Energetics Flashcards
What energy change is breaking bonds associated with?
Energy is taken in to break bonds - Endothermic reaction
What energy change is making bonds associated with?
Energy is released to make bonds - Exothermic reactions
What is an endothermic reaction?
One with an overall positive enthalpy change - energy in breaking bonds>energy out making bonds
What is an exothermic reaction?
One with an overall negative enthalpy change - energy taken in breaking bonds < energy out making bonds
If a reversible reaction is endothermic one way, what type of reaction is the other way?
Exothermic
What is enthalpy change?
The amount of heat energy taken in or given out during any change in a system provided the pressure is constant
What is energy?
The ability of a system to do work
What are the standard conditions for enthalpy change?
100kPa pressure
298K (room temperature or 25°c)
Solutions at 1moldm-³
What is the standard enthalpy of combustion of a substance?
The enthalpy change that occurs when one mole of a substance is combusted completely under standard conditions, all reactants and products being in their standard states.
What is the standard enthalpy change of formation of a compound?
The energy transferred when 1 mol of the compound is formed from its elements under standard conditions, all reactants and products being in their standard states.
Give 2 examples of am exothermic reaction?
Combustion of fuels
Neutralisation
Give an example of an endothermic reaction
Thermal decomposition
How can you calculate enthalpy change from experimental data?
Q=mc△T
m is mass of substance being heated
c is specific heat capacity
△T is change in temperature
What is a flame calorimeter and how does it differ from a simple calorimeter?
Reduces heat lost to the surrounding to give more accurate results: has spiral chimney, made of copper, an enclosed flame and the fuel is burnt in pure oxygen, not air
What does Hess’s Law state?
That the enthalpy change for a reaction is the same regardless of the route taken
What is mean bond enthalpy?
The average value (across different chemicals environments) for the bond dissociation enthalpy of a given bond
Why might experimental methods for enthalpy determination not be very accurate?
Heat is lost to the surroundings
Not in standard conditions
Reaction may not go to completion
Why will using bond enthalpies not be as accurate as using standard enthalpy of combustion/formation?
Bond enthalpies are a mean for the same bond across different molecules
Standard enthalpy of combustion/formation just apply to that molecule, more accurate
What is calorimetry?
An experimental method for finding enthalpy change by measuring temperature change over time
How can you calculate enthalpy change per mole?
△H=q/moles
q=energy change
Why are △H values found using calorimetry never completely accurate?
As energy is easily lost from the system through conduction or convection and inaccuracies in measuring temperature
What is the experimental procedure used to determine △H for a reaction?
- Wash then dry equipment with solution
- Measure initial temp of reactants
- Transfer reagents into cup
- Immerse thermometer bulb in liquid
- Stir mixture
- Record temp at suitable intervals
- Repeat experiment
Are bond enthalpies always positive or negative? Is this endothermic or exothermic?
Positive (endothermic)
What is average bond enthalpy?
The average enthalpy change when 1 mole of bonds in gaseous molecules are broken
How do you calculate enthalpy change from bond energies?
- Calculate bonds broken
- Calculate bonds made
- Enthalpy change = Bonds broken - Bonds made
