Chapter 9 - Enthalpy Flashcards
What is a chemical system?
Refers to the atoms, molecules, or ions making up the chemicals.
What is enthalpy?
Measure of heat energy in a chemical system
or
energy stored within bonds
What is enthalpy change (change in H)?
Difference in enthalpies of reactants of reactants and products
change in H = H(products) - H(reactants)
What is an exothermic reaction?
When a chemical system releases heat energy into the surroundings.
Change in heat is negative
What is an endothermic reaction?
When a chemical system takes in heat energy from the surroundings
change in heat is positive
What is activation energy? (Ea)
Minimum energy required for reaction to take place, by breaking bonds.
Why do reactions with small activation energies take place rapidly?
Energy needed to break the bonds is readily available from the surroundings.
Draw an enthalpy profile diagram for an exothermic reaction
Give the standard conditions?
standard pressure - 101KPa
standard temperature - 298K or 25oC
standard concentration - 1moldm^-3
standard state - physical state of a substance under standard conditions
Define standard enthalpy change of reaction?
enthalpy change that accompanies a reaction in the molar quantities shown in the chemical equations under standard conditions, with all the reactants and products in their standard states.
Define standard enthalpy change of formation?
enthalpy change that takes place when one mole of compound is formed from it’s elements under standard conditions, with all reactants and products in their standard states.
Define standard enthalpy change of combustion?
enthalpy changes that take place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states.
Define standard enthalpy change of neutrilization?
energy that accompanies the reaction of an acid by a base to form one mole of H2O(l) under standard conditions, with all reactants and products in their standard states.
Write the equation of energy change in a reaction
q = mc x change in temp
heat energy = mass of heated substance/ solution x specific heat capacity of substance/solution x change in temperature
(if solid added to liquid - use mass of liquid and if 2 solutions added - use total mass)
Define specific heat capacity
Amount of energy required to heat 1g of a substance by 1 oC.
units - Jg^-1K^-1
Define average bond enthalpy
Average enthalpy change in breaking of one mole of a specified type of covalent bond in a gaseous molecule.
bond enthalpies are always endothermic
How do you calculate enthalpy changes from average bond enthalpies?
Calculating the bond enthalpies of the bond in the reactants and the products.
enthalpy changes = sum of bond enthalpies in reactants - bond enthalpies in product
What does Hess’ law state?
The total enthalpy change of a reaction is always the same, no matter which route is taken.
How do you work out the standard enthalpy changes from the standard enthalpy changes of formation of the reactants and products?
step 1: construct enthalpy cycle between the reactants, the products, and their elements. Form common link between reactants and products
step 2: Add standard enthalpy changes of formation values and calculate unknown enthalpy change.
How do you work out the enthalpy change from enthalpy change of combustion?
step 1: construct the enthalpy cycle between the reactants, products, and their common combustion products , CO2(g) and H2O(l). Arrows pair down as reactants and products combust forming combustion products.
step 2: add enthalpy of combustion values and calculate the unknown enthalpy changes.
What are the 2 rules to help with calculating using enthalpy changes of formation and combustion.
enthalpy change = sum of enthalpy change of formation of products - enthalpy change of formation of reactants.
enthalpy change = sum of enthalpy change of combustion of reactants - sum of enthalpy change of combustion of products.
Why would calculated standard enthalpy change of combustion from an experiment be different to a value in data books?
- Incomplete combustion
- Non-standard conditions
- Specific heat capacity of apparatus/beaker
- Some of substance heated evaporates.
When experiemtally measuring enthalpy change using temperature thermometer, are you measuring temperature of surroundings or the chemicals?
surroundings
