Unit 2.1 - Thermochemistry Flashcards
How do chemists measure energy changes?
When a system changes from one energy state to another
NOT the actual energy of the system
Give 2 examples of reactions that release heat/energy to their surroundings
Burning of fuels
Neutralisation of acids by alkalis
What are energy changes measured in?
kJmol-1
Law of conservation of energy
Energy is neither created nor destroyed in a chemical reaction, but only changed from one form into another form
What do closed systems allow the transfer of?
Energy, not mass
What type of system allows the transfer of energy but not mass?
Closed system
Are all energy changes obvious?
Some are (e.g - coal burning) but some go unnoticed
Enthalpy change definition
At constant pressure and temperature, the heat energy which is gained or lost in a chemical reaction is known as the Enthalpy change
Enthalpy change symbol
ΔH
What type of enthalpy of a substance is impossible to measure?
The absolute enthalpy
What can be calculated as opposed to an absolute enthalpy?
An Enthalpy CHANGE that occurs during a chemical reaction (enthalpy of products - enthalpy of reactants)
How come enthalpy changes are used?
It’s easier to carry out measurements at constant pressure
ΔH unit
kJmol-1
Exothermic reactions
Involve the loss of heat to the surroundings
Enthalpy change of an exothermic reaction + explanation
Negative
Energy is lost
3 examples of exothermic reactions
Combustion
Respiration
Bond forming
Endothermic reactions
Involve the gain of heat to the surroundings
Enthalpy changes of endothermic reactions + explanation
Positive
Energy is gained
Give 4 examples of endothermic reactions
Photosynthesis
Thermal decomposition of calcium carbonate
Ice packs
Bond breaking
Draw and label an exothermic energy graph
(See notes)
Draw an label an endothermic energy graph
(See notes)
What type of enthalpy does the activation energy of the forward reaction - backward reaction of an exothermic reaction give?
Negative
What type of enthalpy does the activation energy of the forward reaction - backward reaction of an endothermic reaction give?
Positive
Which is highest in an exothermic reaction - the enthalpy of products or reagents?
Reagents
Which is highest in an endothermic reaction - the enthalpy of products or reagents?
Products
Observation we would make in an experiment in which an exothermic reaction occurs
Tube feels hot
Observation we would make in an experiment in which an endothermic reaction occurs
Tube feels colder
Hess’s Law
The enthalpy change which takes place in a chemical reaction is independent of the path which the reaction takes
Why is Hess’s Law definitely true?
If it weren’t, it would be possible to CREATE energy in a chemical reaction (conservation of energy!!)
What’s Hess’s Law practically a restatement of?
The law of conservation of energy
Draw and label a stereotypical Hess’s Law energy cycle
(See notes)
Make sure it shows that
ΔH1 = ΔH2 + ΔH3 + ΔH4
What must be known in order to compare the enthalpy changes of various reactions?
Standard conditions
What ARE standard conditions?
Temperature - 298K (25 degrees Celsius)
Pressure - 1.01x10^5 Pa (1 atm)
Concentration of ant solution used under standard conditions
1moldm^-3
What are vital that we write out in this unit?
State symbols
(g) (l) (aq) (s)
How is enthalpy change written to represent standard conditions?
ΔH°
What does ΔH° represent?
Enthalpy change of standard conditions
Standard state of an element
The most stable state of an element at standard conditions
What’s the enthalpy change of formation of elements in their standard states?
Zero
Which elements have an enthalpy change of zero?
Elements in heir standard states
3 examples of elements in their standard states
Al (s), O2 (g), C (s)
What’s the standard state of carbon?
Graphite
C (s)
Enthalpy change of formation symbol
ΔH°f
Enthalpy change of reaction
The enthalpy change when the reactants are converted into products in a chemical reaction
So, the enthalpy change is the overall enthalpy during the reaction
Standard molar enthalpy change of formation
The enthalpy change when 1 mole of a compound is formed in its standard state from its elements in their standard states under standard conditions
What type of reaction is formation?
Usually (not always) exothermic
How is carbon usually taken and why?
As the graphite allotrope - most stable form of carbon
What do we need to remember when balancing equations for standard enthalpy change of formation?
Only 1 mole of product on the RHS of the equation
Enthalpy of formation of elements in their standard states
Zero
What also has an effect on the value of ΔH°f? Give an example
The physical states of the reagents and products
E.g - Formation of steam as opposed to liquid water has a lower value
Why is the value for ΔH°f lower when forming steam as opposed to liquid water?
Enthalpy change for the conversion of liquid water into steam is also required, which is endothermic
Standard enthalpy of combustion definition
The enthalpy change when 1 mole of a substance undergoes complete combustion under standard conditions
All reactants and products are in their standard states
Standard enthalpy of combustion symbol
ΔH°c
What type of reaction is combustion?
Always exothermic
What do reactants react with during combustion?
O2
What do reactants form during combustion?
C02 an H20
What do we need to remember when balancing combustion reactions?
Always only 1 mole of whatever is burning on the LHS of the equation
What do you do to an enthalpy change value if you go in an opposite direction (i.e - clockwise and anticlockwise)?
Subtract the enthalpy change value
What does using Hess’s Law enable us to calculate the enthalpy changes from?
Changes which cannot be measured directly (e.g - lattice enthalpy)
ΔH°of reaction from bond enthalpy
ΔH° of reaction from ΔH°c
ΔH° of formation from ΔH°f
What’s an alternative way in which a reaction could proceed so that we can use Hess’s law?
Convert all the reactants into their constituent elements in their standard states and then convert them into products
Which 2 ΔH°’s are equal to according to Hess’s Law?
Clockwise and anticlockwise
What do we need to know to calculate the enthalpy change of ANY reaction and using which method?
If we know the enthalpies of formation of all the reactants and products, remembering that route 1 = route 2 (clockwise and anti-clockwise) enables us to rearrange the equation and calculate it
The enthalpy of formation of elements in their standard states
Always zero
Equation for using enthalpies of formation to calculate the enthalpy change
ΔH = Σ[ΔHf(products)] - Σ[ΔHf(reactants)]
What type of compounds all burn in air to give CO2 and H20?
Organic compounds
What would be equal in amounts when burning during combustion?
If you b urned all the products, you should get the same amounts of oxidation products such as CO2 and H2O as it you burned the reactants
What would be an alternative way for the following reaction to work out?
C (s) +2H2 (g) —> CH4 (g)
Convert the reactants to CO2 and H2O by combustion and to convert CO2 into methane and oxygen (i.e - reverse combustion)
What do we need to do to the values of enthalpies of formation and combustion if there’s more than 1 mole of that in the equation?
Multiply them up so that there’s enough
Which way do the arrows face in a combustion reaction in a Hess’s law energy cycle?
Towards the combustion products
Which way do the arrows face in an enthalpy of formations calculation?
Towards the actual compounds in the equation fro the elements in their standard states
Which formula with calculating enthalpy change using enthalpy of combustion can be used?
ΔH = Σ[ΔHc(reactants)] - Σ[ΔHc(products)]
(Reactants and products are flipped compared to the enthalpy of formation reaction)
What has to be the reactant for combustion?
a hydrocarbon
What are the substances placed in for measuring the energy change in a reaction in solution?
A calorimeter
Equation for energy changes in solution
Q = -mcΔT
What’s the equation Q = -mcΔT used for?
Measuring the energy change in solution
What’s c in Q = -mcΔT?
Specific heat capacity (constant)
What’s Q in Q= -mcΔT?
Energy change (J)
What’s m in Q= -mcΔT?
Mass of solution (g)
What’s ΔT in Q = -mcΔT?
Temperature change (K or Celsius)
Describe the method for measuring the energy change in a reaction in solution
- A known amount of one of the substances is placed in a calorimeter (e.g - an aqueous solution of known concentration)
- The temperature of the solution is noted
- The other substance is added and the mixture is stirred
- The change in temperature of the mixture is measured my measuring the temperature at set time intervals
- Plot temperature v.s time
- Maximum temperature rise is found by extrapolation
- Energy change is measured using
Q = -mcΔT - The value of Q is scaled up to 1 mole
ΔH = -mcΔT
————
n
(Remember to divide by 1000 too to get the answer in kJmol-1, the unit of energy changes)
The mass of what do we use for calculating enthalpy changed in solution?
The mass of the solution, not the solid
What would 25cm^3 of HCl be in terms of mass and why?
25g as acid is made up of water that has a density of 1gcm^3
cm^3 to g
The same
Equation for the value of Q being scaled up to 1 mole for calculating enthalpy changes in solution
ΔH = -mcΔT
———
n
What’s n in ΔH = -mcΔT
———
n
Moles responsible for energy change Q
Why is there a minus sign in Q = -mcΔT?
Exothermic reaction leads to temperature increase
Which value for the moles do we always choose to divide by when calculating enthalpies in solution?
The smallest moles value (not the one in excess)
How do we know which moles value is in excess?
It’s a larger value than the other
Why is it important for acid to be in excess during a reaction?
For the reactant to fully react for the maximum possible energy change
How can we ensure the maximum possible energy change for a reaction in solution?
Add the acid in excess for the reactant to fully react
What’s the final step when working out the enthalpy change of a solution?
Change from Jmol-1 to kJmol-1 by dividing by 1000
Calorimeter
An apparatus used for measuring the amount of heat involved in a chemical reaction
When does the rule for the enthalpy of elements in their standard states being zero not apply?
During combustion of elements- it’s only for the formation of elements
Name four substances for which the energy of combustion is zero
CO2
H2O
O2
Noble gases
What would happen to the value of the enthalpy change of a reaction if nitric acid were added instead of hydrochloric acid? Why?
It would stay the same
Nitric acid is also monoprotic and so would react in the same ratio as hydrochloric acid
Standard enthalpy change of reaction definition
The enthalpy change that occurs when the quantities of reactants shown in the equation must react under standard conditions with all reactants and products in their standard states
Where should energy change reactions be carried out and why?
In an insulated container to prevent heat energy loss to surroundings
Why can some enthalpy changes not be measured directly?
Other products may form
Elements may not all react with each other under standard conditions
Why would a value for an enthalpy change be lower than expected?
incomplete reactions
heat loss
What do we use as the mass when using q = -mc∆T if we have two liquids?
Add the mass of the two liquids
What tends to happen to the value of the energy change measured in a reaction?
Tend to be underestimated
Why do the energy changes of a reaction tend to be an underestimate of the actual energy change in a reaction?
Due to heat loss to the surroundings
How do we minimise heat loss to the surroundings when measuring energy changes?
Place a lid on the container
Improve insulation
On our endothermic and exothermic graphs for calculating enthalpy changes, what’s our calculation for working out the change in temperature?
∆T = final - initial
What’s the ∆T from the endothermic graph?
negative
What’s the ∆T from the exothermic graph?
Positive
Why do we use extrapolation on graphs to calculate the temperature changes of a reaction?
It takes into account heat loss to surroundings
With diatomic molecules, what does the process of calculating the enthalpy change correspond to?
The breaking of the individual covalent bonds
In what type of molecules does calculating the enthalpy change of bond dissociation correspond to the breaking of individual covalent bonds?
Diatomic gas molecules
Bond dissociation enthalpy
A quantitative measure of the bond enthalpy
What does a greater bond energy for bond dissociation enthalpy represent?
Stronger bonds
What represent stronger bonds with bond dissociation enthalpy?
Greater bond energy
In what type of molecules are average bond energies used to calculate the enthalpy change of a reaction?
More complex molecules
How is the enthalpy change of a reaction determined using bond energies for more complex molecules?
Using average bond energy
Average bond energy
The average energy required per bond to break the bonds in the molecule
Can the bond energy for one compound be used for another compound?
Yes
In reality, are bond energies the same in all compounds? Why?
No, due to the environments of compounds, so we simply use these bond energy figures as estimates
How do we complete calculations with bond energies and to determine what?
Apply Hess’s law to determine the enthalpy change of a reaction
Formula for calculating the enthalpy change of a reaction using bond energies
Enthalpy change of reaction = sum of the bonds broken - sum of the bonds formed
Acronym + definitions to help remember the formula for the enthalpy change of a reaction using bond energies
“BERP”
“Bond enthalpies = R - P”
What should we do when working out the enthalpy change of a reaction using bond energies and why?
Re-write the equation showing all bonds present to reduce the risk of not counting any bonds
Do we use molar ratios when dealing with bond energy for enthalpy changes?
Yes
Average bond enthalpy
The energy required to break 1 mole of a bond in a gaseous species under standard conditions
What’s important to remember when calculating the enthalpy change of a reaction using bind enthalpies when CO2 is involved?
Draw CO2 with double lines!
What often makes it difficult to measure the enthalpy change of a reaction directly?
No water or solution- can’t measure the temperature change
How do you calculate the energy released per gram of fuel burned? Explain
Total energy released
——————————
Mr
kJmol-1
————
gmol-1
kJg-1
(Energy per gram)
What can average bond enthalpies vary depending on?
The environment
What’s the relationship between the enthalpy of formation value and the stability of an oxide?
More negative = more Stable
What assumptions are made when finding enthalpy change values form experimental results?
No heat loss
Reaction is complete
Density of solution is 1gcm^-3
Specific what capacity is the same as water
(Think of the equation)
How would a reactant being damp change our experiment for calculating the enthalpy change of a reaction?
Less exothermic
Less to react
Lower temperature rise
What is it important to show in the diagram for showing the enthalpy change of a reaction?
Thermometer
Lid
Insulated polystyrene container with reagents
Why are averages used for bond enthalpies?
Each individual bond will be in a different environment and so will have a different strength
Why is it difficult to measure the standard enthalpy change of formation of ethanol directly?
Activation energy for the reaction is too high
C, H and O will likely form other products when reacting
C, H and O don’t easily react under these (room temperature) conditions
Why is it difficulty yo determine the enthalpy change of combustion of ethane?
It’s a gas, so there’s practical problems
How can we minimise heat loss to surroundings when using a spirit burner to determine the enthalpy change of combustion, except from better insulating the apparatus?
Reduce the distance between the flame and the beaker
What affect do catalysts have on enthalpy changes?
None
Why should the enthalpy change of combustion of isomers of a molecule all be similar?
Same number of bonds
Same types of bonds
The energy absorbed by breaking bonds and the energy released by forming bonds will be similar
What’s a good reason to maintain an excess of oxygen when burning a fuel?
More energy released
Which equation is used for “heat released” in a reaction?
q = mc(delta)T
(q = heat, so if this is all the question asks for, no need to work out the enthalpy change of reaction)
What is q equal to?
Heat
