Enthalpy Changes Flashcards
If a reaction is exothermic what is the enthalpy change, where does energy go
Negative, surroundings gain energy and the temperature increases
If a reaction is endothermic what is the enthalpy change, where does energy go
Positive, surroundings lose energy and temperature decreases
What is the standard enthalpy change of reaction
The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states
What is enthalpy change of formation
The enthalpy change that takes place when 1 mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states
How do you work out the enthalpy change
H(products)- H(reactants)
What is activation energy
The minimum energy required for a reaction to take place
What is enthalpy change of combustion
The enthalpy change that takes place when 1 mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states
Definition of average bond enthalpy
Energy required to break 1 mold of a specified type of bond in a gaseous molecule
Draw an exothermic energy change
Reactants has a higher enthalpy then products
Surroundings gain energy and temp increases
Draw an endothermic energy change
Enthalpy of products is above the reactants
Surrounding lose energy and get colder
What are standard conditions
Pressure=100kPa
Temperature=25 degrees
Concentration=1moldm-3
State= physical state of a substance under standard conditions
What’s is the standard enthalpy change of reaction
The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states
What is the standard enthalpy change of reaction
The enthalpy change that accompanies a reaction
What is the standard enthalpy change of neutralization
The energy change that accompanies the reaction of an acid by a base to form one mole of h20(l) under standard conditions, with all reactants and products in their standard state
The energy change of surroundings is calculated from 3 quantities, what are they and what are the units
Mass= grams
Specific heat capacity is energy require to raise temp of 1g of substance by 1 degree, water is 4.18Jg-1K-1
Temp of surroundings is determined by thermometer 🔺T= T(final)-T(initial)
What is the equation for calculating energy change
q (Joules)= mc🔺T
What aren’t the experimental enthalpy change of combustion not very accurate
Heat loss to surrounding other than the water (includes beaker but mainly air)
Incomplete combustion of methanol (co and c instead of co2)
Evaporation of methanol from the wick (burner be weighed as soon as possible after extinguishing fire)
Non-standard conditions
All lead to less exothermic then expected
How would you determine the enthalpy change between a solid and a solution or 2 solutions
Use polystyrene cup and a thermometer
Are bond enthalpies always endo or exothermic
Endothermic, always have a positive enthalpy value
How do you find the enthalpy change of reaction
Calculating bond enthalpies of bonds in reactants and products
For gaseous molecules of covalent substances:
£(bond enthalpies in reactants)-£(bond enthalpies in products)
£= reverse E
What does Hess law state
If a reaction can take place by 2 routes, starting and finishing conditions are the same, the total enthalpy change is the same for each route
Eg A= breaking bonds (+110)
B= making bonds (-150)
C= reactants to products =40
A+B=C
Fe2O3= -824
CaO=-635
Construct an enthalpy cycle for enthalpy change of formation
Elements (Fe, Ca, O2)
Reactants B= Fe2O3+3Ca
Products C= 2Fe+3CaO
A= fe2O3+3Ca——->2Fe+3CaO
Route 1: B+A
Route 2:C
Hess law= B+A=C so, A=C-B
A=(3X-635)-(-824)
= -1081kJmol-1
For the enthalpy change of formation and combustion which way are the reactants and products in the equation
Formulation: products-reactants
Combustion:reactants-products
