Energetics
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What is thermochemistry
This is the study of enthalpy changes during chemical reactions
What is enthalpy
This is a measure of the heat content of a substance.
Symbol for enthalpy
H
Symbol for enthalpy change
🔼H Change in heat content at constant pressure
Standard conditions (H°) for enthalpy
100kPa 298K
Endothermic reaction
More energy being put into the reaction to break bonds than is given out
Exothermic reaction
More energy being produced by forming bonds than is used to break the bonds
Enthalpy of products when endothermic
Positive In an endothermic change, energy is absorbed from the surroundings by the reactants to form the products. This means the enthalpy of the products has increased, so enthalpy change is positive
Enthalpy of products in exothermic reaction
Negative In an exothermic change, energy is released to the surroundings by the reactants to form the products. This means the enthalpy of the products has decreases. So, enthalpy change is negative
Standard enthalpy change of reaction, 🔼rH° units
kj/mol Kj mol^-1
Standard enthalpy change of formation, 🔼fH°
Enthalpy change when 1 mole of a substance is formed from its constituent elements with all reactants and products in standard states under standard conditions.
Standard enthalpy change of combustion, 🔼cH°
Enthalpy change when 1 mole of a substance is completely burned in oxygen with all reactants and products in standard states under standard conditions.
Enthalpy of formation of an element by definition = …?
0 This is because we cannot make an element
Standard enthalpy change of neutralisation, 🔼neutH°
Enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard conditions
Another way of saying enthalpy of formation of CO2
Enthalpy of combustion of CO2
Enthalpy change
Change in heat content at constant pressure.
Reaction profile of exothermic reaction and endothermic
How can enthalpy change be found
By measuring the temperature change in a reaction
Equation for finding amount of het energy given out
Equation to find enthalpy change in terms if J/kJ per mole
Why is heat loss a problem in calorimetry
It can lead to errors in the results. So there are techniques designed to reduce heat loss