Energerics Flashcards
In an endothermic reaction, the enthalpy of the system…
… Increases
Define enthalpy change
Enthalpy change is a change in (heat) energy under constant pressure
What is a system?
Reaction
All exothermic reactions have a positive or negative value?
Negative
All endothermic reactions have positive or negative value?
Positive
Why are endothermic reactions have a positive value?
The system has gained energy
Why do all exothermic reactions have a negative value?
The system gives out energy
What are standard conditions?
1 atmosphere (100,000 Pa)
1 moldm-3 (for solutions)
298K or 25C
What signifies standard conditions?
Subway sign
What are standard states?
The standard state of a substance is its most stable state under standard conditions
Define standard enthalpy of formation
Enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
What are the units for enthalpy change?
KJmol-1
In a formation reaction, what is important about the reactants?
They are in their element forms
Define standard enthalpy of combustion
Enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions
What are the products of all combustion reactions?
CO2 and H2O
What is the equation for energy change?
q=mcAt
In the enthalpy change equation, what is m?
Mass
In the enthalpy change equation, what is c?
Specific heat capacity
In a enthalpy change equation, what is At?
Change in temperature
In an exothermic reaction, the temperature…
… Increases
In an endothermic reaction, the temperature…
… Decreases
What is chemical system?
Just means the reactants and products
What 5 things are important when drawing a enthalpy change graph?
Y-axis, scales, plotting, line of best fit, extrapolations
Define Hess’ law
Enthalpy is independent of route taken
In a Hess diagram, the arrows of combustion…
… Go down
If the arrows of the direct route is in the opposite to the indirect route arrows, then..
Use the reverse sign
In a Hess diagram, the formation arrows…
… Go up
Define mean bond energy
Enthalpy change require to break of covalant bonds with all species in gaseous state, averages over a range of compounds
In an exothermic reaction, the enthalpy of the system
Decreases
What is the equation for the bond energy?
Sum of breaking-making
