Module 3: Section 2 - Physical Chemistry Flashcards
Enthalpy changes
Definition of enthalpy change AH (delta H)
Energy transferred between a system and its surroundings at a constant pressure and final temperature is the same as the start temperature
Enthalpy changes
What are the units for enthalpy change
kJ mol ^-1
Enthalpy changes
What is a system
Chemical or chemicals being studied and if system is closed there is no exchange between the system and the surroundings
Enthalpy changes
What does standard enthalpy changes refer to
Reactions done under standard conditions and with everything in their standard stats
Enthalpy changes
What are standard conditions
298 K (25*C)
Pressure of 1atm (100kPa)
Where solutions involved have concentration of 1 mol dm-3
Enthalpy changes
What is exothermic change
Give out heat to surroundings
Enthalpy changes
Examples of exothermic changes
Combustion
Respiration
Hot packs
Enthalpy changes
Is delta H positive or negative for exothermic changes
Negative as the products are at a lower energy level than reactants
Enthalpy changes
On an enthalpy profile diagram for exothermic changes where are the products
Products are below the reactants
Enthalpy changes
What are endothermic changes
Take in energy from the surroundings
Enthalpy changes
Examples of endothermic changes
Melting and vaporisation
Photosynthesis
Enthalpy changes
Is delta H popsoitve or negative for endothermic changes
Positive as products have more energy than the reactants
Enthalpy changes
In and energy profile diagram are the products above or below
Products above reactants
Enthalpy changes
What must happen for a reaction to occur
Bonds must be broken and new ones formed so energy must be supplied to start both endothermic and exothermic reactions
Enthalpy changes
What is activation energy Ea
Minimum amount of energy needed to begin breaking reactants bonds and start a chemical reaction
Enthalpy changes
Units for activation energy Ea
kJ mol-1
Enthalpy changes
How is the symbol of delta H modified fr standard enthalpy change
A H °298(small)
Delta symbol triangle then capital H then degree sign with lien through horizontally then small 298 at bottom
Enthalpy changes
What is the enthalpy change of a reaction
Enthalpy change associated with a stated reaction
Enthalpy changes
Sign for enthalpy change of reaction
AH r
Delta symbol then capital H them small r
Enthalpy changes
What is enthalpy change of combustion
Enthalpy change when one mole of a substance undergoes complete combustion
Enthalpy changes
Sign for enthalpy change of combustion
A H c
Delta sign then capital H the small c
Enthalpy changes
What are enthalpy change of combustion always
Always exothermic so negative delta H
Enthalpy changes
Example of enthalpy change of combustion for ethanol
C2H5OH (l) 3 O2 (g) —> 2 CO2 (g) + 3 H2O (l)
Enthalpy changes
Important for equation of enthalpy change of combustion
Include state symbols, always balanced, COMPLETE combustion
Enthalpy changes
What is the enthalpy change of formation
Enthalpy change when one mole of a compound is formed from its elements
Enthalpy changes
Sign for enthalpy change of formation
A H f
Delta sign then capital H then small f
Enthalpy changes
What is the reaction for enthalpy cage of formation usually
Usually but not exclusively exothermic
Enthalpy changes
Equation for enthalpy of formation
Elements have to be in standard states which have 0 enthalpy
Include state symbols
Carbon usually taken as graphite
Enthalpy changes what is enthalpy change of neutralisation
Enthalpy change when one mole of water is formed from its ions in dilute solution
Enthalpy changes
What type of reaction is enthalpy of neutralisation
Exothermic so negative delta H
Enthalpy changes
Equation for enthalpy of neutralisation
H+ (aq) + OH- (aq) —> H2O (l)
Enthalpy changes
What value is obtained when strong acids react with strong alkalis
-57 kJ mol-1
Calorimetry
What is calorimetry
Practical determination of enthalpy changes usually involving heating or cooling known amounts of water
Calorimetry
What reaction is it when water is heated up in calorimetry
Exothermic
Calorimetry
What reaction is it when water cooled as down in calorimetry
Endothermic
Calorimetry
What equation can be used to calculate energy required to change the temperature of a substance
q = m x c x AT
Calorimetry
What is q in q = m x c x AT
Heat energy/ energy transferred in kJ
Calorimetry
What is m in q = m x c x AT
Mass of solution in kg
Calorimetry
What is c in q = m x c x AT
Specific heat capacity in kJ K-1 kg-1
Calorimetry
What is AT in q = m x c x AT
Change in temperature in K
Calorimetry
What is specific heat capacity of water
4.18 kJ K-1 kg-1
Calorimetry
Equation to calculate enthalpy change
AH =q / moles in reaction
Calorimetry
What is AH in AH = q/moles in reaction
Enthalpy change in kJ mol-1
Calorimetry
What is q in AH = q / moles in reaction
Energy transfer in J
Calorimetry
What is the graphical method to calculate a value of AT (change in temp)
Temperature taken every half minute before mixing the reactants
Reactants mixed after 3 mins
Further reading taken every half minute as reaction mixture cools
Extrapolate the lines and calculate value
Bond enthalpy
What is average bond enthalpy
Energy required to break one mole of a specified type of bond
In a gaseous molecule
Bond enthalpy
What is always required to break bond and what are bond enthalpies always
Energy and always endothermic with a positive enthalpy value
Bond enthalpy
Example. Of bond dissociation ethalpy Cl
Cl2 (g) —> 2 Cl (g)
Bond enthalpy
What does the strength of a bond depend on
It’s environment so mean values are quoted
Bond enthalpy
What type of reaction is making a bond
Exothermic
Bond enthalpy
What does a smaller bond enthalpy mean
Weaker bond so easier to break
Bond enthalpy
What type of reaction is breaking a bond
Endothermic
Bond enthalpy
What determines what type of reaction the overall reaction is
The difference between the energy required for bond breaking and the energy released by a bond making
Hess law
What is Hess Law used for
Determining enthalpy change of difficult reactions
Hess law
What does Hess law state
Enthalpy change is independent of route taken
Hess law
What are diagrams that illustrate Hess law called
Enthalpy cycle
Bond enthalpy
How do you do a calculation for overall change
Using given data workout out energy required to break bonds of reactants and energy required to make bonds in products
Calculate break-make and see if positive or negative value
Hess law
What is it used for calculating
Enthalpy change of reaction
Enthalpy change of combustion
Enthalpy change of formation
Hess law
If given enthalpy of formation what put at bottom and where do the arrows go
Put elements at bottom and arrow upwards away from elements
Hess law
If given enthalpy of combustion, what put at bottom and where do arrows go
Oxides
Down towards oxides
Hess law
If given bond enthalpy what put at bottom and where do arrows go
Gaseous atom
Down towards gaseous atoms
Hess law
If going against arrow what do u do
Minus it
Rates of reaction
What is the basis of rates
The collision theory
Rates of reaction
How must particles collide for a reaction to happen
Must collide in the right direction
Collide with at least a certain amount of kinetic energy to over c0me the activation energy
Rates of reaction
What is the activation energy
Minimum amount of kinetic energy particles need to react as is needed to brews the bonds to start the reaction
Rates of reaction
Distribution of kinetic energy between molecules
Dont all have the same amount
Some have lots and move quickly
Some not about and move slower
Most are somewhere in between
Rates of reaction
What factors affect rate of reaction
Temperature
Concentration pressure
Surface area
Catalysts
Rates of reaction
What happens to rate of reaction as increase temperatures
Rate of reaction increases
Rates of reaction
Why does increasing temperature increase rate of reaction
Particles will have more kinetic energy and move faster, be able to get over activation energy and react
Also moving faster so more collisions will occur
Rates of reaction
Affect of concentration on rate of reaction
Increasing concentration increases rate of reaction
Rates of reaction
Why does increasing concentration increase the rate of reaction
More particles in a set volume so will collide more frequently therefore more chance to react
Rates of reaction
Impact of pressure on rate of reaction
Increasing pressure increases rate of reaction
Rates of reaction
Why does increasing pressure increase rate of reaction
More gas particles in set volume so more frequent and successful collisions
Rates of reaction
Impact of catalysts on rate of reaction
Catalysts increase rate of reaction
Rates of reaction
Why can catalysts spoked up a reactions
Lowers the activation energy by providing a alternate pathways therefore more particles will have enough energy to react
Rates of reaction
Impact of surface areas on rate of reaction
Larger surface area increases rate of reaction
Rates of reaction
Why does increasing surface area increase rate of reaction
More area from them particles to collide with
Rates of reaction
Definition for catalyst
Increases the rate of reaction by providing an alternative reaction pathways with a lower activation energy and is chemically unchanged at the end of the reaction
Rates of reaction
On an energy profile what is the lowered activation energy after a catalysts labelled as
Ec
Rates of reaction
What two things can catalysts be
Homogeneous
Heterogeneous
Rates of reaction
What is a heterogenous catalyst
One that is in a different physical state from the reactants
Rates of reaction
Where does the reaction happen for heterogeneous catalysts
On the surface of the catalyst
Rates of reaction
What does increasing the surface area of a heterogeneous catalyst do
Increase the rate of reaction because the number of molecules that can react at the same time increases
Rates of reaction
What are homogeneous catalysts
In the same physical state as the reactant
Rates of reaction
What are homogeneous catalysts usually used for
An aqueous catalyst for the reaction between two aqueous solutions
Rates of reaction
How does a homogeneous catalyst work
Reactants combine with the catalyst to make an intermediate species, which then react to form the products and reform the catalyst
Rates of reaction
Why are catalysts good for industries
Dramatically lower production costs, give more product in a shorter time and help make better products
Rates of reaction
Example of iron used as a catalyst
In ammonia production
Rates of reaction
What is the Hager process that iron is used in
N2 (g) + 3 H2 (g) <=> 2 NH3 (g)
Rates of reaction
Why is using catalysts good for industry
Lowers production cost
Gives more product in shorter time
Helps make better products
Rates of reaction
Why is iron used in the Haber process
If not for catalyst, temp would have to be raised loads which would be more expensive and reduce the amount of ammonias produced
Rates of reaction
How catalysts help environmental sustainability (2)
Lowers temp and pressure needed so less energy requires so less CO2 released and fossil fuels saves
Greater atom economy so less waste
Rates of reaction
What does the maxwell Boltzmann distribution curve show
Distribution of particles in a reaction
Rates of reaction
Why does the MB distribution curve always start at the origin
No particles have no energy
Rates of reaction
Why does the MB curve never cross the x axis
Particles cannot gave negative energy and energy is infinite
Rates of reaction
What is on the x axis of the MB curve
Molecular energy
Rates of reaction
What’s on the y axis of the MB curve
Number of particles with a particular energy
Rates of reaction
What is the shape of the MB curve
Peak then slopes of but never touches x axis
Rates of reaction
What part of the graph tells us the number of particles that can collide successfully
Area under graph with more energy that the EA
Rates of reaction
Where is the EA on a MB curve
To the right of the x axis
Rates of reaction
what happens to the shape of a MB curve when the temperature is increased
Pearl at most probable energy shifts to right and lowers and tail not as close to x axis
Rates of reaction
Why does increasing the temperature increase rate of reaction including MB distribution
Increasing temperature means area under curve with energy higher than the EA will increase as the particles have more kinetic energy. This means that more particles will collide and be able to overcome the Ea
Also collisions more frequent
Rates of reaction
What happens to the MB curve when catalyst added
EA moves to left but res of curve stays the same
Rates of reaction
Why does adding a catalyst increase the rate of reaction using the MB curve
Catalysts lowers the activation energy by providing g an alternate pathways which Samsung that the area under the curve with more energy than the EA increases. Therefore more particles collide with enough energy to overcome the EA, increasing the rate of reaction
Rates of reaction
Equation for rate of reaction
Rate= change in y axis/ time
Rates of reaction
How do you calculate the rate of reaction from a graph
Draw tangent at time and find change in y/change in x
Dynamic equilibrium
Dynamic equilibrium
Do all reactions proceed to completion
No some end up with a mixture of reactants and products
Dynamic equilibrium
What is the definition for dynamic equilibrium
In a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction and the concentrations of the reactants and products is constant
Dynamic equilibrium
Process of reaching equilibrium
Forward reaction starts fast but slows as reactants gets less concentrated
Initially no backwards reaction by as products form will get faster
Once backwards and forward reactions are equal and opposite the reaction has reached equilibrium
Dynamic equilibrium
Why is a reversible chemical reaction a dynamic process
Everything may appear stationary but reactions are moving both ways
Dynamic equilibrium
How can the position of equilibrium be changed
By changing certain conditions
Dynamic equilibrium
What in equations shows the reactions are reversible
<=> symbol
Dynamic equilibrium
What five things are true when a chemical equilibrium is established
-in a closed system
-equilibrium can be approached from either side
-reaction is dynamic as is moving forwards and backwards
-concentrations remain constant
-forward and backward reaction are equal and opposite
Dynamic equilibrium
What is a closed system
One in which none of the reactants or products escape from the reaction mixture
Dynamic equilibrium
What is an open system
Matter and energy can be lost to the surroundings
Dynamic equilibrium
What’s le chateliers principle
When a change is applied to a system in dynamic equilibrium the system reacts in such a way to oppose the effect of the change
Dynamic equilibrium
What factors can affect the equilibrium position
Concentration
Pressure
Temperature
Dynamic equilibrium
What happens to position if increase concentration on left side
Position of equilibrium moves to right
Dynamic equilibrium
What happens to position is decrease concentration on the left
Position of equilibrium moves to left
Dynamic equilibrium what happens to position if increase concentration on the right
Position of equilibrium moves to the left
Dynamic equilibrium
What happens to position os equilibrium if decrease concentration on the right
Moves to right
Dynamic equilibrium
What happens to position of increase pressure
Moves to side with fewer gaseous molecules
Dynamic equilibrium
What happen to position ifdecrease pressure
Moves to side with more gaseous molecules
Dynamic equilibrium
What happens to position if increase temperature
Moves to endothermic products
Dynamic equilibrium
What happens if decreas the temperature to the positions
Moves toexothemic products
Dynamic equilibrium
What happens to position of equilibrium if add a catalyst
Doesn’t affect position but equilibrium is reached quicker
Dynamic equilibrium ]
What does Kc stand for
Equilibrium constant
Dynamic equilibrium
What is Kc defined as for the reaction aA + bB <=> cC + dD
[C]^c [D]^d
—————-
[A]^a [B]^b
Dynamic equilibrium
What does the notation [X]
The concentration of X
Dynamic equilibrium
What does Kc give you an idea of
How far to the left or right the position of equilibrium is
Dynamic equilibrium \
What is an equilibrium expression
When the products in square brackets raised to number mole is written over the reactants in square brackets raised to the number of moles
Dynamic equilibrium
How do you calculate the value of Kc
Substitute concentrations into the equilibrium expression
Dynamic equilibrium
Why des the units of Kc change
In equilibrium expression each figure is represents concentration in mol dm^-3
Dynamic equilibrium
How do you work out the units of Kc
Since varies for each equilibrium expression, substitute the units mol dm^-3 into the expression and simplify
Dynamic equilibrium
How come some Kc have no units
Concentration units cancel out
Dynamic equilibrium
What can you estimate using Kc
Position of equilibrium
Dynamic equilibrium
Where is the position of equilibrium of Kc is larger than 1
To the right as more products then reactants
Dynamic equilibrium
Where is the position of equilibrium is Kc=1
Lies in centre as equal concentrations of products and reactants
Dynamic equilibrium
Where is the position of equilibrium if Kc is less than 1
To the left because larger concentrations of reactants than products
Dynamic equilibrium
What do you use for moles at equilibrium calculation
ICE
Dynamic equilibrium
What does ICE stand for
Initial
Change
Equilibrium
Dynamic equilibrium
How do you set out a moles at equilibrium question
With equationat top and ICE down side as a table
Put known values into table
Dynamic equilibrium
What units do the values have to be in for ICE
Concentration (mol dm^-3)
If 1:1:1:1 mol ratio can use in mol
Dynamic equilibrium
How do you do an ICE calculation
Work out change for each compound and work out conc for each compound at equilibrium
