Ch3.2 Physical Chemistry Flashcards
What can be measured to determine rate of reaction?
- speed of reactant used up or product formed
How can the formation of product or usage of reactants be measured?
- measure mass with a balance
- measure volume of gas with a gas syringe
What is the formula for calculating rate of reaction?
amount of reactant used or product formed ÷ time taken
How can the rate of a reaction be increased?
- increase temperature
- increase conc of reactant
- increase pressure of reacting gas
- increase surface area of solid reactants (to powder)
- use a catalyst
What is the activation energy of a reaction and how does it relate to collisions?
- activation energy is the minimum energy needed for a reaction to occur
- this is the amount of energy particles must collide with for the reaction to happen
How does changing concentration and pressure affect collisions?
- increasing pressure means more particles in same volume
- higher chance of collisions
How does changing the particle size or pressure within a reaction affect collisions?
- more reactant particles in the same volume
- higher chance of colliding
What is a homogenous catalyst?
- a catalyst in the same phase as the reactant
- ie. liquid catalyst in a liquid reaction
Give an example of a homogenous catalyst
enzymes in bodily fluids
What is a heterogenous catalyst?
- a catalyst in a different state to reactant
- eg. gas reactants passed over a solid catalyst
How could a catalyst be heterogenous even though all the substances are in the same state?
If all are liquids but they are immiscible (don’t mix) then it’s still heterogenous as they are in different layers
What is a dynamic equilibrium?
Forward and back reactions occurring at equal and opposite rates, so it looks as though nothing happens when there’s constant change
What are the features of a dynamic equilibrium?
- only in a closed system
- dynamic at molecular or ionic level
- visible/macroscopic properties are constant
What is Le Chatelier’s principle?
When any of the conditions affecting the position of a dynamic equilibrium are changed, the position of the equilibrium will shift to minimise the change
What happens to the concentration of products/reactants when equilibrium moves left/right?
- equilibrium shifts to right: conc of products increases
- equilibrium shifts to left: conc of reactants increases
What is the reaction to the action:
increase in conc of reactants
Equilibrium shifts to the right
What is the reaction to the action:
decrease in conc of reactants
Equilibrium shifts to the left
What is the reaction to the action:
increase in pressure
Moves in direction of side with less moles of gas produced to decrease pressure again
What is the reaction to the action:
decrease in pressure
Moves towards the side with more moles of gas produced to increase pressure again
What is the reaction to the action:
increase temperature
Moves in endothermic direction
What is the reaction to the action:
decrease temperature
Moves in exothermic direction
What is the reaction to the action:
catalyst added
Reaches equilibrium faster
Describe the stages of the Haber Process
1: hydrogen and nitrogen are obtained from natural gas and air respectively. They are pumped into a compressor through a pipe
2: gases are compressed to about 200 atmospheres in the compressor
3: gases are pumped into a tank with layers of catalytic iron beds at 450 degrees C. Some of hydrogen and nitrogen react to form ammonia
4: unreacted gases go into a cooling tank. ammonia is liquefied and removed to pressurised storage vessels
5: unreacted hydrogen and nitrogen are recycled back into the system and started over again
Why does the yield of ammonia change during the Haber Process?
Yield of ammonia will change with changes to temperature and pressure
What must companies consider when they want to make a profit from forming ammonia?
Availability and cost of raw materials
Is the Haber Process cheap or expensive to start?
Cheap
- raw materials are readily available and cheap to purify
What is the process if the cost for a process is too high?
No longer economically viable
- many processes require heat and pressure in huge amounts which is expensive
What temperature is the Haber process performed at and why?
- +450 degrees C
- higher temp favours reverse reaction as it is endothermic
- higher yield of reactants made
- lower temp favours forward reaction so higher yield of products made
- 450 degrees is a compromise condition as a lower temp would be too slow but higher would produce too much of reactants
Is the haber process endo or exo thermic?
Endothermic (takes in heat)
Is bond breaking endo or exothermic?
Endothermic
Is bond making endo or exothermic?
Exothermic
Describe why the pressure condition of the Haber process is what it is
- pressure: 200 atm
- lower pressure favours production of reactants
- higher pressure favours production of products
- high pressure can be dangerous and expensive equipment is needed
- 200 atm is a compromise of less products being made for expense and safety purposes
What catalyst is used during the Haber process?
Iron
Describe how a catalyst affects the Haber process
- iron is the catalyst used
- doesn’t affect the point of equilibrium, just the rate at which it is achieved
- speeds up the forward and backward reactions by the same amount
- gives a higher yield at a lower temperature due to a lower activation energy
What happens during the Haber process without a catalyst?
- higher temp
- higher costs
- decreasing yield
What does a higher temp cause in a reaction?
More molecules will have the activation energy
What does a catalyst cause in a reaction?
Lowers activation energy so more molecules are likely to have it
What are CFCs?
- chlorofluorocarbons
- made by Thomas Midgely
- destroy ozone layer
What is the difference between nitrogen dioxide and dinitrogen tetraoxide?
Nitrogen dioxide is brown
Dinitrogen tetraoxide is colourless
What yield of ammonia do the compromise conditions of the Haber process produce?
18.3% yield at 200 atm and 450 degrees C
What are the uses of ammonia?
- 80% used for making fertilisers such as ammonium sulphate
- smaller % used for making nitric acid (used to make explosives) and polymers such as nylon
What symbol is used for the equilibrium constant?
Kc
Where is the equilibrium if the Kc is between 0 and 1?
It lies to the left
Where is the equilibrium if the Kc is over 1?
It lies to the right
What does the term enthalpy mean?
Used to distinguish between the heat energy taken in or out of a reaction compared to other types of energy
What symbol is used to represent the term enthalpy?
∆H
Compare endo and exothermic reactions
- chemical reactions in which the system loses heat energy to its surroundings are exothermic
- chemical reactions that absorb energy from its surroundings are endothermic
What are enthalpy changes measured in?
kilojoules per mole
What is bond enthalpy?
The amount of energy covalent bonds have locked up in them. This is the amount of energy it will take to break one mole of these bonds.
What are standard conditions?
- the agreed criteria that scientists use to measure enthalpy cycles
- 25 degrees C
- 298 K
- 100 kPa
What does ΔH⁰c mean?
one mole of a substance has completely combusted
What does ΔH⁰f mean?
one mole of a substance has formed from its constituent elements in their standard states
What does ΔH⁰neut mean?
one mole of water forms when an acid reacts with a base
What does ΔH⁰sol mean?
one mole of a substance is dissolving
What does ΔH⁰r mean?
one mole of a substance is reacting
(the reaction isn’t combustion formation, solution or neutralisation)
Describe the process of cup calorimetry
- we mix solutions in a polystyrene cup and measure the amount of energy transferred to or absorbed from the water
What do you need to carry out a calorimetric calculation?
- vol of solution
- mass of any solid reactant added
- initial and final temps of solution
Describe the stages of a calorimetric calculation
- work out energy transfer in joules using q = mcΔt
- change J into kJ
- work out number of moles of chemical involved
- divide kilojoules by moles
When would can calorimetry be used compared to cup?
- when the temperature change would melt the polystyrene cup
What is bomb calorimetry?
- a technique that overcomes the limitations of cup and can calorimetry
What does en enthalpy cycle allow?
- provides an indirect method of measuring the enthalpy change of a chemical reaction
What is another term for an enthalpy cycle?
A Hess cycle
What does Hess’ law state?
- the enthalpy change in a chemical reaction is independent of the route it takes
- (if the start and end points are the same the overall enthalpy change will be the same)
What does it imply if the examiners provide us with ΔH⁰c values?
- that we are separately completely combusting the reactants and products in the original reaction
What is the equation used for calculating enthalpy change?
q = mcΔt
q is energy change in J
m is mass
c is specific heat capacity (4.18)
Δt is temp change
Define bond enthalpy
The energy required to break 1 mole of gaseous bonds into gaseous atoms
What is enthalpy change of formation?
- the enthalpy change when 1 mole of a substance is formed from its constituent elements
What is enthalpy change of combustion?
- the enthalpy change when 1 mole of substance is completely burned in excess oxygen
Describe the first law of energy
- energy can neither be created nor destroyed but it can be converted from one form to another
- all chemical reactions are accompanied by some form of energy change
- changes can be very obvious but in many cases go unnoticed
Give 2 examples of exothermic reactions
- combustion of fuels
- respiration
Give 2 examples of endothermic reactions
- photosynthesis
- thermal decomposition
What is enthalpy change of formation?
- the enthalpy change when one mole of water is formed from its ions in dilute solution
During calorimetry, how does the temp change of the water determine the nature of the reaction?
- water heats up = exothermic
- water cools down = endothermic
Define enthalpy change of formation
The enthalpy change of formation is the enthalpy change when 1 mole of a substance is formed from its elements in their normal state under standard conditions
Define enthalpy change of combustion
The enthalpy change of combustion is the enthalpy change when 1 mole of a substance is completely burned in an excess of oxygen under standard conditions
Define enthalpy change of neutralisation
The enthalpy change of neutralisation is the enthalpy change when 1 mole of water is formed in a reaction between an acid and a base under standard conditions
What do spectroscopic techniques use?
- they use various parts of the electromagnetic spectrum to analyse the structure of matter
Describe IR spectroscopy
- infrared radiation makes chemical bonds in covalently bonded substances vibrate
- each type of bond vibrates in response to a different frequency of infrared radiation
- we can use these absorptions to identify the bonds
Compare a sample cell and a control cell within IR spectroscopy
sample cell:
NaCl disc, smear sample ground up in Nujol onto disc, place second NaCl disc on top
control cell:
same as above without sample
Why are NaCl discs used during IR spectroscopy?
- NaCl contains purely ionic bonds so won’t flare up spectrometer
Why isn’t water used for grinding up the sample used in IR spectroscopy?
It will dissolve the NaCl discs
What is the vibration value of O-H in alcohols and phenols?
3200-3600
What is the vibration value of O-H in carboxylic acids?
2500-3300
What is the vibration value of C=O in aldehydes, ketones and carboxylic acids?
1630-1820
Name 2 oxidising agents
- potassium dichromate K2Cr2O7
- potassium manganate KMnO4
Name a reducing agent
- sodium borohydride NaBH4
How can a primary alcohol be oxidised?
- oxidised with distillation to aldehyde
- oxidised with reflux to aldehyde then carboxylic acid
How can a secondary alcohol be oxidised?
- oxidised to a ketone only
How can a tertiary alcohol be oxidised?
It cannot be oxidised
What can happen in the high energy environment of the ionisation chamber in mass fragmentation spectra?
- 2 things:
- the parent molecules may have an electron knocked off, turning into molecular ions
- the parent molecules will get broken up into a series of smaller pieces which we call fragments
How can the mass fragmentation spectra be used?
- used to work out the structure of the original molecule
- computer scales the peak of the most abundant fragment to 100 (base peak)
What is the molecular ion peak in a mass fragmentation spectra?
- the peak at which the original molecule’s Mr would be
- there may be peaks higher than this but this is due to carbon and hydrogen both having heavier isotopes
Describe the blue litmus wet test
- tests for carboxylic acid/phenol
- carboxylic acid turns it red
- phenol turns it pink
Describe the acidified potassium dichromate wet test
- tests for aldehyde/primary alcohol/secondary alcohol
- orange to green if positive
Describe the 2,4-dinitrophenylhydrazine wet test
- test for aldehyde/ketone
- positive = orange precipitate
Describe the sodium carbonate wet test
- tests for carboxylic acids
- fizzes and produces gases that turn limewater milky
Describe the neutral iron (III) chloride wet test
- test for phenol
- brown to purple
Describe the Tollens wet test
- test for aldehyde
- makes silver mirror
Describe the sodium metal wet test
- test for alcohol/carboxylic acid
- fizzing (more vigorous with carboxylic acid)
Describe the bromine water wet test
- test for C=C bond
- orange to colourless
Describe the warmed with ethanoic acid and sulphuric acid wet test
- test for alcohol
- positive result: fruity smell produced (ester formed)
What is the formula for percentage error?
% error =
error margin/actual measurement
x 100
