Energetics Flashcards
Exothermic Processes
Energy is released from the chemical to the surroundings during the process
The temperature of the surroundings increases
ΔH is negative
Endothermic Processes
Energy is taken in by the chemical from the surroundings during the process
The temperature of the surroundings decreases
ΔH is positive
Enthalpy Changes definiton
heat energy change at constant pressure (kJ mol-1)
Standard Conditions
- 100 kPa pressure
- 298 K temperature
- 1.0 mol dm^3 concentration
- When an enthalpy change is measured under standard conditions it is written as ΔHθ.
Enthalpy level diagrams
Breaking bonds requires energy and is an endothermic process
Making bonds releases energy and is an exothermic process
*need initial input of energy- Ea
Ea definiton
Activation Energy, Ea, is the minimum needed energy to start a reaction
Mean bond enthalpy definition
heat energy required to break one mole of a covalent bond into gaseous atoms, averaged over a range of different compounds
E.g. O2(g) → 2O(g)
- always endothermic
- more positive bond enthalpy, larger the amount of energy needed to break bond= stronger bond
Standard enthalpy of formation
The enthalpy change when one mole of substance is formed from its constituent elements under standard conditions with all reactants and products being in their standard states.
e.g. H2(g) + ½O2(g) → H2O(l)
-ΔHf of all elements in standard state = 0
-why would reaction not show ΔHf - forms two moles not one
Hess cycle = ↑
Standard enthalpy of combustion
The enthalpy change when one mole of a substance is completely burnt in excess oxygen under standard conditions, all reactants and products being in their standard states
e.g. CH4(g) + 2O2(g) CO2(g) + 2H2O(g)
- Substances that cannot be combusted like water, carbon dioxide and most other oxides, have zero enthalpy of combustion
- always exo, always negative
- difficult to measure = incomplete combustion
Hess cycle = ↓
Enthalpy of Reaction
any reaction that does not match any of the above
Hess’ Law definition
the enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products.
Bond enthalpy Calculation
BERP
reactants - products
Oxygen bond
O2 only substance with O=O bond
Why is databook value different
databook derived from different compounds
calorimetry equations
Q=mcΔT (Q/1000)
ΔH=Q/n
Combustion calorimetry
Temp of water ↑ = exo, -ΔH
Temp water ↓ - endo, +ΔH
use mass of water
Calorimetry with two solutions reacting
- use total solution vol when calculating mass
- use moles of limiting factor - lower mol
Calorimetry- adding solid to solution
- only use mass of water
- don’t use excess moles
Assumptions made in calorimetry
energy transferred from reacting chemicals is equal to energy released by reaction
Combustion Calorimetry Sources of error
- Heat loss to the surroundings
- Incomplete combustion of the fuel
- Heat energy transferred to the metal calorimeter
- Some fuel evaporates
Improvements to Minimise Sources of Error
* Add a lid – reduces heat loss
* Insulate sides of calorimeter – reduces heat loss
* Reduce distance between flame and beaker – reduces heat loss
* Put sleeve around flame to protect it from draughts
Solution Calorimetry Sources of error
- Heat loss to the surroundings
Improvements to Minimise Sources of Error
* Add a lid – reduces heat loss
* Insulate calorimeter – reduces heat loss
HEAT LOSS BIGGER ISSUE THAN UNCERTAINTY
Enthalpy Change Using a Cooling Curve
1)
-measure water with measuring cylinder (100cm3)
-into polystyrene cup
-add known mass of MgCl
2)
-record initial temp until constant
-min 5 solid added
- record temp at regular intervals
-cooling till trend established
3)
Q=mcΔT
ΔH=Q/n
