Module 3: Chapter 9 (Enthalpy)
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What is meant by enthalpy?
Enthalpy (H) is the measure of heat energy present in a chemical system
What is meant by enthalpy change?
Enthalpy change (ΔH) is the amount of energy released taken in during a chemical reaction
What is meant by an exothermic reaction? And draw the enthalpy profile for one
An exothermic reaction (-ΔH) releases energy into the surroundings
What is meant by an endothermic reaction? And draw the enthalpy profile for one
An endothermic reaction (+ΔH) takes in energy from the surroundings
What is meant by activation energy?
Activation energy (EA) is the minimum energy required for a reaction to take place. It is the energy input required to break bonds
What is meant by standard conditions?
The ΔH of a reaction can vary depending on the conditions used. Data books and tables will always give the ΔH value obtained under standard conditions (⦵)
What are the standard conditions?
• 100kPa (close to 1 atm pressure) • 298 K (25 degrees celsius) • standard state - the physical state of a substance under standard conditions
What is meant by the enthalpy change of reaction?
Enthalpy change of reaction (ΔrH) is the enthalpy change associated with a stated equation in the molar quantities shown in the chemical equation (under standard conditions)
What is meant by the enthalpy change of formation?
Enthalpy change of formation (ΔfH) is the enthalpy change associated with the formation of 1 mole of a compound from its elements (under standard conditions) • the ΔfH for an element will always be 0 as no chemical change is actually taking place
What is meant by the enthalpy change of combustion?
Enthalpy change of combustion (ΔcH) is the enthalpy change associated with the complete combustion of 1 moles of a substance (under standard conditions)
What is meant by the enthalpy change of neutralisation?
Enthalpy change of neutralisation (ΔneutH) is the enthalpy change associated with the formation of 1 mole of water from a neutralisation reaction between an acid and a base
What is always the ΔneutH for the reaction between a strong acid and a strong alkali?
-57 kJ mol^-1
What is the equation for energy transferred?
q = m c ΔT • the specific heat capacity of water is 4.18 Jg^-1k^-1 MUST DIVIDE BY 1000 TO CONVERT J INTO KJ
What is meant by average bond enthalpy?
The energy required to break one mole of a specified type of bond in gaseous molecules - average bond enthalpies are calculated from actual bond enthalpies found in different environments
Bond breaking is ….(?)
endothermic
Bond making is ….(?)
exothermic
How do you use bond enthalpies to calculate energy released?
• draw structures of reactants and products in reaction • identify the bonds broken and made • calculate total energy released • add together to find ΔrH
What does Hess’s law state?
If a reaction can take place by 2 routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route
Draw an enthalpy cycle
(check in textbook)
What is the rule if you are calculating enthalpy changes of formation?
Arrows point upwards elements at the bottom
What is the rule if you are calculating enthalpy changes of combustion?
Arrows point downwards combustion products at the bottom
Why are bond enthalpies always endothermic?
Bond enthalpies involve bond breaking which requires energy from the surroundings
What is the calculation when using enthalpy changes of formation for (ΔrH =)?
ΔrH = ΣΔfH products - ΣΔfH reactants
What is the calculation when using enthalpy changes of combustion (ΔrH =)?
ΔrH = ΣΔcH reactants - ΣΔcH products