Energetics

Question 1

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

Answer 1

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

Question 2

the experimentally determined values for the enthalpy of combustion does not always agree with values in the data booklet, suggest two reasons why

Answer 2

(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.

Question 3

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

Answer 3

The rate is too slow or the activation energy may be too high or we could form carbon dioxide and water instead.

Question 4

Suggest one reason why extracting vanadium like this is expensive with ca in an equation

Answer 4

As ca is expensive to extract as it has to be extracted using electrolysis

Question 5

2 hazards in the process (vcl2+h2–>c+2hcl)

Answer 5

HCl is toxic and hydrogen is flammable

Question 6

What are the only two liquids at standard conditions

Answer 6

Mercury and bromine

Question 7

Where are most of the gases found

Answer 7

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.

Question 8

What does bond enthalpy mean

Answer 8

Heat energy change needed to break a covalent bond and it is averaged over a range of compounds

Question 9

State a change in the volume of water that would cause a reduction in the heat loss and explain

Answer 9

increase in volume=
smaller increase in T above room temp
or increased contact between calorimeter and water
smaller heat loss by evaporation

Question 10

Define bond energies if a specific bond is mentioned

Answer 10

The enthalpy change to break 1 mol of … bonds averaged over a range of compounds

Question 11

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

Answer 11

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

Question 12

How to improve a enthalpy reaction

Answer 12

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

Question 13

Two hazards and why does this process produce pure vanadium

Answer 13

Hcl is toxic, h2 is flammable..

Only other product formed is h2 which is a gas and is easily seperated

Question 14

Suggest one reason that the enthalpy change cannot be measured directly

Answer 14

As impossible to add the precise amount of water

Question 15

Define bond disassociation enthalpy as applied to chlorine

Answer 15

Enthalpy change to break the bond in 1 mol of chlorine.

Question 16

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

Answer 16

Water has a known density of 1.0gcm-3
Therefore a volume of 50.0cm3 could be measured out

Question 17

Give the meaning of the term mean bond enthalpy

Answer 17

Energy needed in breaking COVALENT BONDS averaged over a range of molecules.

Question 18

Set up a practical for calorimetry

Answer 18

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

Question 19

Give one reason why the bond enthalpy that you calculated is different to the mean bond enthalpy from a data book

Answer 19

As data book values are averaged over a range of different compounds not just….

Question 20

Identify one substance that is manufactured directly from the sulfur dioxide formed in this reaction

Answer 20

So3 or gypsum caso4 etc

Question 21

Why is somethings value not zero

Answer 21

If its not in its standard state

Question 22

The uncertainity in each of the temp readings from the thermometer used in this experiment was +-0.1 calculate hthe precentage uncertainty for this

Answer 22

0.2/measurement i got so the 0.6 0.2 because i did the measurement twice and then times by 100

Question 22

Suggest a change to the exp that would minimise heat loss

Answer 22

Insulate a glass beaker

Question 23

Suggest and explain another change to this exp that would decrease percentage uncertainity in the use of the same thermometer

Answer 23

Increase magnitude of temp change by increasing the conc of an acid

Question 23

How do you work out the heat released of smth if you have a enthalpy and a dm3

Answer 23

Work out the mass and moles adn then you just do the enthalpy multiplied by the moles

Question 24

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

Answer 24

As these two enthalpy changes are for the same reaction

Question 25

Explain why this thermometer was adequate for this exp

Answer 25

As the heat loss is more significant or the temp rise is bigger than the uncertainity

Question 26

Explain why the enthalpy of atomisation of chlorine is exactly half the bond disassociation enthalpy of chlorine for atomisation

Answer 26

As only 1 mole of chlorine atoms not 2 mols is formed

Question 27

Explain the enthalpy of formation of cif3 that you calculated and why is it diff from a data book value

Answer 27

As the bond enthalpy for the cl-f bond would be diff than the cif3

Question 28

Suggest why the na-cl bond enthalpy is not found in any data book

Answer 28

As it is IONIC not covalent

Question 29

Standard enthalpy of formation

Answer 29

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.

Question 30

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

Answer 30

The product yield is very small (1)
Yield does not justify cost of producing high temp (1)

Question 31

suggest why the catalyst became less efficient if the carbon dioxide and hydrogen contained impurities

Answer 31

impurities in the compounds of sulfur could block the active sites of the catalyst.

Question 32

By reference to its structure, suggest why a large amount of heat energy is
required to produce free carbon atoms from solid carbon.

Answer 32

macromolecular

Question 33

why is a thin layer of palladium used inside cat convertors

Answer 33

increases surface area so less metals are required less money

Question 35

Explain the meaning of the terms mean bond enthalpy and standard enthalpy
of formation.
Mean bond enthalpy ………………………………………………………………………….
……………………………………………………………………………………………………….
……………………………………………………………………………………………………….
Standard enthalpy of formation ..

Answer 35

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;

Question 36

State why the standard enthalpy of formation, ∆Hf
ο
, of oxygen is zero

Answer 36

by definition
∆Hf is formation from an element

Question 37

Explain why your answer to part (b) is a less accurate value than your answer to
part (d).

Answer 37

bond enthalpies are mean/average values

from a range of compounds

Question 39

suggest why the enthalpy change of an exp cannot be measured directly

Answer 39

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