Topic A2 - Energetics
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What are the standard conditions for measuring enthalpy?
100kPa pressure, 298K
What is the nature of energy change when bonds are broken?
Endothermic as energy is absorbed
What is the nature of energy change when bonds are made?
Exothermic as energy is released
How is enthalpy change calculated?
Enthalpy change = bonds broken - bonds made
What is activation energy?
Minimum energy required for a reaction to take place between two colliding reactant particles
Define the enthalpy change of reaction.
Enthalpy change associated with a stated reaction (in molar quantities shown in chemical equation) with all reactants and products in standard states and react completely under standard conditions
What is the enthalpy change of formation?
Enthalpy change when one mole of substance is produced from its elements in their standard states under standard conditions
What is the enthalpy change of combustion?
Enthalpy change when one mole of substance is burned completely in oxygen under standard conditions
What is the enthalpy change of neutralization?
Enthalpy change when solutions of acid and alkali react together under standard conditions to produce one mole of water
What does calorimetry measure?
Measures temperature change over time to find enthalpy change
What is the equation used to calculate heat transfer?
Q = mcΔT
What does ‘c’ represent in the equation Q = mcΔT?
Specific heat capacity (energy required to raise 1g of substance by 1K without a change of state)
How is ΔH calculated in terms of heat transfer?
ΔH = - (Q) / moles
What is a source of error in calorimetry?
Heat loss via conduction or convection
How can heat loss in calorimetry be reduced?
By putting a lid on calorimeter or insulating outsides (e.g. polystyrene)
What is another source of error in calorimetry?
Inaccuracies in measuring temperature
What is meant by the term average bond enthalpy?
Average enthalpy change when 1 mole of (stated) bonds of gaseous covalent bonds is broken
What is enthalpy?
measure of heat energy in a chemical system (chemical system = atoms, molecules or ions that make up the chemicals)
In Q = mcΔT, m is the mass of…
the materials that are changing temperature
Give 4 reasons why experimental values for the enthalpy change of combustion are less exothermic than data book values
• Heat loss to surroundings - mainly air surrounding the flame (or anything other than the water) • Incomplete combustion of fuel (CO and C may be produced instead of carbon dioxide) • Evaporation of fuel from the wick (use a cover to reduce this error) • Non-standard conditions (data book value is standard conditions)
Why is the enthalpy change of formation of NO positive/ endothermic?
more energy is required for bond breaking that is is for bond making
What would be the increase in temperature if double the moles of reactants are used?
-temperature would remain the same -as there is double the energy spread over the double the volume