A comprehensive in-depth, specification led, revision booklet of the 2015 spec chemistry AS level course. Written during AS studies by an AS student, meaning it is written in an easily comprehensible stance for any student wishing to find extra resources to revise from.
1. Know that standard conditions are 100kPa and a specified temperature, usually 298 K
2. Know that the enthalpy change is the heat energy change measured at constant pressure
Chemical and heat energy
The first law of thermodynamics states that, doing a chemical reaction, every cannot be crete or
destroyed. However, one form of energy can be conmveced into another.
1) Chemical Energy:
Chemical energy is made up of two components:
• Kinetic energy, which is a measure of the motion of the particles (atoms, molecules or ions) in a
substance
• Potential energy, which is a measure of how strongly thee participles interact with one another
(i.e. both attract and repel one another).
2) Heat Energy:
Is the portion of the potential energy and the kinetic energy of a substance that is responsible for
the temperature of the substance
The heat energy of a substance is directly proportional to its absolute temperature (i.e. the
temperate measured in Kelvin).
Enthalpy and enthalpy changes
Enthalpy is a measure of the total energy of a system
You cannot determine the enthalpy of a system, but you can measure the enthalpy change (∆H)
that takes place during a physical or a chemical change.
The enthalpy change of a process is the heat energy that is transferred between the system
and the surroundings at constant pressure.
Exothermic and endothermic precess and reactions
Exothermic - Where heat energy is transferred from the system to the surroundings
Endothermic - Where heat energy is taken into the system from the surroundings
It is important to note that the ∆rHϴ is for the formula written only. As in each case per mole
regards to per mole of the reaction not the reactants or products.
3. be able to construct and interpret enthalpy level diagrams showing an enthalpy change,
including appropriate signs for exothermic and endothermic reactions.
Activation energy is not shown in enthalpy level diagrams but it is shown in reaction profile
diagrams.
How to construct and interpret enthalpy level diagrams
In an exothermic reaction, the final enthalpy system is less than its initial enthalpy. The reverse is
true for endothermic processes. This is shown in these two enthalpy level diagrams.
The change in enthalpy, ∆H is given by:
∆H = H products - H reactants
For an exothermic reaction H reactants > H Products so, ∆H is negative
For an endothermic reaction H reactants < H products so, ∆H is positive
Key points when constructing an enthalpy level diagram:
1) Should only label the vertical axes, with enthalpy (H), the horizontal can be left or put with
progress of reaction
2) The formula for both reactants and products should be given including their state symbols
3) Values for ∆H should be given, including the correct sign
4) It is not essential to show the activation energy in an enthalpy level diagram, but is should be
shown in an enthalpy profile diagram
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, A Cook 2016
5. understand experiments to measure enthalpy changes in terms of:
i Processing results using the expression:
energy transferred = mass x specific heat capacity × temperature change
(Q=mcΔT)
ii Evaluating sources of error and assumptions made in the experiments
Students will need to consider experiments where:
o substances are mixed in an insulated container and the temperature change is measured
o enthalpy of combustion is measured,such as using a series of alcohols in a spirit burner
o the enthalpy change cannot be measured directly.
Standard enthalpy change of combustion, ∆CHϴ
What is meant by standard enthalpy change of combustion?
The standard enthalpy change of combustion ∆cHϴ, is the enthalpy change measured at 100kPa
and a specified temperature, usually 289K, when one mole of a substance is completely burned in
oxygen.
Experimental determination of enthalpy change of combustion of a liquid
To find the enthalpy change of combustion of a liquid, a known was of the liquid is burned and the
heat energy produced is used to heat a known volume of water.
Procedure:
1. A spirit burner containing the liquid under test is weighted
2. A known volume of water is added to a copper can
3. The temperature of the water is measured
4. The burner is lit
5. The mixture is constantly stirred with a thermometer
6. When the temperature of the water has reached approx. 20˚C above its initial temperature,m
the flame is extinguished and the burner immediately reweighed
7. The final temperature is measured
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