to break bonds than is released when bonds are made, ΔH is positive
If you need less, ΔH is negative
What is bond dissociation enthalpy
The energy needed to break oppositely charged ions apart in ionic compounds
The energy needed to separate positive nuclei to the negative charge of the shared electrons
The bond dissociation enthalpy is the amount of energy you need per mole
Always involve bond breaking in gaseous compounds
What is average bond enthalpy
The energy needed to break one mole of bonds in the gas phase, averaged over many different compounds
Why are average bond enthalpies not exact
It’s an average for a much bigger range of molecules
E.g. O-H average bond enthalpy isn’t just for water, it includes alcohols and carboxylic acids
How to investigate enthalpy changes in a lab
Must know:
The number of moles for the chemical that’s reacting
The change in temperature
For exothermic reactions:
E.g. to find the enthalpy for a combustion, you burn it
The burning fuel heats water
Can work out the heat absorbed by water if you know the mass of water, the temperature change of the water, and the specific heat capacity of water
Ideally all the heat released by the fuel is absorbed by water but some is always lost
This is calorimetry
What is calorimetry
Can be used to calculate an enthalpy change for a reaction that happens in a solution, such as neutralisation or displacement
What does the equation q=mcΔT stand for
q= heat lost of gained (in joules). Same as enthalpy change is pressure is constant
m= mass of water in the calorimeter, or solution in the insulated container (g)
c= specific heat capacity of water (4.18 J g-1 K-1)
ΔT= change in temperature of the water solution (K)
In a laboratory experiment, 1.16g of organic liquid fuel was burned
The heat released raised the temperature of 100g or water from 295.3K to 357.8K
Calculate the standard enthalpy of combustion of the fuel
It’s Mr is 58.0
Calculate amount of heat in joules
q=mcΔT
q= 100 x 4.18 x (357.8-295.3) = 26,125J (26.125 kJ)
Standard enthalpy of combustion involves 1 mole of fuel, so find out how many moles of fuel produced this heat
n = 1.16/58 = 0.02 mole of fuels
Heat produced by 1 mole of fuel = -26.125/0.02 = -1310 kJ mol-1
What happens in calorimetry for neutralisation reactions
For a neutralisation reaction, known quantities of acid and alkali are combined in an insulated container, and the temperature change is measured
What is the standard enthalpy change of reaction
The enthalpy change for a reaction with the quantities shown in the balanced equation
The value should be quoted along with the equation
What are the standard conditions for enthalpy change
Standard pressure: 100kPa
Standard temperature: 298K
Standard concentration: 1 mol dm-3
Standard state: physical state of a substance under standard conditions
What is the standard enthalpy change of formation
The enthalpy change when one mole of a substance is formed from its elements in their standard states under standard conditions of 298K and 100 kPa
Make sure 1 mole of substance in equation
Standard enthalpy change of combustion
The enthalpy change when one mole of a substance is completely burned in excess oxygen under standard conditions of 100kPa and 298K
Make sure 1 mole of substance in equation
Standard enthalpy change of neutralisation
Enthalpy change when one mole of water is formed from the reaction of an acid with a base or when 1 mole of water is formed from reaction H+(aq) + OH-(aq) = H2O(l) under standard conditions of 298K and 100kPa
Make sure 1 mole of water in equation
Why is the enthalpy change of neutralisation always -57 kJ mol-1
Same reaction occurring each in terms of ions
Always H+(aq) + OH-(aq) = H2O(l)
What is Hess’ law
If a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy is the same for each route
How can Hess’ law be used to calculate enthalpy change
The two routes from the reactants to products are:
Route 1: A + B arrows
Route 2: C arrow
By Hess’ law the total enthalpy change for both routes is the same
A + B = C
Equation to find enthalpy change of formation
ΔrH = Σ ΔfH products - Σ ΔfH reactants
Calculate the ΔH for ZnCO3 = ZnO + CO2
ΔfH: ZnCO3: -812, ZnO: -348, CO2: -393
ΔrH = Σ ΔfH products - Σ ΔfH reactants
Σ ΔfH products: (1x-348) + (1x-393) = -741
Σ ΔfH reactants: (1x-812) = -812
-741 - -812 = 71kJ mol-1
Endothermic
Calculate the ΔH for 2CO + O2 = 2CO2
ΔfH: CO: -111, CO2: -393
ΔrH = Σ ΔfH products - Σ ΔfH reactants
Σ ΔfH products: (2 x -393) = -786
Σ ΔfH reactants: (2 x -111) = -222
-786 - -222 = -564 kJ mol-1
Exothermic
Equation for enthalpy change of combustion
ΔrH = Σ ΔfH reactants - Σ ΔfH products
How to work out missing bond enthalpy
Write out enthalpy arrow diagram, e.g. formation where arrows go into raw elements
Work out total enthalpy of each arrow including X
Using Hess’ law: X + other bond enthalpies on that side = other arrows
Rearrange to find X
Where do the arrows point in enthalpy diagrams for different enthalpy changes
Formation: arrows down into elements away from equation
Combustion: arrows down into CO2 and H2O
What does the rate of reaction measure
Rate of chemical reaction measures how fast a reactant is being used up or how fast a product is formed
Definition of rate of reaction
The change in concentration of a reactant or a product in a given time
How to work out rate of reaction
Rate (mol dm-3 s-1) = change in concentration(mol dm-3) / s
How to find rate of reaction on a graph
Draw tangent and find gradient of this to find rate
When is the rate of reaction at its fastest
The rate of a reaction is fastest at the start of the reaction, as each reactant is at its highest concentration, line is steepest here
Why does the rate of reaction decrease over time
Rate of reaction slows down as the reaction proceeds, because there reactants are being used up and their concentration decreases (meaning less frequent collisions), this is shown by the line becoming less steep and starting to curve
Why does the rate of reaction graph level out
Once one of the reactants has been completely used up, the concentrations stop changing and the rate of reaction is 0, the line plateaus
What factors affect the rate of reaction
Concentration (or pressure when reactants are gases)
Temperature
Use of a catalyst
Surface area of solid reactants
What is collision theory
The collision theory states that two reacting particles must collide for a reaction to occur
What is an effective collision
A collision that leads to a chemical reaction
What leads to an effective collision
The particles collide with the correct orientation
The particles have sufficient energy to overcome the activation energy barrier of the reaction
How does concentration affect rate of reaction
Increasing the concentration increases the overall collision frequency as there’s more particles per unit volume
Therefore in a given period of time there will be more effective collisions and an increased rate of reaction
How does pressure affect rate of reaction for gases
Keeping a gas at a lower volume the pressure increases, therefore the rate of reaction increases
Concentration of the gas molecules increases as the same number of gas molecules occupy a smaller volume
Gas molecules are close together so will collide more frequency
How to measure rate of reaction
Monitoring decrease in concentration of reactant
Following increase in concentration of a product
What methods are used to measure the rate of reaction with reactions that produce gas
Monitoring the volume of gas produced at regular time intervals using gas collection
Monitoring the loss of mass of reactants using a balance
Volume of gas produced and mass loss are both proportional to the change in concentration of a reactant or product
So the change in volume with time or the mass loss with time both give a measure of the rate of reaction
Practicals methods to measure the rate of reaction with gas
Inverted measuring cylinder filled with water
Gas syringe
Monitoring mass loss on a balance
Advantages and disadvantages of inverted measuring cylinder method
Easier to read than gas syringe as it slower moving
Lower precision
Advantages and disadvantages of gas syringe method
Precise measurements
Gas syringe can get stuck
Advantages and disadvantages of mass balance method
Easy to set up and losing gas is no problem
CO2 may dissolve back into the solution
What is activation energy for collision theory
Minimum amount of kinetic energy particles need to react
Particles need this much energy to break bonds to start the reaction
How does activation affect how easily reactions occur
Reactions with low activation energies often happen easily
Whereas reactions with high activation energies occur less easily
What does a Boltzmann distribution graph show
Shows the different proportions of kinetic energy of particles
Most particles moving at a moderate speed so the frequency of them is highest
Can mark activation energy to show how some particles will not have enough energy
How does temperature affect the rate of reaction
Increasing temperature means particles have more kinetic energy and will move faster meaning a higher collision frequency
This means a greater proportion of molecules will have at least the activation energy and be able to react
What are catalysts
A catalyst increases the rate of a reaction by providing an alternative reaction pathway with a lower activation energy
The catalyst is chemically unchanged at the end of the reaction
How many different chemical reactions does a catalyst work for
Many catalysts usually only work on a single reaction
How do catalysts affect a Boltzmann distribution
The line for activation energy moves to the left towards the majority of particles
How do catalysts affect the enthalpy profile
The curve for the activation energy is reduced with the catalyst
What are heterogenous catalysts
This catalyst is in a different physical state from the reactants
The reaction happens on the surface of the heterogenous catalyst
Usually solids in contact with gaseous reactants or aqueous reactants
What happens on a heterogenous catalyst
Reactant molecules are adsorbed (weakly bonded) onto surface of catalyst, where the reaction takes place
The reactant molecules leave the surface of the catalyst by desorption
What are homogeneous catalysts
Homogenous catalysts are in the same physical state as the reactants
Usually a homogenous catalyst is an aqueous catalyst for a reaction between two aqueous solutions
What happens on a homogenous catalyst
A homogenous catalyst works by forming an intermediate species
The reactants combine with the catalyst to make an intermediate species
This then reacts to form the products and reform the catalyst
How are catalysts good for industries
Catalysts lower production costs
Provide more product in a shorter time
Help make better products
Example of a catalyst used in industry
Iron is used as a catalyst in ammonia production
Without iron, the temperature would have to be raised to make the reaction happen quick enough
This would cost more and reduce the yield of ammonia
How can catalysts change the properties of a product
E.g. poly(ethene)
Properties of poly(ethene) without a catalyst: less dense, less rigid
Properties of poly(ethene) made with a catalyst: more dense, more rigid, higher melting point
How do catalysts benefit environmental sustainability
Using catalysts means that lower temperatures and pressures can be used
Energy is saved so less CO2 is released and fossil fuel reserves are preserved
Catalysts reduce wastes by allowing a different reaction to be used with a better atom economy
How can catalytic converters act as environmentally beneficial catalysts
Catalytic converters on cars are made from alloys of platinum and palladium, they reduce the pollution released into the atmosphere by speeding up the reaction:
2CO + 2NO -> 2CO2 + N2
Features of a Boltzmann distribution
No molecules have 0 energy - the curve starts at the origin
The area under the curve is equal to the number of molecules
There is no maximum energy for a molecule - the curve does not meet the x-axis at high energy (curve would need to reach infinite energy to meet the x-axis)
Effect of increasing temperature on the Boltzmann Distribution
More molecules have an energy greater than or equal to the activation energy
Greater proportion of collisions will lead to a reaction, increasing the rate of reaction
Frequency of collisions will be increased as molecules are moving faster, but increased energy of molecules is more significant
Effect of catalysts on the Boltzmann Distribution
Catalysts provide an alternative route with a lower activation energy
Greater proportion of molecules will have an energy greater or equal to the activation energy with the catalyst
Activation energy with catalyst moves to the left of the curve where there’s a greater proportion of molecules
What is a reversible reaction
A reaction that can go in both ways
A + B ⇌ C
A + B = C
C = A + B
How does a reaction reach dynamic equilibrium
As the reactants get used up the forwards reaction slows down and as more product is formed, the reverse reaction speeds up to reform reactants
These reactions can continue until the forwards reaction and reverse reaction are occurring at the same rate
The amounts of reactants and products won’t be changing any more so their concentrations will stay the same
What is dynamic equilibrium
A reversible reaction where the forwards and reverse reactions are occurring at the same rate in a closed system
What is a closed system
No substances can get in or out of where the reaction is taking place
What happens when you change the concentration, pressure or temperature of a reaction at equilibrium
The position of equilibrium will shift
It will produce different amounts of reactants and products at equilibrium
Depending on which way the position of equilibrium shifts, what happens to the amounts of reactants and products
Position of equilibrium moves to the right:
More products produced
Position of equilibrium moves to the left:
More reactants are produced
Le Chatelier’s principle
If there’s a chance in concentration, pressure or temperature, the equilibrium will move to help counteract the change
What happens to a reaction at equilibrium when the concentration is changed
Increasing the concentration of a reactant will result in the equilibrium trying to reduce the amount of the extra reactant
It makes more product and shifts position of equilibrium to the right
Increasing the concentration of a product means that the equilibrium will increase the rate of the reverse reaction to reduce the amount of extra product
Equilibrium shifts to the left
What happens to a reaction at equilibrium when the temperature is changed
Increasing the temperature means that the product will shift in the endothermic direction to absorb the heat that has been added
Decreasing the temperature means that the equilibrium shifts in the exothermic direction to replace the heat
What happens to a reaction at equilibrium when the pressure is changed
Increasing the pressure shifts the equilibrium to the side with fewer gas molecules to reduce the pressure
Decreasing the pressure shifts the equilibrium to the side with more gas molecules which increases the pressure again
This only applies to gases
What effect do catalysts do to the position of equilibrium
Catalysts have no effect on the position of equilibrium
They speed up the forward and reverse reactions by the aame amount
Can’t increase yield but means that equilibrium is reached faster
What does the equilibrium constant show
Shows how far to the left or right the equilibrium is
Expression for the equilibrium constant
General reaction: aA + bB ⇌ dD + eE
Kc = [D]^d [E]^e / [A]^a [B]^b
Remember [ ]
Putting the equation H2(g) + I2 (g) ⇌ 2HI (g) into equilibrium constant and finding Kc with correct units
HI = 0.8moldm-3, H2 = 0.1moldm-3, I2 = 0.1moldm-3
Kc = [HI(g)]^2 / [H2(g)]^1 [I2(g)]^1
Kc = 0.80^2 / 0.10 x 0.10 = 64
Units: (mol dm-3)^2 / (mol dm-3) (mol dm-3)
All units cancel out so there are no units
Kc = 64 no units
How to estimate the position of equilibrium using Kc
The larger the value of Kc is, the further to the right equilibrium is and the more products there are relative to reactants
The smaller the value of Kc the further to the left equilibrium lies and the more reactants there are relative to product
Why is the value from enthalpy change experiments never accurate
Heat is released to the surroundings
Incomplete combustion
Non standard conditions
