AS level chemistry - CH2 - Unit 2 - WJEC (Wales) specification:
These notes are based on the WJEC curriculum but can be used for other specifications. Very in-depth in all topics (8 units) describing basic chemistry.
Thermochemistry: Unit 2.1
Energetics
Enthalpy he total energy content of a system at a constant pressure (KJmol-1)
T
Energy required to break 1 mole of bonds
Enthalpy change The change in energy (ΔH)
Exothermic reaction where surroundings increase in temperature (the system has lost energy)
A
Negative enthalpy change (-ΔH)
Endothermic reaction where surroundings decrease in temperature (the system has gained energy)
A
Positive enthalpy change (+ΔH)
Standard conditions
● Fixed conditions for comparing enthalpy change
○ Enthalpy changes based on the conditions
● Elements that meet the standard conditions have 0 enthalpy
1. Temperature of 298K (25o)
2. Pressure of 1atm (101000Pa)
3. All substances are in their standard state
Standard enthalpy change tandard molar enthalpy change where all substances meet the standard
S
(ΔHθ) conditions
Standard molar enthalpy he enthalpy change when one mole of substance is formed from its
T
θ
change of formation (ΔfH
) constituent elements in their standard states under standard conditions
Standard molar enthalpy he enthalpy change when one mole of substance in its standard state is
T
θ
change of combustion (ΔcH
) completely combusted in oxygen under standard conditions
Carbon dioxide and water are the only products
C2H OH + 3O2 → 2CO2 + 3H2O
5
Standard enthalpy change of T he enthalpy change in a reaction between the number of moles of
θ
reaction (ΔrH
)
reactants shown in the equation for the reaction under standard conditions
Calculated from standard enthalpy changes of formation of all reactants
and products
θ
- ∆𝑟𝐻
= Σ∆𝑓𝐻(𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠) − Σ∆𝑓𝐻
𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠
Hess’s law
● The total energy change that occurs when a system changes from one state to
another is independent of the route taken to bring about the change
● An interpretation of the law of conservation of energy
35
,Average bond enthalpy
Bond breaking Anexothermicprocess - energy is absorbed
Bond formation Anendothermicprocess - energy is evolved
● Enthalpy change for a particular bond depends on the structure of the molecule
○ Therefore the values for bond enthalpy are usually an average
θ
ΔfH as an indication stability
θ
● The more negative the ΔfH
(exothermic) the easierthe compound is to form
○ More stable
Calorimetry
● Used to measure enthalpy changes
Temperature he average kinetic energy of the particles in a system
T
Independantto the number present
Measured by a thermometer
Heat he total energy in a given amount of substance
T
Dependanton how much of the substance is present
Measured using calorimetry
Specific heat capacity (c) The heat needed to raise the temperature of 1g of substance by 1K
● Measures the heat transferred to surrounding (not the heat of the system directly)
○ We assume all heat is exchanged with the solution only
36
, ΔrH between magnesium and sulphuric acid
Procedure 1
. Accurately weigh out approximately 0.11g of magnesium powder
2. Measure 50cm3 of sulphuric acid using a buretteand pour into a polystyrene cup
- Acid is inexcessbut the volume must be accurateto know the mass heated
3. Stir continuously using the thermometer and measure temperature every 30 seconds
- Must be at least 3 concordant readings
4. Add magnesium after 3.5 minutes
5. Stir continuously and measure temperature every 30 seconds until minute 7
6. Record results
Results ass of acid = 50g (50cm3 * 1gcm-3)
M
Specific heat capacity = 4.18
Temperature change = 9.7Co
Moles of magnesium = 4.53*10-3 (0.11/24.3)
−(50×4.18×9.7)
∆𝐻 = −3
4.53×10
37
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