More on Enthalpy Changes Flashcards
Why are the enthalpy changes of exo./endothermic reactions +ive or -ive?
- Enthalpy change is the overall change in energy of the breaking and making of bonds in a reaction.
- If you need more energy to break the bonds than is released when bonds are made ∆H is +ive. If you need less, ∆H is -ive.
Is bond making/breaking exo./endothermic?
- You need energy to to break bonds, so bond breaking is endothermic. ∆H is +ive.
- Energy is released when bonds are formed so bond making is exothermic. ∆His -ive.
How are the particles within atoms/ions attracted to each other?
- Ions are attracted to each other through ionic bonding (attraction between +ive and -ive ions).
- In covalent molecules, the +ive nuclei are attracted to the -ive charge of the shared electrons in a covalent bond.
How is the attraction between atoms/ions broken?
You need energy to to break an attraction - stronger bonds need more energy to break. The amount of energy you need per mole is called the bond dissociation enthalpy.
What do you need to remember about breaking the attraction between atoms/ions?
The amount of energy you need per mole to break an attraction is called the bond dissociation enthalpy. Bond dissociation enthalpies always involve bond breaking in gaseous compounds. This makes comparisons fair.
What do you need to remember about the bond enthalpies within molecules?
Although molecules may contain two of the same type of bond, these do not necessarily have the same bond enthalpies.
e.g. H2O Water has two O-H bonds. You’d think it would take the same amount of energy to break them both… but it doesn’t:
The first bond, H-OH(g): E(H-OH) = +992kJ mol-1
The second bond, H-O(g): E(H-O) = +428 kJ mol-1 (OH- is easier to break apart because of the extra electron repulsion).
So, the average bond enthalpy is (492 + 428)/2 = +460 kJ mol-1
What is the av. bond enthalpy of a reaction?
The average bond enthalpy is the the energy needed to break one mole of bonds in the gas phase, averaged over many different compounds.
Why aren’t average bond enthalpies exact?
The average bond enthalpy is the the energy needed to break one mole of bonds in the gas phase, averaged over many different compounds.
e.g.
If we calculate the average bond energy in water it comes out as +460 kJ mol-1:
The data book says the bond energy for the O-H bond in a water molecule is +463 kJ mol-1. It’s a bit different because it’s the average for a much bigger range of molecules, not just water. For example, it includes the O-H bonds in alcohols and carboxylic acids.
What two things are needed to calculate the enthalpy change of a reaction?
- the number of moles of the stuff that’s reacting.
- the change in temperature.
How is enthalpy change calculated in the lab?
calorimetry experiments
How could you calculate the enthaply change of a flammable liquid? (the set up)
As the fuel burns it heats the water. You can work out the heat absorbed by the water if you know the mass of the water, the temperature change of the water (ΔT) and the specific heat capacity of water (=4.18Jg-1K-1).
What is specific heat capacity?
The SHC of a substance is the amount of heat energy needed to raise the temperature of 1g of that substance by 1K.
Why can you never calc. the exact enthalpy change of combustion?
In practice, you always lose some of the heat (to the apparaus and the surroundings).
What are the calorimetry experiments carried out in solution?
- exp. one
The the enthaply change (of combustion) of a flammable liquid, and the enthalpy changes of neutralisation or diplacement reactions.
for a neutralisation reaction, combine the quantities of acid and alkali in an insulated container and measure the temperature change. The heat given out can be calculated using the formula q = mcΔT.
What is the equation for calculating enthalpy change?
