A2 energetics 1 Flashcards
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
The enthalpy change that takes place when one mole of a substance is formed from its elements under standard conditions, with all reactants and products in their standard states.
Define Enthalpy Change of Combustion
one mole of substance is completely oxidised
in excess oxygen
at 298k and 100kpa
The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states.
Define Enthalpy Change of Neutralisation
one more of water formed
in the reaction of an acid with an alkali
at 298k and 100kpa
The enthalpy change that accompanies the reaction of an acid and a base to form one mole of liquid water under standard conditions, with all reactants and products in their standard states.
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
The enthalpy change when 1 mol of gaseous covalent bonds are broken. endothermic
Explain why reactions are exothermic.
Bond breaking absorbs energy, bond making releases energy, and more energy is released than absorbed (H is negative).
Explain why reactions are endothermic.
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.
Hess cycle for combustion and equation
Hess cycle for formation and equation
Theoretical Hess cycle for neutralisation
endothermic key words
Ah= +ve
chemical system gains energy
surrounding lose energy
temp to surrounding decreases
enthalpy definition
heat energy in chemical system
AH
broken - made
products - reactants
kjmol-1
exothermic is always
-ve
reactants - products
endothermic
products - rectants
percentage error
error in measurement/ measurement taken x100
exothermic
making bonds
endothermic
+ve
breaking bonds
explain Ah are different in terms of. bonds broken and bonds formed
more energy required to break bone in reactants than to make
explain why AH different from data book value and suggest how to modify to improve accuracy
- value could be less exothermic
- less heat lost to surroidnig ns
- incomplete combustion
- use a digital thermometer
states the conditions of temp and pressure used for standard enthalpy mesaurements
298 K
100kpa
temp increases
exo
negative value
-ve
ive sign
higher pressure - for industry
safety risk
uses a lot of energy
more expensive
lower temp- for industry
slower rate
