Enthalpy Changes Flashcards
What is enthalpy
(H) is a measure of the thermal energy stored in a chemical system
T
What is the chemical system
Te atoms , molecules or ions making up the chemicals- everything else in the surroundings
Enthalpy change
(Change in H) , the change/difference in thermal energy between that stored in products and stored in reactants
Units of enthalpy
KJ mol-1
How do you work out change in enthalpy
H products - H reactants
Energy stored in the system - not making or breaking bonds
Positive or negative , depending on whether the products contain MORE or LESS energy than the reactants
Why do you use triangle H *
To sho the measurements were made under standard conditions and that the elements were in their standard states
1 atm/ 100kPA and 25 degrees/ 298K
Conservation of energy
Energy cannot be created or destroyed
Exothermic reaction
Change in enthalpy is NEGATIVE
Chemical system loses energy
Surroundings gain energy
Temperature of surroundings increases
E.g. Combustion of fuels
Endothermic reaction
Change in enthalpy is positive Chemical system gains energy Surroundings loose energy Temperature of surroundings decreases E.g. Thermal decomposition of CaCO3
Standard enthalpy change of reaction
The energy change associated with a. Givern reaction under standard conditions
Occurs in the molar quantities in the equation
HStandard enthalpy change of formation
Energy change that take place when one mole of a compound is formed from its elements in their standard condition
Enthalpy of formation of an element
0 kJ mol-1
Standard enthalpy change of combustion
Energy change hat takes pace when one mole of a substance is completely combusted
Standard enthalpy change of neutralisation
Energy change associated with the formation of 1 mole of water from neutralisation
Bond dissociation enthalpy
Amount of energy needed t break a bond per mole
Average bond enthalpy
Energy needed to break me mole of bonds in the gas phase averaged over many different compounds
Calorimertry?
Using q=mc change in T
What do q, m, c and change in t stand for
Q= heat exchanges with the surroundings, joules
M= mass of the substrate heated or cooled, grams
C= the specific heat capacity of the substance that is heated or cooled-energy required to raise the temperature of 1g of a substance by 1k, expressed as Jg-1k-1
Change in temp , K
273K
0 degrees C
How do you use calorimetery to measure enthalpy change
Use q=mct to calculate energy change for quantities used
Work out the moles of the reactants used
Divide q by the number of reactant not in excess to give change in H
Add a sign and a unit (kJmol-1)
How do you change Jmol-1 to kJ mol -1
Divide by 1000
Bomb calorimeter
Equipment that MINIMISES heat loss
Uses pure oxygen , to ensure COMPLETE combustion is achieved
Ensures it accurate
Specific heat capacity of water
4.18 J g-1k-1 in any reaction where the reactants are dissolved in water
How to work out density of solution
SAME as water - 1 gcm-3
E.g. 25cm 3= 25G
What is calorimeter
Type of experiment where stances are mixed in insulated container and temp rise is measured
Errors in calorimetric method
- energy transfer from surroundings (loss)
-approximation in specific heat capacity of soltiuion - assumes all solutions have same as water
Ignoring energy absorbed by apparatus
Reaction may be in compete or sow
Density f solution is taken to be the same as water
3 ways to measure enthalpy changes
Calorimetric method - using experiments
Hess’ law - can determine them INDIRECTLY
Bond enthalpies - no experiment has to be done
Average bond enthalpy
Energy required to break one mole of a specified type of bond in a gaseous molecule
Strength of caovalent bond
Why are bond enthalpies always endothermic
Positive
- breaking bond - energy is always required and more energy is used then given out
Limitations of bond enthalpies to calculate enthalpy change
An average bond enthalpy si calculated from actual bond enthalpies in different chemical environments
BUT - it can depend
How to work out enthalpy change from bond enthalpies
= sum of bond enthalpies in REACTANTS- PRODUCTS
WHAT IS hess’ law
The total enthalpy hinge for a reaction si independent of the route by which the chemical change takes place
If a reaction can take place by 2 routes+the starting and finishing conditions are the same - enthalpy change is Same for each route
How do you work out enthalpy change of e.g. Formation
Write a balanced symbol equation for the equation
Draw enthalpy cycle- elements at the bottom and all arrows going to equation
Add enthalpies
Follow the arrows and add them
OR I use the equation =sum of products-sum fo reactants
Combustion elements products
Co2 adn h20
What are standard conditions
100kPa and 25 degrees C
