Topic 8 - Energetics I Flashcards
What is enthalphy change?
The heat change in a reaction at constant pressure.
What is the symbol for enthalpy change?
ΔH
What is the unit for enthalpy change?
kJ/mol
What is the symbol ΔH° used for?
- Enthalpy change under standard conditions and with the elements in their standard state
- The ° indicates this (a circle with a line through it).
What are standard states of elements?
Their physical states under standard conditions.
What are standard conditions?
- 100kPa (about 1 atm)
* Specified pressure (normally 298K)
What is an exothermic reaction?
One that gives out heat energy.
What happens to the temperature of the system in an exothermic reaction?
It goes up.
Give an example of an exothermic reaction.
Combustion of a fuel.
What is an endothermic reaction?
One that takes in heat energy.
What happens to the temperature of the system in an endothermic reaction?
It goes down.
Give an example of an endothermic reaction.
Thermal decomposition of CaCO3.
What is an enthalpy level diagram?
A diagram that shows the relative energies of the reactants and products in a reaction.
Describe what an enthalpy level diagram looks like.
- Enthalpy on the y-axis, no x-axis
- Horizontal line for reactants and products -> No line joining the two
- Arrow between the two lines, showing ΔH
What is another name for an enthalpy level diagram?
Energy level diagram
How does enthalpy relate to how stable a substance is?
The lower a substance’s enthalpy, the more stable it is.
Describe the enthalpy level diagram for an exothermic reaction.
- Reactants line above products line
* Downwards arrow between the two, showing ΔH
Describe the enthalpy level diagram for an endothermic reaction.
- Reactants line below products line
* Upwards arrow between the two, showing ΔH
What is a reaction profile diagram?
A diagram used to show how enthalpy changes throughout a reaction.
Describe what a reaction profile diagram looks like.
- Enthalpy on y-axis, Progress of reaction on x-axis
- Horizontal line for reactants and products
- Curve between the two -> Peak up to activation, then falls to products
- Arrow up to peak = Activation energy (Ea)
- Arrow from reactants to products = ΔH
What is the activation energy of a reaction?
The energy that must be supplied to break bonds in the reactants and start a chemical reaction.
What is the symbol for activation energy?
Ea
What is the ΔH value for an exothermic reaction?
Negative
What is the ΔH value for an endothermic reaction?
Positive
Describe the reaction profile diagram for an endothermic reaction.
- Products line is above reactants line
- Curve from reactants to peak above products, then falls to products
- Arrow upwards from reactants to products, showing ΔH
- Arrow upwards from reactants to peak, showing Ea
Describe the reaction profile diagram for an exothermic reaction.
- Products line is below reactants line
- Curve from reactants to peak above products, then falls further to products
- Arrow downwards from reactants to products, showing ΔH
- Arrow upwards from reactants to peak, showing Ea
How is the stability of a substance shown on an enthalpy level diagram or reaction profile diagram and why is this?
- The height of the reactants or products line shows the how unstable a substance is.
- Because a higher enthalpy corresponds with a low stability.
In an exothermic reaction, are the reactants or products more stable?
Products (since they have a lower enthalpy)
In an endothermic reaction, are the reactants or products more stable?
Reactants (since they have a lower enthalpy)
Looking at an enthalpy level diagram and reaction profile diagram, how can you find the:
• Activation energy
• Enthalpy change
ENTHALPY LEVEL DIAGRAM:
• Activation energy -> Can’t be found!
• Enthalpy change -> From reactants to products line
REACTION PROFILE DIAGRAM:
• Activation energy -> From reactants line to peak of curve
• Enthalpy change -> From reactants to products line
Can you measure the actual enthalpy of a system?
No, only the enthalpy change of a reaction.
Why must enthalpy change be measured under standard conditions?
Changes in enthalpy are affected by temperature and pressure.
What is 298K in *C?
25*C
What are the different types of enthalpy change?
- Standard enthalpy change of reaction
- Standard enthalpy change of formation
- Standard enthalpy change of combustion
- Standard enthalpy change of neutralisation
What is the standard enthalphy of reaction?
The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions.
What is the standard enthalpy of formation?
The enthalpy change when 1 mole of a ompound is formed from its constituent elements in their standard states, under standard conditions.
Give an equation for the standard enthalpy of formation of ethanol.
2C (s) + 3H2 (g) + 1/2O2 (g) -> C2H5OH (l)
What is the standard enthalpy of combustion?
The enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions.
What is the standard enthalpy of neutralisation?
The enthalpy when an acid and an alkali react together to form 1 mole of water, under standard conditions.
When defining what different type of ΔH are, what is it important to remember?
Include “under standard conditions” at the end of each definition.
What is the symbol for the standard enthalpy of reaction?
ΔrH°
What is the symbol for the standard enthalpy of formation?
ΔfH°
What is the symbol for the standard enthalpy of combustion?
ΔcH°
What is the symbol for the standard enthalpy of neutralisation?
ΔneutH°
To work out the enthalpy change in an experiment, what things do you need to know?
- Number of moles of the substance reacting
* Change in temperature
Is the heat change of a system equal to the enthalpy change?
Only if the pressure is kept constant.
What is calorimetry?
Measuring temperature changes.
How can you measure the temperature change in a reaction between two liquids or solids?
1) Set up a polystyrene beaker with a thermometer and a lid.
2) Mix the reactants inside and measure the temperature change every 30 seconds.
3) Use the graph method to minimise error (see flashcard).
(See pg 106 of revision guide)
What is the problem with measuring the temperature change of a reaction between two solids or liquids?
Lots of heat is lost to the surroundings (or gained, if the reaction is endothermic).
What method can be used to minimise the error in measuring the temperature change of a reaction between two solids or liquids?
1) Measure the temperature throughout the reaction every 30 seconds
2) Plot a graph of temperature against time -> There will be a curve
3) Extrapolate the curve from the RHS of the peak or trough to the left until the start of the reaction
4) Read off the temperature at the time when the reaction started
5) Compare this with the initial temperature reading to find the temperature change
(See pg 106 of revision guide)
Remember to practise drawing temperature-time graphs for a reaction and extrapolating to find the temperature change.
Pg 106 of revision guide
How can you measure the temperature change in a combustion reaction?
1) Set up a calorimeterr with the fuel underneath it.
2) Measure the temperature change of the reaction.
Describe the structure of a calorimeter.
- Spirit burner inside combustion chamber with air supply
- Combustion chamber surrounded by water tank
- Water tank has a stirrer and thermometer inside
(See diagram pg 106 of revision guide)
What is the problem with using a calorimeter to measure the temperature change of combustion?
Some heat is lost to the surroundings and apparatus.
What is the equation for the energy change in a calorimetry experiment?
Q = m x c x ΔT
Where:
• Q = Energy change (J)
• m = Mass of substance being heated (g)
• c = Specific heat capacity of substance being heated (J/g/K)
• ΔT - Change in temperature of substance being heated (K)
What is specific heat capacity?
The amount of energy needed to raise the temperature of 1g of a substance by 1K.
What is the specific heat capacity for water?
4.18 J/g/K
In the equation “Q = mcΔT”, what is m the mass of?
The substance being heated, NOT the fuel.
How can you work out the enthalpy change per mole of a calorimetry reaction?
- Measure the temperature change using the right method
- Calculate the energy change (Q) using “Q = mcΔT”
- Work out the number of moles of the limiting reactant / fuel
- ΔH = Q / Moles
In a lab experiment, 1.16g of an organic liquid fuel was completely burned in oxygen. The heat formed during this combustion raised the temperature of 100g of water from 17.5C to 80.0C. Calculate the standard enthalpy of combustion of the fuel. Its Mr is 58.0.
1) Q = m x c x ΔT
Q = 100 x 4.18 x (80.0 - 17.5) = 26125J = 26.125kJ
2) Moles = 1.16 / 58.0 = 0.0200 mol
3) ΔcH° = 26.125 / 0.0200 = -1310kJ/mol
Give some reasons why the final ΔcH° value in an experiment might be smaller than the quoted value.
1) Some heat may have escaped through the calorimeter
2) Incomplete combustion of fuel
3) Conditions not standard
Remember to ask teacher about the definition of ΔH - is it defo per mole.
Do it!
State Hess’s Law.
The total enthalpy change of a reaction is always the saem, no matter which route is taken.
What is Hess’s Law useful for?
Working out enthalpy changes that you can’t find directly by doing an experiment.
What are the diagrams for Hess’s Law called?
Enthalpy cycles.
How does an enthalpy cycle diagram work?
- Write the reaction you’re trying to calculate along the top
- Look at the values you are given
- Write the intermediate step along the bottom and draw the arrows for the values you are given
- Add any reverse arrows as dotted lines
- Work out the overall enthalpy change, remembering to multiply by the number of moles where necessary
In enthalpy cycles, when writing elements along the bottom line, what must you remember?
They must be in their standard state and as molecules (not atoms).
In enthalpy cycles, what are the two enthalpies you might need to use?
- Enthalpies of formation
* Enthalpies of combustion
SO2 (g) + 2H2S (g) -> 3S (s) + 2H2O (l)
Work out the standard enthalpy of reaction, given that:
ΔfH°[SO2 (g)] = -297 kJ/mol
ΔfH°[H2S (g)] = -20.2 kJ/mol
ΔfH°[H2O (l)] = -286 kJ/mol
-235 kJ/mol
See pg 108 of revision guide
2NH4NO3 (s) + C (s) -> 2N2 (g) + CO2 (g) + 4H2O (l)
Work out the standard enthalpy of reaction, given that:
ΔfH°[NH4NO3 (s)] = -365 kJ/mol
ΔfH°[CO2 (g)] = -394 kJ/mol
ΔfH°[H2O (l)] = -286 kJ/mol
-808 kJ/mol
See pg 108 of revision guide
2C (s) + 3H2 (g) + 1/2O2 (g) -> C2H5OH (l)
Work out the standard enthalpy of reaction, given that:
ΔcH°[C (s)] = -394 kJ/mol
ΔcH°[H2 (g)] = -286 kJ/mol
ΔcH°[C2H5OH (l)] = -1367 kJ/mol
-279 kJ/mol
See pg 108 of revision guide
Are the enthalpies of formation and combustion the only things that you might include in an enthalpy cycle?
No, you could also you different reactions with a common product, like neutralisation reactions.
Experimentally, how can you find the enthalpy of reaction for:
CaCO3 (s) -> CaO (s) + CO2 (g)
- You can’t simply measure the temperature change, because the reaction is endothermic but requires heating
- Instead, carry out a neutralisation reaction with HCl of both CaCO3 and CaO
- Then draw an enthalpy cycle to work out the enthalpy of reaction
CaCO3 (s) -> CaO (s) + CO2 (g)
Work out the standard enthalpy of reaction, given that:
ΔH1 = CaCO3 + 2HCl -> CaCl2 + CO2 + H2O
ΔH2 = CaO + 2HCl -> CaCl2 + H2O
Top line of cycle: CaCO3 (s) + 2HCl -> CaO (s) + CO2 (g) + 2HCl
Bottom line of cycle: CaCl2 + H2O + CO2
Therefore, the enthalpy change = ΔH1 - ΔH2
(See pg 109 of revision guide)
What is the standard enthalpy of formation for elements?
0
Remember to revise Hess’s law and enthalpy cycle diagrams.
Pg 108-109 of revision guide.
Is breaking bonds endothermic or exothermic?
Endothermic
Is forming bonds endothermic or exothermic?
Exothermic
Does breaking bonds give a positive or negative ΔH value?
Positive
Does forming bonds give a positive or negative ΔH value?
Negative
If the energy in breaking bonds is greater than in forming them, is ΔH positive or negative?
Positive
If the energy in breaking bonds is smaller than in forming them, is ΔH positive or negative?
Negative
Why is energy needed to break bonds?
You need energy to overcome the attraction between the bonded atoms.
What is bond enthalpy?
The amount of energy required to break 1 mole of a type of bond in a molecule in the gas phase.
What is it important to remember when defining bond enthalpy?
The molecules are in the gas phase.
Are bond enthalpies positive or negative?
Positive, because breaking bonds is always an endothermic process.
What is mean bond enthalpy?
The energy needed to break one mole of bonds in the gas phase, averaged over many different compounds.
Is the bond enthalpy of a certain bond always the same?
No, it varies between compounds, so it is usually quoted as a mean bond enthalpy in data books.
Compare the bond enthalpy of the two O-H bonds in water?
- First OH -> +492 kJ/mol
- Second OH -> +428 kJ/mol
The second bond is easier to break because of the extra electron repulsion.
What is the equation for the enthalpy change of a reaction in terms of bond enthalpies?
Enthalpy change = Sum of bond enthalpies of reactants - Sum of bond enthalpies of products
(Enthalpy change = Energy absorbed to break bonds - Energy realeased in making bonds)
N2 (g) + 3H2 (g) -> 2NH3 (g)
Work out the overall enthalpy change given that:
N≡N -> 945 kJ/mol
H-H -> 436 kJ/mol
N-H -> 391 kJ/mol
Bonds broken: 945 + (3 x 436) = 2253 kJ/mol Bonds formed: 6 x 391 = 2346 kJ/mol Enthalpy change: ΔH = 2253 - 2346 = -93 kJ/mol
In enthalpy calculation using bond enthalpies, why might there be some variation between the calculated value and the actual value?
1) The specific bond enthalpies of the molecules may be slightly different from the mean values given
2) In an experiment, the molecules may not be in their gaseous state, so the bond enthalpies are different
N2 (g) + 3F2 (g) -> 2NF3 (g)
The enthalpy change of the reaction is -264.2 kJ/mol. Find the mean N-F bond enthalpy, given that:
N≡N -> 945 kJ/mol
F-F -> 158 kJ/mol
Bonds broken: 945 + (3 x 158) = 1419 kJ/mol Bonds formed: 6 x N-F Enthalpy change: -264.2 = (945 + 474) - (6 x N-F) N-F = [945 + 474 -(-264.2)] / 6 = +281 kJ/mol
