C9- Enthalpy Flashcards
What is enthalpy
a measure of heat energy in a chemical system
What is a chemical system
atoms, molecules or ions making up the chemicals
exothermic reaction
Energy is transferred from the chemical system to the surroundings
results in a temperature increase
negative enthalpy change
Endothermic reaction
energy is transferred from the surroundings to the chemical system
Results in a temperature decrease
positive enthalpy change
What is activation energy
minimum energy required for a reaction to take place
Standard conditions
Pressure, temperature, concentration
Pressure= 100kPa
Temperature= 25c or 298K
concentration= 1M
Standard enthalpy change of reaction
define
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
Standard enthalpy change of formation
Define
Enthalpy change that takes place, when one mole of a compound is formed from its elements under standard conditions with all reactants and products in their standard states
Standards enthalpy change of formation of an element
value
always zero
standard enthalpy change of combustion
define
Enthalpy change that takes places when one mole of a substance reacts completely with oxygen under standard conditions with all reactions and products in their standard states
Standard enthalpy change of neutralisation
energy change that accompanies reaction of an acid by a base to form one mole of water under standard conditions and all reactants and products in their standard states
Specific heat capacity
define
The energy required to raise the temperature of 1g of a substance by 1K
How to experimentally determine the enthalpy change of combustion of a fuel
beaker of water of known mass over a spirit burner
weigh spirit burner before and after usage
heat water by 15c
measure change in mass of spirit burner
use formula q=mcT
Heat energy change formula
q= m c T
Mass
specific heat capacity Jgk
Change in temperature c or K
Problems with accuracy of experimental determination of enthalpy change of combustion
heat lost to surroundings other than water
incomplete combustion, forms soot on bas of beaker
evaporation of fuel from wick (place cap on right after extinguishing)
Non standard conditions
Impact of problems with accuracy of experimental determination of enthalpy change of combustion
leads to the enthalpy change being less exothermic than was expected
How to experimentally determine enthalpy change of reaction
solid added to solution in a polystyrene cup
Take temperature readings every 30 seconds
Plot values on graph
How to find maximum temperature on a time/temperature graph
Extrapolate from each section of graph, intersection of lines is the max temp
How to determine enthalpy change of neutralisation experimentally
In a polystyrene cup add two solutions
record temperature every 30 seconds
plot values on a graph
Enthalpy change units
KJmol^-1
Limitations of using average bond enthalpies
An average bond enthalpy is calculated using bond enthalpies from different chemical environments, not the actual bond enthalpy of the bond
What are standard conditions
1atm and 25c
298K 100kPa
Hess’ Law
If a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy change for each route is the same
How to calculate enthalpy change using enthalpy of formation vs combustion
Combustion= reactants - products
Formation= products - reactants
