9 Enthalpy Flashcards
What does ∆H mean? (1 mark)
Heat energy change in constant pressure.
What does ∆Hᶱ mean? (1 mark)
Heat energy change in standard conditions.
What are the standard conditions? (2 marks)
101kPa and 298K
What are the units for enthalpy change? (1 mark)
KJmol-1
In an exothermic reaction, are the reactants or products higher? (1 mark)
Reactants
What are the two axis on an enthalpy profile diagram? (3 marks)
Enthalpy (KJmol-1)
Reaction progress
Define activation energy. (2 marks)
Minimum energy needed to begin breaking bonds and start the chemical reaction.
What is the symbol for activation energy? (1 mark)
Ea
What is enthalpy of reaction? (2 marks)
∆H when molar amounts of reactants as stated in the equation react under standard conditions.
What is the symbol for enthalpy of reaction? (1 mark)
∆rHᶱ
What is enthalpy of formation? (2 marks)
∆H when one mole of a substance is formed under standard conditions.
What is the symbol for enthalpy of formation? (1 mark)
∆fHᶱ
What is enthalpy of combustion? (2 marks)
∆H when one mole of a substance is completely burnt (in oxygen) under standard conditions.
What is the symbol for enthalpy of combustion? (1 mark)
∆cHᶱ
What is enthalpy of neutralisation? (2 marks)
∆H when one mole of water is formed when an acid and alkali react under standard conditions.
What is the symbol for enthalpy of neutralisation? (1 mark)
∆neutHᶱ
What is the equation for enthalpy change? (4 marks)
Heat given out = mass of substance heated x specific heat capacity x change in temperature
q= mc∆T
What are the units in the equation for enthalpy change? (4 marks)
q- J
m- g
c- Jg-1k-1
T- K or degrees
How can you measure ∆H in combustion reactions? (3 marks)
Flammable liquid is burnt inside a flame calorimeter.
The fuel, in a bottle with wick, is burned so that it heats a known mass of water.
The change in temperature of the water is measured.
What are 3 possible problems with how we measure ∆H in combustion reactions? ( 3 marks)
Incomplete combustion (less energy given out)
Evaporate
Heat loss to surroundings
How can you measure ∆H for reactions in solution? ( 2 marks)
Reaction carried out in an insulated beaker and change in temperature is measured.
Name three possible problems with how we measure ∆H for reactions in solutions? (3 marks)
Heat absorbed by container
Heat lost to surroundings
The specific heat capacity is assumed to be 4.185Jg-1k-1
What is bond enthalpy? (2 marks)
The energy required to break a covalent bond in the gaseous state. Endothermic. (+)
What is average bond enthalpy? ( 2marks)
One mole of covalent bonds is broken in the gas phase averaged over several different conditions.
How do you figure average bond enthalpy? ( 2 marks)
∆H of reactants - ∆H of products
or Hess’ cycles
What is Hess’ law? (1 mark)
Total enthalpy change for a reaction is independent of the route taken (same conditions).
What are the two ‘shortcut’ equations for ∆fHᶱ and ∆cHᶱ? (2 marks)
∆H = ∑∆fH products - ∑∆fH reactants
∆H = ∑∆cH reactants - ∑∆cH products
What is the ∆fHᶱ of elements? (1 mark)
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