Enthalpy changes Flashcards
What is enthalpy?
A measure of heat in a chemical system
Why do most chemical reactions have an enthalpy change?
Reactants and products have different bonds and structures so different amounts of energy
What’s a system?
The reactants and products
What’s the surroundings?
The rest of the universe
What is a chemical reaction that is exothermic?
Energy transferred from the system to the surroundings, ΔH is negative
What is a chemical reaction that is endothermic?
Energy transferred from the surroundings to the system, ΔH is positive
How to draw enthalpy level diagram for exothermic reaction?
Enthaply on y axis
Progress of reaction on x axis
Reactants higher than products
ΔH is the difference between the 2 lines
The activation energy is the distance from the reactants to the peak of the curve
How to draw enthalpy level diagram for exothermic reaction?
Enthaply on y axis
Progress of reaction on x axis
Reactants lower than products
ΔH is the difference between the 2 lines
The activation energy is the distance from the reactants to the peak of the curve
Activation energy definition?
The minimum energy required to start a reaction by the breaking of bonds
Why do most chemical reactions have an activation energy?
It’s necessary to break bonds in the reactant molecules
Equation for heat energy change in a reaction?
q= m x c ΔT
What’s q?
Heat energy change for the reaction carried out, measured in Joules
What’s m?
Mass of material for which the temperature change is measured in (eg the amount of water in solution)
What’s c?
Heat capacity of material for which temp is measured in eg. in water of solution = 4.18
what is ΔT?
Temp change measured in kelvin or degrees
Why is water used normally as the heat sink?
High heat capacity
Non hazardous
Readily available
How do you find the enthalpy change?
Divide the energy change by the amount of moles of your reactant, units are ΔH = kJ mol-1
What conditions do enthalpy changes need to measured in?
Temperature 298K or 25 degrees
Pressure 100kPa
Concentration 1 mol/dm^3
Definition of standard enthalpy change of combustion Δcomb H (circle with line going through)?
The enthalpy change that takes place when 1 mole of substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states
Definition of standard enthalpy change of formation Δform H (circle with line going through)?
The enthalpy changes that occurs when 1 mole of compound is formed from it’s elements under standard conditions, with all reactants and products in their natural states
Definition of standard enthalpy change of nuetralisation Δnuet?
The enthalpy change that accompanies the reaction of an acid and a base to form 1 mole of H2O under standard conditions, with all reactants and products in their standard states
What’s hess’s law?
If a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route.
How to draw a hess cycle?
Line from reactants to products of the unknown energy change
Then have a line from the reactants and from the products to different step
Can add the total forward enthalpies to find the missing enthalpy
What’s the average bond enthalpy?
The average enthalpy change that takes place when breaking by homolytic fission 1 mole of a given type of bond in the molecules of a gaseous species
How to work out enthalpy change when given the average bond enthalpy?
Draw the displayed formula of the reactants
and products
Break every bond in the reactants and calculate enthalpy change
Do the same for bond formation in products
Do bonds broken energy - bonds formed energy
