Energetics Flashcards
What is Enthalpy change?
Enthalpy change is the heat energy change measured under conditions of constant pressure
What is the symbol for enthalpy change?
∆H^Ɵ
(circle with a line means in standard conditions)
What are standard conditions classed as?
100kPa pressure and 298K (25C) temperature
What are the units for enthalpy change?
kJmol-1
When breaking bonds, explain if the bond enthalpy is positive or negative.
- Energy needs to be absorbed to break bonds
- Bonds are broken in the reactants which is an endothermic process
- So enthalpy change is positive
When making bonds, explain if the bond enthalpy is positive or negative.
- Energy is released when bonds are formed
- Bonds are made when products are being produced which is an exothermic reaction
- so enthalpy change is negative
What is classed as an endothermic reaction?
If the energy needed to break bonds is > than the energy released from forming bonds
What is classed as an exothermic reaction?
If the energy released from forming bonds > than the energy needed to break bonds
How do you calculate the enthalpy change?
total energy needed to break bonds - total energy released from forming bonds
What are Calometry reactions used for?
They are used to work out the bond enthalpy change for a specific reaction
Tell me about the calorimetry reaction for combustion.
- An alcohol/fuel is burnt to increase the temperature of the water
- Make sure to weigh the fuel before and after (to see the mass burnt)
- Most energy from the fuel is transferred into the water, however, some is lost to its surroundings too (since it is a flame)
- To combat this, a lid is placed on top to prevent heat loss and windshields are placed on either side of the flame to prevent wind from moving the flame
What is the difference between enthalpy change and heat energy change?
enthalpy change is a specific type of heat energy change that occurs at constant pressure and is used to describe chemical reactions, while heat energy change is a more general term for energy transfer due to temperature differences
Explain fully how to calculate the enthalpy change.
You need 2 things, the energy transferred and the moles of the fuel burnt / used.
To get the energy transferred use the equation: q=mc∆T
To get the moles of the fuel burnt / acid used, use the equation: mass/mr or conc x vol
Explain the equation for energy transferred.
q=mc∆T
q = energy transferred
m = mass of water / solution (total mass of BOTH liquids mixed)
c = shc of water ( 4.18Jg^-1K^-1 )
∆T = temp change
what are the units for shc?
Jg^-1K^-1
what is the shc of water?
4.18Jg^-1K^-1
Tell me about the calorimetry reaction for solutions.
- Fill a polystyrene cup with an amount of acid
- measure the temperature of it
- add an amount of alkali to the acid and stir
- calculate the temperature change
Why is Hess’s law used?
To work out the enthalpy changes that you can’t determine by doing an experiment.
What is Hess’s law?
The total enthalpy change of a reaction is independent of the route taken.
What is the enthalpy change of formation?
The enthalpy change when 1 mole of a substance is formed from is elements in their standard states under standard conditions.
What is the definition of standard enthaply of combustion?
Enthalpy change when one mole of a substance burns completely in oxygen With all substances in their standard states (at stated temperature and 100kPa)
Why might the enthalpy of combustion be less exothermic than that calculated using enthalpies of formation?
- Incomplete combustion
- Heat loss
