Energetics Flashcards
What is enthalpy change?
The heat energy change at a constant pressure
What are the units for enthalpy change?
KJ mol-1
What is the enthalpy of a reaction?
The heat energy stored in a chemical reaction
What is an exothermic reaction? Is ΔH +ve or -ve?
Heat energy is given out to the surroundings, causing the temp of surroundings to increase.
The reactions system looses energy so ΔH is -ve?
What is an endothermic reaction? Is ΔH +ve or -ve?
Heat energy is taken in from the surroundings, so temp of surroundings decreases.
The reactions system gains energy, so ΔH is +ve
Give two important examples of expthermic reactions
Combustion of fuels
Respiration
Give two important examples of endothermic reactions
Photosynthesis
Thermal decomposition of calcium carbonate
What key features should an enthalpy profile diagram show?
Products
Reactants
Activation energy
Enthalpy change
What are enthalpy profile diagrams?
Diagram with heat energy against time, within a system
Shows the enthalpy changes of a reaction
Box surrounding with thermometer to show surroundings
What is the activation energy?
The minmum energy required to start a reaction, by the breaking of bonds
What is bond enthalpy?
The heat energy required to break one mole of a covalent bond within a molecule in the gaseous state
Why are bond enthalpys always positive values?
Becasue heat energy is ALWAYS required to break a bond
What is mean bond enthalpy?
The heat energy required to break one mole of a covalent bond, averaged for that type of bond within a range of different compounds in the gaseous state.
Why might the mean bond enthalpy used rather the bond enthalpy?
Because the bond enthalpy for the same covalent bond may be different in different compounds
Give the equation for enthalpy change?
ΔH= energy requored to break bonds (energy in) - energy released when new bonds form (energy out)
What are the standard contitions considered in standard enthalpy changes?
298K, 100KPa
What is a standard state?
The physical state of a compound when it is in standard conditions
What is the standard enthalpy change of formation?
The enthalpy change when one mole of a compound is formed from its constituent elements, when all reactants and products are in the standard states
What is the standard enthalpy change of formation of an element?
0
How does the standard enthalpy change of formation of a compound tell you how stable it is?
The more negative the value, the stable the compound is
What is the standard enthalpy change of combustion?
The enthalpy change when one mole of a compiund reacts completely in oxugen, with all reactants and products in the standard states
State the first law of thermodynamics
Energy cannot be created or destroyed, but it can be changed from one form to another
State Hess’s law
The enthalpy change of a chemical reaction is independent of the route taken.
What is Hess’s law used for?
Used to measure the enthalpy change of a reaction that cannot be measured directly, using the enthalpy cycle
Why might it not be easy to measure the enthalpy change of a reaction directly?
- the rate of reaction may be too slow
- there may be a very high activation energy
When using Hess’s law, which way should the arrows piint when using combustion or formation data?
Formation- arrows point upwards
Combustion- arrows point downwards
What is the formula for q (change in heat)?
q = mc∆T
What is the firmula for enthalpy change?
∆H = q / n
Why might the measured enthalpy change of combustion of a experiment not be accurate?
Due to heat loss or incomplete combustion
