Enthalpy Flashcards
What is enthalpy?
The heat energy stored in a chemical system
Units for 🔺H
KjMol^-1
What is an exothermic reaction?
One that transfers energy from system to surroundings
What value will 🔺H have for an exothermic reaction?
Why?
Negetive
Energy lost
Give examples of exothermic reactions
Combustion
Respiration
(oxidation reactions)
Define endothermic reaction
Energy transferred from surroundings to system
What value will 🔺H have for an endothermic reaction?
Why?
Positive
Energy gained
Give examples of endothermic reactions
Thermal decomposition
Photosynthesis
What is activation energy?
Minimum amount of energy need for a reaction to start
What does a larger activation energy show?
Slower rate of reaction
What do all standard enthalpy changes require?
Standard conditions and all reactants and products in their standard states
What are the standard conditions?
100KPa
25°c /298K
Moldm^-3
Standard state= physical state under standard conditions
Define standard enthalpy change of reaction
Enthalpy change of a reaction in molar quantities as shown by the equation under standard conditions with all reactants and products in their standard states
Define standard enthalpy change of formation
Enthalpy change for formation of one mole of a substance from its constituent elements under standard conditions with all reactants and products in their standard states
Define standard enthalpy change of combustion
Enthalpy change of burning one mole of a substance completely in an excess of oxygen (complete combustion) under standard conditions with all reactants and products in their standard states
What is the 🔺fH of an element?
0
Define standard enthalpy change of neutralisation
Enthalpy change of a neutralisation reaction (acid + base) to form one mole of water under standard conditions with all reactants and products in their standard states
Why is the standard enthalpy change of neutralisation the same regardless of acid and base used?
Always OH + H =H2O
Is bond breaking or bond making exo or endothermic
Breaking = endothermic
Forming = exothermic
Define average bond enthalpy
Average amount of energy need to break one mole of a specific bond in gaseous molecules
What value do average bond enthalpies always have?
Positive
Bond breaking=endothermic
What are the disadvantages of using average bond enthalpies?
Same bond requires different amounts of energy to break in different conditions
Average = not specific
How can you calculate 🔺rH of a reaction using average bond enthalpies?
Why is this not a standard enthalpy change?
(Sum of reactant bond enthalpies) - (sum of product bond enthalpies)
Not standard because bond enthalpy = in gaseous molecules (not always standard conditions)
What is Hess’ law?
If a reaction can occur by two routes that start and end in the same conditions then the enthalpy changes or each are equal.
Why is Hess’ law useful?
Hard to determine 🔺H directly
Allow indirect determination
How do you calculate 🔺cH experimentally?
Using spirit burner to heat water by burning substance
Hwo do you calculate 🔺r H or 🔺neutH experimentally?
Using insulated beaker to determine temp change of solution
What errors may occur when calculating 🔺H experimentally?
Heat lost to surroundings
Incomplete combustion
Substance evaporating from wick before reweighing
Non-standard conditions
How may errors be reduced when calculating 🔺H experimentally?
Draught excluder/screen
Oxygen supply
Insulation
Lid
Why might a cooling curve be used in calculating 🔺H experimentally?
Adjust value for loss of heat to surroundings to calculate temp change
Extrapolate using graph
Why is enthalpy change hard to measure?
Other products form
High activation energy means reaction doesnt take place
What is the disadvantage to using average bond enthalpy?
Average bond strength in lots of different molecules
When calculating enthalpy change how do you know which number of moles to divide by?
Use moles of limiting reactant
When calculating enthalpy change of neutralisation what moles do you divide KJ by?
The moles of OH- or H+
