Define Enthalpy Change of Reaction
The enthalpy change when the number of moles of a substance in a balanced equation for a reaction react at 298k and 100kpa
Define Enthalpy Change of Formation
One mole of a substance is formed
from its elements in their standard state
at 298k and 100kpa
Define Enthalpy Change of Combustion
one mole of substance reacts completly with oxygen
at 298k and 100kpa
Define Enthalpy Change of Neutralisation
one more of water formed
in the reaction of an acid with an alkali
at 298k and 100kpa
What enthalpy do exothermic reactions have?
Negative enthalpy.
What enthalpy do endothermic reactions have?
Positive enthalpy.
What is the enthalpy characteristic of heating water, and what is the problem with accurate readings when water is heated?
Heating water is endothermic. Enthalpy cannot be measured.
draw an energy profile diagram for an exothermic reaction
draw an enthalpy profile diagram for an endothermic reaction
Define Activation Energy
The minimum energy required to break all the bonds in the reactants, allowing the reaction to take place.
What are standard conditions?
298K and 100kPa.
What is the standard state of a substance?
The physical state of a compound under standard conditions.
Define Average Bond Enthalpy
energy required to break 1 mole of a specified type of bond in gaseous molecule
endothermic +ve
Explain why reactions are exothermic.
- less eneergy required to break bonds
- more enrgy required to make bonds
- prodcuts less energy thasn reactants
- heat given off to surroudings
bond enthalpy of bonds being made is higher than bond enthalpy of bonds being broken
Explain why reactions are endothermic.
- more energy required to break bonds
- products have more enrgy tahn reactants
- heat energy absorobed by reaction from surroudings
Bond breaking absorbs energy, bond making releases energy, and more energy is absorbed than released (H is positive).
bond energy calculation equation
What must always be remembered when structuring bond enthalpy calculations?
Always write the total enthalpy of bonds broken and bonds made.
Steps to a combustion calorimetry calculation?
- Determine energy transferred to water
- Determine moles of fuel used
- Determine enthalpy.
combustion calorimeter equation for energy transferred to water
Combustion/solution calorimetry equation for determination of enthalpy (symbol + word)
What are the reasons why calculated enthalpy data differs for both solution and combustion calorimetry, and how can this effect be reduced?
- Heat loss to surroundings - use bomb calorimeter
- Data book uses standard values - use standard conditions
- Evaporation of water from beaker - place lid over beaker.
What are the reasons why calculated enthalpy data differs for combustion calorimetry, and how can this effect be reduced?
- Incomplete combustion of fuel - burn in plentiful oxygen
- Evaporation of alcohol from wick - place cap over wick when not burning.
What must always be remembered when carrying out Hess cycle calculations?
Add/Subtract data values according to arrow direction.
What must you always bear in mind when considering solution calorimetry and molar ratios?
Multiply H by however many moles of reagent you calculated in the equation.
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A1 Atomic structure112
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Formula17
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A2 energetics 144
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A3 inorganics77
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A4 kinetics 124
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A5 Equilibrium27
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A6 kinetics 272
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acids 239
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B1 moles, equations59
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B2 redox and periodicity73
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B5 Energetics 287
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A8 - Redox15
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A9- Transition metals34
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B3 AlkAnes49
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AlkEnes46
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alcohols30
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B4- haloalkanes23
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organics 1 synthesis25
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B4 organic analysis52
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Benzene59
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phenol and directing groups43
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Carbonyl16
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carboxylic acids9
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esters6
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acyl chlorides and acid anhydrides10
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amines15
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amino acids and amides8
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optical isomers7
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polymers7
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organic practicals31
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tests14
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reaction pathway43
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HARD31
