Energetics I Flashcards
What is meant by system?
The materials or chemicals being studied.
What is meant by surroundings?
Everything around the system-in theory the whole universe (includes what you do the reaction in eg: test tube).
What is a closed system?
A system where matter cannot be exchanged with the surroundings but energy can.
What is enthalpy change?
The overall energy exchanged with the surroundings, when change happens at constant pressure and the final temperature in the same as the initial temperature.
The symbol and units for enthalpy change are:
ΔH, kJmol^-1
What are the axis labels for an energy level diagrams?
Energy on the y-axis and course of reaction on the x-axis.
Equation for energy change:
E=mcθ
Where: E is energy change in J. m is mass of solution in g. c is the specific heat capacity of water in Jg^-1K^-1. θ is change in temperature in K.
What is the specific heat capacity of water?
4.18 Jg^-1K^-1
Or 4.2 if stated in the question.
Assumptions made when calculating energy change in a solution?
1g of solution is 1cm^3.
All energy is transferred to solution.
The solution has the same SHC as water.
What apparatus do we use to obtain accurate enthalpy changes for combustion?
A bomb calorimeter.
Why does a bomb calorimeter work well for calculating accurate enthalpy changes of combustion?
It is designed to ensure the sample burns completely with an adequate oxygen supply and energy losses are avoided. A higher pressure is used to ensure the sample burns completely.
Values of enthalpy changes vary with what factors?
Temperature, pressure and concentration.
Why do we have standard conditions for enthalpy changes?
To ensure that all enthalpy changes are measured evenly without varying temperature, pressure or concentration.
Standard conditions are:
A pressure of 100 kPa (approx pressure of atm at sea level).
A stated temperature (usually 298K).
Substances in their most stable state at 100 kPa and stated temperature.
Solutions with a conc of 1 moldm^-3.
In thermochemistry the states of substances are important so we must what when writing symbol equations?
Give state symbols.
Which allotrope is used for carbon for standard enthalpy changes and why?
Graphite as it is more energetically stable at 298K.
Define the standard enthalpy change of combustion.
The enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions into standard products.
Define the standard enthalpy change of formation.
One mole of compound is formed from its elements into standard states under standard conditions.
What is the standard enthalpy change of neutralisation.
The enthalpy change when an acid and an alkali react to form one mole of water under standard conditions.
State Hess’s Law.
The enthalpy change accompanying a chemical change is independent of the route by which chemical change occurs.
Is bond breaking endothermic or exothermic?
Endothermic.
Is bond making endothermic or exothermic?
Exothermic.
What is meant by bond enthalpy?
The energy required to break all the bonds in 1 mole of a substance in the gaseous state.
When calculating enthalpy changes by bond enthalpies what is the main assumption made?
That all bond enthalpies are the same as the average bond enthalpy for that bond.
