5 Energetics / Thermochemistry Flashcards
What is temperature a measure of?
We say that temperature is a measure of the average kinetic energy of the particles
What is heat a measure of?
heat is a measure of the energy content of a substance
Why do particles have kinetic energy?
The particles have kinetic energy because they are moving
The faster they move the more energy they have and the higher the temperature of the substance
What is energy?
Energy is a measure of the ability to do work
What 3 types of systems are there?
Systems come in three types: open, closed and isolated:
What is the rarest type of system?
Isolated systems are very rare; most chemical reactions are open systems
What is the Law fo Conservation of Energy?
Although energy can be exchanged between open and closed systems and the surroundings, the total energy of the process cannot change
What is enthalpy?
The total chemical energy inside a substance is called the enthalpy (or heat content)
Why does enthalpy change in a chemical reaction?
When chemical reactions take place, changes in chemical energy take place and therefore the enthalpy changes
When is a reaction exothermic?
A reaction is exothermic when the products have less enthalpy than the reactants
Where is energy transferred in an exothermic reaction?
Heat energy is given off by the system to the surroundings
How is the temperature of the surroundings and system affected in an exothermic reaction?
The temperature of the surroundings increases
The temperature of the system decreases
What is the sign of delta H and why for exothermic?
There is an enthalpy decrease during the reaction so ΔH is negative
Are exothermic reactions thermodynamically possible?
Exothermic reactions are thermodynamically possible (because the enthalpy of the reactants is higher than that of the products)
When is a reaction endothermic?
A reaction is endothermic when the products have more enthalpy than the reactants
Where is heat taken from in an endothermic reaction?
Heat energy is absorbed by the system from the surroundings
How is the temperature of the surroundings and system affected in an endothermic reaction?
The temperature of the surroundings decreases
The temperature of the system increases§
What is the sign of delta H for an endothermic reaction and why?
There is an enthalpy increase during the reaction so ΔH is positive
What must be done to compare changes in enthalpy?
To compare the changes in enthalpy between reactions, all thermodynamic measurements are made under standard conditions
What are the standard conditions?
These standard conditions are:
A pressure of 100 kPa
A concentration of 1 mol dm-3 for all solutions
Each substance involved in the reaction is in its standard state (solid, gas or liquid)
What is the temperature for standard conditions?
Temperature is not part of the definition of standard state, but a temperature of 298 K (25 oC) is usually given as the specified temperature
What symbol is used to show that a reaction has been carried out under standard conditions?
To show that a reaction has been carried out under standard conditions, the symbol ⦵ is used
Eg. ΔHꝊ = the standard enthalpy change
What is the standard enthalpy change of reaction?
the enthalpy change when the reactants in the stoichiometric equation react to give the products under standard conditions (both exo and endo)
What is the standard enthalpy change of formation?
the enthalpy change when one mole of a compound is formed from its elements under standard conditions (both)
What is the standard enthalpy change of combustion?
the enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions (exo only!)
What is the standard enthalpy change of neutralisation?
the enthalpy change when one mole of water is formed by reacting an acid and alkali under standard conditions
What is calorimetry?
Calorimetry is a technique used to measure changes in enthalpy of chemical reactions
What can a calorimeter be made out of (v. simple)?
A calorimeter can be made up of a polystyrene drinking cup, a vacuum flask or metal can
What is the specific heat capacity?
The energy needed to raise the temperature of 1 g of a substance by 1 K is called the specific heat capacity (c) of the liquid
what formula is used to calculate the energy transferred?
q = mc∆T
q = heat transferred
m = mass of water IN GRAMS
c = specific heat capacity
∆T = temperature change IN KELVINS
(once calculated, divide by number of moles to get kJ per MOLE)
What is the principle of calorimetry experiments?
The principle of these calorimetry experiments is to carry out the reaction with an excess of one reagent and measure the temperature change over the course of a few minutes
What are 5 assumptions in calorimetry in solution? (q=mct)
- That the specific heat capacity of the solution is the same as pure water, i.e. 4.18 J g-1 K-1
- That the density of the solution is the same as pure water, i.e. 1 g cm-3
- The specific heat capacity of the container is ignored
- The reaction is complete
- There are negligible heat losses
What may be delayed in reactions that are not instantaneous?
For reactions which are not instantaneous there may be a delay before the maximum temperature is reached
What may be a limitation of the reaction not being instantaneous?
During that delay the substances themselves may be losing heat to the surroundings, so that the true maximum temperature is never actually reached
What can be used to determine maximum enthalpy change when the reaction is not instantaneous?
To overcome this problem we can use graphical analysis to determine the maximum enthalpy change
When should the line (temperature) be extended to find the maximal temperature?
Plot the graph and extrapolate the cooling part of the graph until you intersect the time at which the second reactant was added
What is an assumption when extrapolating the temperature line?
An assumption made here is that the rate of cooling is constant
What is the principle used in enthalpy of combustion experiments?
The principle here is to use the heat released by a combustion reaction to increase the heat content of water
Is all heat transferred to the combustion calorimeter?
NO!
Some heat is lost to the surroundings
Some heat is absorbed by the calorimeter
What could be done to minimise heat losses in combustion calorimetry?
To minimise the heat losses the copper calorimeter should not be placed too far above the flame and a lid placed over the calorimeter
What can be used to reduce draughts (combustion calorimetry)?
Shielding can be used to reduce draughts
What are the two main sources of error in combustion calorimetry?
In this experiment the main sources of error are
Heat losses
Incomplete combustion
What can Hess’s law be used to calculate?
Hess’s law can be used to calculate the standard enthalpy change of a reaction from known standard enthalpy changes
What does Hess’s law state?
The total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same.”
What is the simplified meaning of Hess’ law?
This means that whether the reaction takes place in one or two steps, the total enthalpy change of the reaction will still be the same
What can enthalpy of formation be used for with regards to Hess’ law?
We can use enthalpy of formation of substances to find an unknown enthalpy change using a Hess cycle
In this type of cycle the elements are always placed at the bottom of the diagram
Why are arrows always pointing upwards in the enthalpy of formation?
n this cycle the arrows will always be pointing upwards because the definition of the enthalpy of formation must go from elements to compounds
This means the Hess’s Law calculation of ΔH will always be in the same arrangement
What is a chemical bond?
A chemical bond is a force of attraction between two atoms
Is breaking bonds endo or exo?
Breaking the bond requires the input of energy it is therefore an endothermic process
end’o the bond
What is the energy change required to break the bond dependent on?
The energy required to break a particular bond is called the bond dissociation enthalpy
This is usually just shortened to bond enthalpy or bond energy
Is bond breaking endo or exo?
Bond formation is the opposite of bond breaking and so energy is released when bonds are formed
It is therefore an exothermic process
What is the relationship of energy released when a particular bond is broken with the energy taken when the bond is broken?
same magnitude
When is a reaction exothermic, in terms of comparing bond breaking and making?
If more energy is released when new bonds are formed than energy is required to break bonds, the reaction is exothermic
IN an exothermic reaction, are the products or reactants more stable?
The products are more stable than the reactants
When is a reaction endothermic, in terms of comparing bond breaking and making?
If more energy is required to break bonds than energy is released when new bonds are formed, the reaction is endothermic
IN an endothermic reaction, are the products or reactants more stable?
The products are less stable than the reactants
What are bond energies affected by?
Bond energies are affected by other atoms in the molecule (the environment)
What is the average bond energy? (simple terms - why is it an average?)
Therefore, an average of a number of the same type of bond but in different environments is calculated
What is the formal definition of the average bond enthalpy?
‘The energy needed to break one mole of bonds in a gaseous molecule averaged over similar compounds’
How is the average bond enthalpy of CH found?
The average bond enthalpy of C-H is found by taking the bond dissociation enthalpy for the whole molecule and dividing it by the number of C-H bonds
Are all C-H bonds the same in terms of easiness to break in a compound?
NO
The first C-H bond is easier to break than the second as the remaining hydrogens are pulled more closely to the carbon
What is this value then compared with?
This value is also compared with a range of similar compounds to obtain an accepted value for the average bond enthalpy
What are bond energies used to find?
Bond energies are used to find the ΔHrꝋ of a reaction when this cannot be done experimentally
What is the formula for calculating average bond enthalpy of a reaction?
reactants - products / enthalpy change for bonds broken + enthalpy change for bonds formed
what is an energy profile?
An energy profile or energy level diagram is a diagram that shows the energies of the reactants, the transition state(s) and the products of the reaction with time
What is the transition state?
The transition state is a stage during the reaction at which chemical bonds are partially broken and formed
Is the transition state un/stable?
The transition state is very unstable – it cannot be isolated and is higher in energy than the reactants and products
What is the activation energy?
The activation energy (Ea) is the energy needed to reach the transition state
What can activation energy be defined as?
‘the minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction’
In an exothermic reaction are the reactants closer/farther away from the transition state and why?
In an exothermic reaction, the reactants are higher in energy than the products
The reactants are therefore closer in energy to the transition state
Is activation energy greater/smaller for exothermic reactions compared to endothermic reactions?
This means that exothermic reactions have a lower activation energy compared to endothermic reactions
In an endothermic reaction are the reactants closer/farther away from the transition state and why?
In an endothermic reaction, the reactants are lower in energy than the products
The reactants are therefore further away in energy to the transition state
Is activation energy greater/smaller for endothermic reactions compared to exothermic reactions?
This means that endothermic reactions have a higher activation energy compared to exothermic reactions
What can be studied in terms of bond enthalpy and what does this explain?
A study of bond enthalpy can explain why ozone and oxygen in the atmosphere play very different roles in the flow of energy
What is the significance of studying ozone?
These processes have a profound effect on the amount of solar radiation reaching ground level
What influences the amount of energy needed to break the bonds in ozone?
The structure of oxygen and ozone molecules influences the amount of energy needed to break their bonds:
Which bond is stronger, O2 or O3?
The double bond in oxygen is stronger than the delocalised π bonds in ozone
Compare the bond order of oxygen in o2 and o3?
We say the bond order of oxygen is 2 and the bond order of ozone is 1.5
What is the difference in the wavelength needed to break the bonds in O2 and O3?
Both bonds are broken by ultraviolet radiation but the bond in oxygen requires radiation of higher energy and shorter wavelength than the bond in ozone
What does high energy UV radiation do in the stratosphere?
High energy UV radiation in the stratosphere breaks the oxygen-oxygen double bond creating oxygen atoms
What are free radicals? in terms of ozone
These oxygen atoms have unpaired electrons- they are known as free radicals
What do the free radicals do in the atmosphere?
The free radicals are highly reactive and quickly attack oxygen molecules forming ozone in an exothermic reaction, which raises the temperature of the stratosphere
Give the reaction for the production of oxygen free radicals
O2 (g) → O⋅ (g) + O⋅ (g) ∆H +ve, UV light, λ < 242 nm
Give the equation for ozone formation
O⋅ (g) + O2 (g) → O3 (g) ∆H - ve
Does o3 or o2 require less energy to break, and what are the consequences of this?
Ozone requires less energy to break than oxygen
It produces an oxygen molecule and an oxygen free radical (contributes to ozone depletion)
Give the equation for ozone depletion in terms of UV light breaking bonds
O3 (g) → O⋅ (g) + O2 (g) ∆H +ve, UV light, λ< 330 nm
Give the equation for ozone depletion in terms of reaction with free radical
O3 (g) + O⋅ (g) → 2O2 (g) ∆H - ve
What type of reaction is the reaction of ozone depletion with free radicals?
The radical reacts with another ozone molecule making two molecules of oxygen in an exothermic reaction
What maintains the temperature in the stratosphere?
The temperature in the stratosphere is maintained by the balance of ozone formation and ozone depletion in a process known as the Chapman Cycle
Is the stratosphere a closed system? Why?
It is not a closed system as matter and energy flow in and out, but it is what is called a steady state
What has interfered with the steady state of the stratosphere?
Unfortunately, chemicals we have introduced into the atmosphere have interfered with this steady state resulting in ozone depleting at a faster rate than it is replaced
What is an example of a chemical depleting ozone?
Amongst these chemicals are chlorofluorocarbons (CFCs) found in refrigerants, propellants and solvents
Are CFCs still used, despite being greatly damaging?
CFCs are greatly damaging to stratospheric ozone and have been largely replaced by safer alternatives following the 1985 Montreal Protocol
What has the depletion of ozone led to?
The depletion of ozone has allowed greater amounts of harmful UV light to reach the surface of the Earth
What has UV light been linked to?
UV light has been linked to greater incidence of skin cancer and cataracts as well as the destruction of phytoplankton and reduced plant growth
