Energetics Flashcards
To work out heat energy released when they give you concentration (in cm) and volume in dm and a enthalpy change in - values what do we do
Firstly work out the mass of it so use comovo to work out the mass by doing c times v. The c was in cm so convert the vol to cm as well by multiplying it by 1000. Then times them and then use that value and divide it by the mr to get the moles. Then do the moles times the enthalpy change bc enthalpy change is for one mole so that manny is that times that. Remember units kj
the experimentally determined values for the enthalpy of combustion does not always agree with values in the data booklet, suggest two reasons why
(Heat loss or incomplete combustion)
You can say, because the reaction may have occurred under non standard conditions or incomplete combustion may have occured. Also the data values are an average.
Glucose can be formed according to the equation below, the enthalpy change of formation of glucose cannot be determined directly suggest why this is the case
The rate is too slow or the activation energy may be too high or we could form carbon dioxide and water instead.
Suggest one reason why extracting vanadium like this is expensive with ca in an equation
As ca is expensive to extract as it has to be extracted using electrolysis
2 hazards in the process (vcl2+h2–>c+2hcl)
HCl is toxic and hydrogen is flammable
What are the only two liquids at standard conditions
Mercury and bromine
Where are most of the gases found
On the right hand side so from nitrogen onwards row underneath chlorine onwards then bromine onwards and all the noble gases. The rest are solids.
What does bond enthalpy mean
Heat energy change needed to break a covalent bond and it is averaged over a range of compounds
State a change in the volume of water that would cause a reduction in the heat loss and explain
increase in volume=
smaller increase in T above room temp
or increased contact between calorimeter and water
smaller heat loss by evaporation
Define bond energies if a specific bond is mentioned
The enthalpy change to break 1 mol of … bonds averaged over a range of compounds
If it gives you a reaction and tells you to work out the enthalpy of water for example and then asks you why hydrogen would be the same and why would they differ
As it is the same reaction with the same reactants and products differ as there may be a difference in the actual bond enthalpies and the mean bond enthalpies of The o-h bond or didn’t occur at standard conditions
How to improve a enthalpy reaction
Insulate a beaker, use a lid to reduce heat loss. Record the temp for a suitable time before adding the metal to ensure accurate initial temp. Stir water. Put temp results against time on a graph, establish a clear temp change
Two hazards and why does this process produce pure vanadium
Hcl is toxic, h2 is flammable..
Only other product formed is h2 which is a gas and is easily seperated
Suggest one reason that the enthalpy change cannot be measured directly
As impossible to add the precise amount of water
Define bond disassociation enthalpy as applied to chlorine
Enthalpy change to break the bond in 1 mol of chlorine.
A 50.0g sample of water was used in this exp. explain how you could measure out this mass of water without using a balance
Water has a known density of 1.0gcm-3
Therefore a volume of 50.0cm3 could be measured out
Give the meaning of the term mean bond enthalpy
Energy needed in breaking COVALENT BONDS averaged over a range of molecules.
Set up a practical for calorimetry
Insulate the calorimeter to prevent heat loss, make sure the thermometer is fully immersed, lid on the beaker, continuous stirring, record the temp for a suitable amount of time to establish an accurate initial temp. Adjust cooling to get an accurate final temp to calculate an accurate change in temp
Give one reason why the bond enthalpy that you calculated is different to the mean bond enthalpy from a data book
As data book values are averaged over a range of different compounds not just….
Identify one substance that is manufactured directly from the sulfur dioxide formed in this reaction
So3 or gypsum caso4 etc
Why is somethings value not zero
If its not in its standard state
The uncertainity in each of the temp readings from the thermometer used in this experiment was +-0.1 calculate hthe precentage uncertainty for this
0.2/measurement i got so the 0.6 0.2 because i did the measurement twice and then times by 100
Suggest a change to the exp that would minimise heat loss
Insulate a glass beaker
Suggest and explain another change to this exp that would decrease percentage uncertainity in the use of the same thermometer
Increase magnitude of temp change by increasing the conc of an acid
How do you work out the heat released of smth if you have a enthalpy and a dm3
Work out the mass and moles adn then you just do the enthalpy multiplied by the moles
State why the value quoted in part c for the standard enthalpy of formation is the same as the value for the standard enthalpy of combustion
As these two enthalpy changes are for the same reaction
Explain why this thermometer was adequate for this exp
As the heat loss is more significant or the temp rise is bigger than the uncertainity
Explain why the enthalpy of atomisation of chlorine is exactly half the bond disassociation enthalpy of chlorine for atomisation
As only 1 mole of chlorine atoms not 2 mols is formed
Explain the enthalpy of formation of cif3 that you calculated and why is it diff from a data book value
As the bond enthalpy for the cl-f bond would be diff than the cif3
Suggest why the na-cl bond enthalpy is not found in any data book
As it is IONIC not covalent
Standard enthalpy of formation
Enthalpy change AT A CONSTANT PRESSURE when 1 mol of a compound is formed from its constituent elements when all reactants and products are in their standard states.
The value of Kc for this reaction is 1 × 10–5 at 1500 K.
Explain the significance of this value for an industrial chemist interested in manufacturing
nitrogen monoxide by the direct combination of the elements
The product yield is very small (1)
Yield does not justify cost of producing high temp (1)
suggest why the catalyst became less efficient if the carbon dioxide and hydrogen contained impurities
impurities in the compounds of sulfur could block the active sites of the catalyst.
By reference to its structure, suggest why a large amount of heat energy is
required to produce free carbon atoms from solid carbon.
macromolecular
why is a thin layer of palladium used inside cat convertors
increases surface area so less metals are required less money
Explain the meaning of the terms mean bond enthalpy and standard enthalpy
of formation.
Mean bond enthalpy ………………………………………………………………………….
……………………………………………………………………………………………………….
……………………………………………………………………………………………………….
Standard enthalpy of formation ..
enthalpy (or energy) to break (or dissociate) a bond;
1
averaged over different molecules (environments);
1
enthalpy (or heat energy) change when one mole of a compound;
1
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is formed from its elements;
1
in their standard states;
State why the standard enthalpy of formation, ∆Hf
ο
, of oxygen is zero
by definition
∆Hf is formation from an element
Explain why your answer to part (b) is a less accurate value than your answer to
part (d).
bond enthalpies are mean/average values
from a range of compounds
suggest why the enthalpy change of an exp cannot be measured directly
impossible to add precise amount of water
very difficult to measure the temp rise of a solid
difficult to prevent solid dissolving
copper sulfate solution will form
