Energetics
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Temperature Definition
Measure of the Average Kinetic Energy in a substance
The enthalpy of formation
The enthalpy change when one mole of a compound / chemical / substance; [1] Is formed from its elements (under standard conditions); [1] And the standard conditions are: 298 K; [1] 1 mol dm-3 solutions; [1] 100 kPa; [1] Substances in their standard state; [1
The enthalpy change of Combustion
The enthalpy of combustion is defined as the enthalpy change when 1 mole of a substance reacts completely with oxygen under standard conditions.
Can a Neutralisation Reaction be an enthalpy change of combustion or formation?
Nein
When a sample of Ammonium Chloride is added to a small beaker and stirred, the temperature drops as the ammonium chloride slowly dissolves into water? Is this exothermic or endothermic
A temp drop –> Reaction is colder in the surroundings Heat energy will flow from the surroundings into the beaker Heat energy flowing into the system = Endothermic System gains energy from surroundings Delta H positive
Explain all three types of Systems
An open system allows the transfer of matter and energy across the boundary A closed system only transfers energy across the boundary Isolated systems do not allow the transfer of matter or energy across the boundary
The definition of average bond enthalpy is:
The energy needed to break one mole of bonds; [1] In a gaseous molecule averaged over similar compounds; [1]
Enthalpy Change Equation
ΔHr = products - reactants
Bond dissociation energy is:
The amount of energy required to break one mole of a specific covalent bond in the gas phase;
The equation that is required to calculate the enthalpy change of reaction, ΔHr, given enthalpy of formation, ΔHf, data is
ΔHr = ∑ΔHf products - ∑ΔHf reactants; [1]
Hess’ Law
the enthalpy change for any chemical reaction is independent of the route taken providing that the starting and final conditions, reactants and products are the same
How to carry out Q = MCDELTA T experiment and the shortcomings of it?
i) To conduct this experiment: You would need a beaker, a thermometer and a balance; (1] You need to measure the mass of water, the mass of ammonium nitrate and the temperature before and after the reaction; [1] To find the enthalpy of reaction you would need to multiply the mass of the water x specific heat capacity of water (4.18 J g* K*’) ×temperature change; [1] il) A major source of error would be: Heat loss from the apparatus is a major source of error; (1] The value q of will be less than expected; (1]
Definition of Accuracy
An accurate result is how close something is to the expected value
Definition of Accuracy
An accurate result is how close something is to the expected value
Definition of Precision
How many decimal places used
Improvements that can give more accurate results?
Use a cover on the beaker Use a lid Stirrer
Calorimetry Practical Diagram:
SEE
Calculated Value might be different?
Reaction was incomplete Heat loss to the surroundings
Standard Enthalpy of Neutralisation
• Energy Change when one mole of water is formed • With all the reactants and products in their standard state • 298 K, 100 KPA
Enthalpy of Atomisation
The enthalpy change when 1 mole of gaseous atoms is formed from its elements under standard conditions