Chapter 9 - Enthalpy Flashcards
What is Enthalpy (H)?
A measure of the heat energy in a chemical system - the energy stored within bonds
Why can Enthalpy not be measured?
Because it is the energy of the system - the atoms, molecules and ions making up chemicals so cannot directly be measured
Enthalpy CHANGE can be measured
Difference in enthalpies ΔH?
H(products) - H(reactants)
Can be positive or negative depending on whether the products contain more or less energy than the reactants
Conservation of energy?
Law of conservation of energy states that energy cannot be created or destroyed
What happens when a chemical reaction takes place?
Energy is transferred between system and surroundings - system being the chemicals and surrounding being the apparatus/lab
What is the universe?
System + surroundings
How can Enthalpy change be measured?
Measuring energy transfer between system and surroundings
From system to surroundings = exothermic
From surroundings to system = endothermic
Exothermic
Heat out of the system to surroundings
ΔH is negative
Temperature of surroundings increases as they gain energy
System loses energy
Reactants higher than products in Enthalpy profile diagram
Endothermic
Heat into the system from the surroundings
ΔH is positive
Chemical system gains energy
Surroundings lose energy
Temperature of surroundings decreases
Products are higher than reactants in Enthalpy profile diagram
What is activation energy?
The energy input required to break bonds - minimum energy required for a reaction to take place
Bonds need to be broken by taking in energy - endothermic
How to label activation energy on Enthalpy diagrams?
From reactants to peak of curve
How does activation energy affect reaction?
Small activation energy reactions take place very rapidly because the energy needed to break these bonds is readily available from the surroundings - very large activation energies may present an energy barrier so the reaction takes place extremely slowly or even not at all
What do chemists use to keep all data similar?
They use standard conditions for physical measurements like Enthalpy changes - close to typical working conditions of temperature and pressure
Standard conditions?
A standard physical value, like enthalpy, is shown in data tables using a special standard sign Θ (indices)
Units are kj/mol
Standard pressure
100kPa
Standard temperature
298K ; 25 degrees Celsius
Standard concentration (for solutions only)
1m mol dm^-3
Standard state
Physical state of a substance under standard conditions
What is the standard Enthalpy change of a reaction?
Enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states
What does a standard Enthalpy change of a reaction always refer to?
A stated equation - and it’s value depends on the balancing numbers, for formation this may be 1 mol and this changes if the moles of the equation are doubled
Then the Enthalpy change is also doubled
Standard Enthalpy change of formation?
Enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states
Why is it wrong to use whole balancing numbers for the Mg + 1/2O2 -> MgO equation?
Would not match the 1 mol definition of formation of MgO ; BALANCE THE EQUATION TO GIVE 1 MOLE OF THE PRODUCT
All elements have an Enthalpy change of formation of?
0kJ/mol - already in their elemental form
Standard Enthalpy change of combustion
Enthalpy change that takes place when 1 mol of a substance reacts completely with oxygen under standard conditions, with all the reactants and products in their standard states
When a substance reacts completely with oxygen the products are?
Oxides of the elements in that substance
In the case of combustion - what requires 1 mol?
The reactant - do not balance without fractions… need reactant to be 1 mol of C4H10
Standard Enthalpy change of neutralisation?
Energy change that accompanies the reaction of an acid by a base to form 1 mole of H2O(l), under standard conditions, with all reactants and products in their standard states
When writing standard Enthalpy definitions what must you do?
LIST OUT THE STANDARD CONDITIONS (TEMP/CONC/PRESSURE ETC)
Neutralisation reactions are generally?
Exothermic
What is special about the standard Enthalpy change of neutralisation?
Involves the reaction of H+ (aq) with OH- (aq) to form one mole of H2O ; therefore the value is the same for all neutralisation reactions (-57 kj/mol)
How to get from degrees Celsius to Kelvin?
+273
What is the thermometer a part of?
The surroundings - thus you measure temperature change of surroundings
1K rise in temperature
= 1 degrees Celsius rise in temperature - thus temperature change for both is exactly the same
What 3 variables are needed in calculating energy change?
Mass, SHC and temperature change
Mass (q=mΔct)
Measured by simply weighing ; measured in grams and is of the materials that are changing temperature
SHC
Energy required to raise the temperature of 1g of a substance by 1K ; this value varies depending on if it is a conductor or an insulator
If conductor then small value
If insulator then large value (takes time and thus energy for it to actually transfer energy and heat up)
What is SHC of water?
4.18J/g/K
Temperature change ΔT
T(final) - T(initial)
Thermometer reading
What is the formula for energy change?
q=mcΔT
The final value is in Joules
What is m in the energy change formula?
The mass that CHANGES TEMPERATURE not mass of the reactants
Enthalpy change of combustion
1) Measure out a certain volume of water and pour this into the beaker ; record the initial temperature of the water to the nearest 0.5 degrees
2) Add methanol to the spirit burner and weigh it
3) Place the spirit burner under beaker (clamped) and burn the methanol while stirring the water with thermometer
4) After about 3 minutes (more than 15 degrees increase in temperature) ; extinguish flame, put on cap and re weight immediately
5) Assume wick has not been burnt
Determination of standard Enthalpy change of combustion
MASS OF WATER WHICH HAS INCREASE IN TEMPERATURE (FROM VOLUME BECAUSE DENSITY IS 1g/CM^-3)
CHANGE IN TEMPERATURE (THERMOMETER)
4.18 - SHC
MULTIPLY THEM ALL AND DIVIDE BY 1000 TO CONVERT INTO kJ
DIVIDE THIS BY THE NUMBER OF MOLES OF METHANOL BURNT (MASS/MR)
THIS GETS YOU ENERGY CHANGE/MOL
HAS GAINED TEMPERATURE SO EXOTHERMIC THUS NEGATIVE SIGN
Why are experimental combustion values not that accurate?
1) Heat is dissipated to the surroundings ; OTHER than water - to the air surrounding the flame
2) Incomplete combustion of methanol ; carbon monoxide and carbon produced instead of carbon dioxide… can see the carbon layer of soot on the beaker
3) Evaporation of methanol from the wick - burner must be weighed as soon as possible after extinguishing the flame. Otherwise some methanol may have evaporated from the wick
4) Non-standard conditions - unlikely to be identical to standard conditions
FIRST 3 REASONS MEAN THE VALUE IS LESS EXOTHERMIC THAN EXPECTED
How to minimise errors from heat loss and incomplete combustion?
Use of drought screens and input of oxygen gas
How to determine Enthalpy change of a reaction?
Between 2 solutions or a solid and a solution
Can be determined using plastic cups made of polystyrene foam - cheap and waterproof ; offer insulation against heat loss to the surroundings
What is special about carrying out reactions between aqueous solutions?
Solution itself is the surroundings - chemical particles react when they collide and energy transfer is between particles and water in the solution ; a thermometer in the solution will record any temperature change, allowing heat energy change to be calculated using q = mcΔT
How to determine standard Enthalpy change for solid and a solution?
Work out q=mcΔT with the data given
Divide that by the moles of the limiting reactant
Stoichometry if need be
What are cooling curves?
An alternative method that has been adapted to correct for heat loss
Start by getting a constant temperature first (of your first reactant)
Add in your second reactant and immediately start the timer and thermometer
Record temperature every 30 seconds until temperature has fallen for several minutes
Extrapolate by drawing a line back (of the cooling) against a vertical line straight up from when the solutions were mixed - THIS WILL GIVE YOU TEMPERATURE CHANGE TO BE USED IN AN EXPERIMENT
Do graph question pg 133
Done 😊
Determination of Enthalpy change of neutralisation?
Same thing as that of reaction but this time two solutions react
Figure out q and use mass (volume of BOTH SOLUTIONS ADDED)
Moles of both - should be the same (use limiting reactant if not)
Definition of standard Enthalpy change of neutralisation is 1 MOL OF H2O therefore use stoichometry
DIVIDE Q/moles
Put sign
Average bond Enthalpy?
Energy required to break 1 mole of a specified type of bond in a gaseous molecule
Bond breaking?
Endothermic
Requires energy
Have a positive Enthalpy value
What is average bond Enthalpy?
A measurement of covalent bond strength
Limitations of AVERAGE bond enthalpies?
Varies depending on the chemical environment of the bond ; average is calculated from actual bond enthalpies in different chemical environments
Bond making?
Releases energy
Exothermic
Enthalpy change is negative
Overall reaction?
Determined by the difference between energy required for bond breaking and the energy released by bond making
Exothermic
Energy released when making bonds is greater than energy needed to break the bonds
Endothermic
Energy needed to break the bonds is more than energy released when making the bonds
Enthalpy change of a reaction?
Sum of bond enthalpies in reactants - sum of bond enthalpies in products
What is required for average bond Enthalpy calculations?
Need all species to be gaseous molecules - this often means that the calculated standard Enthalpy change of a reaction is not agreeing with standard conditions thus is NOT a standard Enthalpy change ; WOULD STILL NEED TO TAKE INTO ACCOUNT ENTHALPY CHANGE FOR H2O (G) CONDENSING INTO H2O (L) - EXOTHERMIC
