3.1.4 - Energetics Flashcards
What is enthalpy
Heat energy
What is enthalpy change
Heat energy change measured under conditions of constant pressure
Calculation for enthalpy change
Enthalpy of products - enthalpy of reactants
What is a exothermic reaction
Gives out heat energy to surroundings
Explain an exothermic reaction and show it’s graph
- loss of enthalpy
- enthalpy change = negative
- temperature goes down
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What is endothermic reaction
Takes energy from surroundings
Explain endothermic reaction and it’s graph
- gain of enthalpy
- enthalpy change is positive
- temperature goes down
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What are standard conditions
Pressure of 100kPa
And a stated temperature
Define standard enthalpy change of formation
The enthalpy change when 1 mol of a compound is formed from its elements under standard conditions, all reactants and products being in standard states
What is enthalpy of formation of an element
Zero
Define standard enthalpy of combustion
The enthalpy change when 1 mol of a substance is completely burned in excess oxygen under standard conditions, all reactants and products being on standard conditions
What type of reaction is standard enthalpy of combustion
Always exothermic so is always negative
Define heat capacity
Amount of heat required to raise temperature by 1 K
Equation for heat capacity
q= C 🔺t
Define specific heat capacity
Amount of heat required to raise 1kg of a substance by 1K
Equation for specific heat capacity
q = m c 🔺t
Equation for enthalpy change
🔺H = -q/n
What does it mean if enthalpy change is positive and what does it mean if it is negative
- is exothermic
+ is endothermic
Assumptions made in calorimetry
All energy is transferred
Heat is not escaping to surroundings
All heat is absorbed
State hess’s law
The overall enthalpy change is independent of route take
Using standard enthalpies of formation
🔺rH
Reactant ----->products
\ /
\🔺fH / 🔺fH
\ /
Elements in standard
StatesCalculating standard enthalpies of formation
Elements
Stand states ---> compound
\ /
\ 🔺cH / 🔺cH
\ /
Combustion productsDefine mean bond enthalpies
The energy needed to break 1 mol of particular gaseous covalent bond averages over several compounds where the bond exists
What is symbol for mean bond enthalpies
-
E
Define dissociation energy
Refers to specific bond in a specific comping
What are the only molecules with same bond enthalpies
Diatomic molecules
How to use mean bond enthalpies
1) draw the molecule
2) write and equation using hess’s law
3) bond broken - bonds formed
