Enthalpy Flashcards
What happens in exothermic reactions?
Products have less enthalpy than reactants
Chemical energy is transferred into thermal energy
The chemicals lose energy
Energy that is lost is gained by surroundings increasing temp
As a result enthalpy change (∆H) is negative
What happens in endothermic reactions?
Products have more enthalpy than reactants
The chemicals gain energy
Energy that is gained by chemicals is lost by surroundings decreasing temp
As a result enthalpy change (∆H) is positive.
What is enthalpy?
Is the heat energy contained in chemicals e.g reactants or products
What is enthalpy of reaction?
When reactants react in their molar ratio according to the equation
What is enthalpy change?
The heat energy change in a chemical reaction at a constant pressure
What is an example of an equation for empathy of reaction?
2CO + O2 →2CO2
What is enthalpy of combustion?
When one mole of substance is burnt completely in excess oxygen
What is an equation for enthalpy of combustion?
C + O2→CO2
What is enthalpy of formation?
When one mole of a compound is formed from its elements in their standard states
What is the equation for enthalpy of formation?
C + 1/2O2 →CO
What are ‘standard conditions’ for enthalpy?
101KPa (1 atmosphere)
25°c
How to calculate energy change?
q= mc∆T
What does q stand for and what are its units?
energy change
joules
What does m stand for and what are its units?
Total mass of solution (what ever thermometer is touching)
grams
What does c stand for and what are its units?
specific heat capacity (water mostly unless told otherwise)
4.18 Jg-1 K-1
What does ∆T stand for and what are its units?
Change in temperature
degrees c
What sign do you need to add at the end?
+ or -
for endo or exo
What is the next step?
equartion?
∆H= Q(KJ)/ N moles
you might need to use n=cv first if solution to work out moles
How to work out moles in limiting reagent ?
Mass of limiting reagent/ mr of limiting reagent
What are problems of calculating enthalpy changes experimentally?
reaction rate is too slow ti allow measurement of enthalpy change
reaction may produce unwanted products (isomers)
and heat is lost to surroundings
Ways to reduce heat being lost to surroundings?
add lid to polystyrene cup
use insulator around cup
A reaction is exothermic when?
more energy is released during bond forming than energy taken in during bond breaking.
A reaction is endothermic when?
more energy is absorbed during bond breaking than energy is released during bond breaking
How to work out enthalpy change of combustion? ∆Hc
use q= mc∆T
then calculate number of moles
calculate energy transfer ∆H= Q (KJ)/ n
What are errors of calculating ∆Hc?
Heat is lost to surroundings
incomplete combustion
evaporation from wick
non-standard conditions being used
Methods for reducing heat loss?
adding lid to beaker
using draft shields around the sides
How to work out bond enthalpy?
1- if in structural draw out in displayed
2- list bonds that are broken in reactants then do same for in products.
3- reactants subtract products
What is the definition of bond enthalpy?
enthalpy change to break one mole of a covalent bond in the gaseous phase
Limitations of using average bond enthalpys?
doesn’t take in to account specific atoms that surround the bond.
its only an average value.
Enthalpy change with an acid and base definition?
when an acid reacts with a base in their standard states to produce 1 mole of water
Enthalpy change of neutrallisation equations?
NaOH + HNO3 –> NaNO3 + H20
