Bond Enthalpies and Energy Cycles

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  • Created by: Lotto65
  • Created on: 04-03-17 11:29
What is an alternative way of calculating enthalpy changes?
Bond enthalpies
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What is the definition of bond enthalpy?
The energy needed to break the bonds in a gaseous substance
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Is the value for the energy to make bonds positive or negative?
Negative - energy is given out
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Is the value for energy to break bonds positive or negative?
Positive - energy is put in
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What does 'evolved' mean?
Given out
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In diatomic molecules, can we know the enthalpy precisely?
Yes - only two atoms
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Why does the enthalpy of certain bonds vary in different compounds?
It varies depending on the surrounding atoms
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What is average bond enthalpy?
The energy needed to break one mole of a bond in a gaseous molecule averaged over similar compounds
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When do we use average bond enthalpy?
For bond enthalpies of compounds as the actual value varies slightly with each
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What do we need to know in terms of bond enthalpies to calculate the enthalpy change of the reaction?
The bond enthalpy values for all the bonds in the reactants and the products
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What is the unit of bond enthalpy?
kJ/mol
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Are double bonds stronger or weaker than single bonds?
Stronger
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How does the ozone protect Earth from damaging UV radiation?
It absorbs high and low energy UV light to break the O-O bond within it
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What is the requirement to use bond enthalpies to calculate enthalpy change?
All reactants and products must be in the gaseous state
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Why do all the reactants and products need to be in the gaseous state to use bond enthalpies?
If water was a liquid, more heat would be evolved as you would have to include enthalpy change of vaporisation of water too
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How did they calculate average bond enthalpies?
Took the average of bond enthalpies from a number of different compounds containing that bond
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Are average bond enthalpies accurate?
No
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What is the particular issue with calculating the enthalpy of combustion of a substance?
Using bond enthalpies requires the reactants and products being in the gaseous state while enthalpy change of combustion requires the reactants and products being in their normal states - different results
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What is a born haber cycle?
A cycle showing the formation of an ionic compound
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Why do the enthalpy changes of formation of ionic compounds have a negative value overall?
A large lattice enthalpy
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What is the standard enthalpy of atomisation?
The standard enthalpy change when one mole of gaseous atoms is formed from the element in its standard state under standard conditions
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How can you calculate the standard enthalpy of atomisation of diatomic molecules?
Half the bond dissociation enthalpy
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What is lattice enthalpy?
The enthalpy of an endothermic process to turn a crystalline solid into its gaseous ions or the exothermic process of turning gaseous ions into a crystalline solid
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What can you determine from the sign of the lattice enthalpy value?
If the lattice is broken (+) or formed (-)
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What does lattice enthalpy depend on?
The size of the ions and the charge of the ions
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The smaller the ion...
The greater the lattice enthalpy
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The greater the charge...
The greater the lattice enthalpy
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Why is a lot of energy required to melt sodium chloride?
Strong electrostatic forces of attraction in the lattice
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What is an easier way of breaking down an ionic lattice?
Dissolve the salt in water at room temperature
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What is the enthalpy change of solution?
The enthalpy change when one mole of an ionic substance dissolves in water to give a solution of infinite dilution
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What are the two stages involved in dissolving an ionic compound?
1) Break down the lattice into gaseous ions 2) Hydrate the gaseous ions into aqueous ions
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Breaking down an ionic lattice is highly endothermic/ exothermic
Endothermic
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What is the energy called to hydrate gaseous ions into aqueous ions?
Hydration energy
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The process of hydrating gaseous ions to form aqueous ions is a highly endothermic/ exothermic process
Exothermic
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The smaller the ion (hydration)...
The greater the hydration energy
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The greater the charge on an ion (hydration)...
The greater the hydration energy
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Hydration energy is very similar to...
Lattice enthalpy
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For sodium chloride, what is the numerical difference between lattice enthalpy and hydration energy?
7kJ/mol
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What does the solubility of salts rely on?
The size of the lattice enthalpy compared to the hydration energy
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Why is it not a good idea to add water to sulfur trioxide?
It is not ionic so there is no lattice enthalpy so the highly exothermic hydration energy is very strong that the resulting sulfuric acid can boil
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Card 2

Front

What is the definition of bond enthalpy?

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The energy needed to break the bonds in a gaseous substance

Card 3

Front

Is the value for the energy to make bonds positive or negative?

Back

Preview of the front of card 3

Card 4

Front

Is the value for energy to break bonds positive or negative?

Back

Preview of the front of card 4

Card 5

Front

What does 'evolved' mean?

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