Enthalpy Flashcards
What is enthalpy
The heat content stored in a chemical system. It is sometimes thought of as energy stored within bonds
What are the units of energy
Joules
What is the enthalpy change
Is a heat exchange with the surroundings during a chemical reaction. Heat exchange occurs because reactants and products have different enthalpies
What happens if a reaction is exothermic
Energy is transferred from the system to the surroundings. When a reaction is negative it is exothermic
What happens if a reaction is endothermic
Enthalpy of products is greater than reactants so enthalpy is positive as its transferred from the surroundings to the system
What is the activation energy
The minimum energy required to start a reaction
What are the units for standard conditions
Pressure is 100 kPa
Temperature 298 K
Concentration 1 mol dm-3
What is the enthalpy change of combustion
The enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions
What is the enthalpy change of formation
The enthalpy change when 1 mole of a compound is formed from its consistuent elements in their standard states
What is the enthalpy change of reaction
The enthalpy change that accompanies a reaction in their molar quantities expressed in a chemical equation under standard conditions
What is enthalpy change of neutralisation
The enthalpy change that accompanies the neutralisation of an aqueous acid by an aqueous base to form one molle of water under standard conditions
What measurements are taken for determination of enthalpy change for a polystyrene calorimeter
Mass of water
Temperature of water
Volume or concentration and mass of solutes reactants
What measurements are taken when you use a spirit burner ( for combustion)
Mass of water
Initial and final temp of water
Initial and final mass of fuel
What is the equation of heat exchange
q= mc delta T
What happens if temperature increases
The surrondings increase in heat energy system to surrondings so exothermic
What happens if temperature decreases
The surrondings decrease in heat energy so surrondings to system so its endothermic
How do you evaluate if the volume of water was doubled
Firstly the mass would doulbe up however the temperature change would double down
This means q would be the same
No change in n as the system isnt affected
delta H wouldnt change
What are the procedural errors for both calorimeters
Heat loss to all the surrondings
Not done under standard conditions
What are the procedural errors for combustion experiments
Liquid fuel may evaporate
Heat capacity of the beaker isnt considered
Incomplete combustion may occur
What evaluation do you make if the lid on the cup isnt used for exothermic reaction
The problem- Heat loss to surrondings
Delta T would decrease so q would decrease
So delta H would be less exothermic than it should be
What evaluation do you make if the liquid fuel evaporates
There is no impact on q as the mass of water and temperature change are not affected
However n is bigger than it should be. As some fuel is evaporated as well as combusted
This means that delta H would be less exothermic than the actual value
What evaluation do you make if incomplete combustion occurs
Delta T would be smaller as less energy is released for incomplete combustion so q would be smaller than it should be
There is no impact on n as all the fuel combusts
Since q is smaller and n is the same Delta H would be less exothermic than the actual value
What happens if some methanol had evaporated from the wick before the final weighing
The calculated mass of methanol reacted would be higher than the actual value.
So calculated moles of methanol would be higher
Since Delta H is q over n Delta H would be less exothermic than the actual value
What happens if the student used the Mr of ethanol instead of methanol
The Mr would be larger than methanol
Moles of methanol calculated is is smaller is n is mass divided by Mr
Since delta H is q divided by n delta H is more exothermic than the actual value
What does Mexo Bendo mean
Bond making is exothermic
Bond breaking is endothermic
What happens in exothermic reactions in terms of bond breaking and making
In exothermic reactions the energy released from bonds made is greater than the energy absorbed from bonds broken
What happens in endothermic reactions in terms of bond breaking and making
In endothermic reactions the energy absorbed from bonds broken is greater than the energy released from bonds made
What is the average bond enthalpy
The enthalply change when one mole of gaseous bonds are broken at 298 K
Why do all bonds have a positive enthalpy value
Bonds are electrostatic attractions which require energy to break so are always endothermic
What is the bond enthalpy equation
The sum of bond enthalpies of bonds broken - the sum of bond enthalpies of bonds made
If the bond enthalpy of bonds broken are larger than those of bonds formed what will be the sign of delta H be
When the bond enthalpies of reactants are larger than products then delta H is larger than 0
When the bond enthalpies of products is larger than reactants then delta H is smaller than 0
Why are the true values for energy change often slightly different from the values calculated using bond enthalpies
Reason 1- Bond enthalpies are averaged out over different molecules
Reason 2- Reactions may not be under standard conditions as it is only for gaseous bonds
Why would actually measuring the enthalpy change in a reaction be difficult
1) A high activation energy
2) A slow reaction rate
3) More than one reaction is taking place
What does Hess Law state
If a reaction can take place by more than one route and the initial and final conditions are the same the total enthalpy change is the same
What happens if you use a Hess cycle with combustion data
If you follow the direction of an arrow you ADD
If it is opposite to the direction of an arrow you SUBTRACT
