Module 3 chapter 9- Need to finish calormetry Flashcards
What is enthapy?
When a chemical reaction occurs there is normally a change in energy,
The enthapy change of a reaction is the heat change in a reaction at constant pressure
UNITS of enthalpy
KJmol-1
what are the standard conditions?
1- 100kpa pressure
2- 298K (25degrees)
What is endothermic reaction?
Absorb energy from the surroundings
Products are higher in energy than reactants
Enthalpy is positive
EXAMPLE of endo
Thermal decomposition of calcium carbonate
What is exothermic?
- Release energy to the surroundings
-Reactants are higher in energy than the products
- Change in enthalpy is negative
Example of exothermic
Combustion of ethane
4 type of enthalpy changes you need to know of
1- Standard enthalpy change of reaction
2- Standard enthalpy change of naturalization
3- Standard enthalpy change of formation
4-Standard enthalpy change of combustion
What is standard enthalpy change of reaction?
The enthalpy change of a reaction to the molar quantities in the equation under standard conditions
What is a standard enthalpy change of neutralization?
The enthalpy change when an acid and alkali react to form 1 mole of water under standard conditions
What is standard enthalpy change of formation?
The enthalpy change when 1 mole of a compound is formed from its elements its their standard states under standard conditions
What is standard enthalpy change of combustion?
The enthalpy change when 1 mole of a substance is completely burned in oxygen to make co2 and H20 under standard conditions
Bonds making and breaking
Bonds are broken and made during a reaction
What happens when bonds are broken
to break a bond, energy need to be absorbed
Bonds are broken in reactants and this is endothermic process so enthalpy change is positive
if more energy is need to break bonds than energy given out, this reaction is endothermic
What happens when bonds are made?
When bonds are made energy is released
Bonds are made when products are being produced and this is an exothermic process so enthalpy change is negative
if more energy is released when bonds are formed than what was needed to break initial bonds the reaction will be exothermic
mean bond enthalpy
Bond enthalpy varies for even the same time of bond, the energy needed to break the first C-H will be different to the energy needed to break the second C-H
We can measure the enthalpy of breaking all 4 bonds and divide by 4 to get a mean and that is our mean bond enthalpy
What is the bond dissociation enthalpy?
The energy needed per mole to break attractive forces in bonds is called the BDE.
Only happens in gaseous compounds
How to work out enthalpy change using mean bond enthalpy
Total energy released from forming bonds (exothermic - products)
Calorimetry (practical technique)
Calorimetry is used to work out the enthalpy change of combustion of a fuel
How can energy from calorimetry experiment be calculated?
q = mc(delta)T
q= heat energy lost or gained (J)
m= mass of water or/in? solution (g)
c= specific heat capacity pf water (4.18Jg-1K-1) given in exams
deltaT= change in temp (K)
IMORTANT!
heat energy is different to enthalpy
How to work out enthalpy from energy ?
1- work out energy (covert to KJ)
2- Work out the moles of fuel
3- Work out enthalpy (q/moles)
What is Hess’s Law
The total enthalpy change of a chemical reaction is independent of the route taken
Why do we have Hess’s law?
Germain Hess came up with a law to work out enthalpy changes you cant find out by doing an experiment
What are the two Hess’s cycles?
1- Formation cycles
2- Combustion cycles
One thing to remember about Hess’s cycles
Start anywhere on the cycle and add up the numbers until you go back where you started. FLIP THE SIGN IF YOU GO AGAINST THE DIRECTION OF THE ARROW. It should add up to ZERO
