Enthalpy Flashcards
What is enthalpy
The heat energy stored in chemical bonds. It’s a measure of energy content of a substance at a constant pressure
What happens if the enthalpy increases
- reaction is endothermic (heat taken in fro the surroundings)
- change is +ve
What happens in the enthalpy decreases
- Reaction is exothermic ( heat lost to the surroundings)
- change is -ve
Enthalpy change equation
Enthalpy of products - enthalpy of reactants
Standard enthalpy change
Pressure
Temperature
100kPa (1 atmosphere)
298k (25 degrees)
Common exothermic reactions
- combustion
- respiration
- self heating cans
Common endothermic reactions
- photosynthesis
- cooling energy packs
- thermal decomposition
How to work out enthalpy change on a graph
Bond enthalpies of bonds broken - bond enthalpies of bonds made
What is the standard enthalpy of formation
The enthalpy change when one mole of a compound is formed in its standard state from its element in their standard states
Features of standard enthalpy of formation
- usually, but not alway exothermic
- only one mole of product on the right hand side of the equation
- elements in their standard states have zero enthalpy of formation
Standard enthalpy of combustion
The enthalpy change of one mole of a substance undergoes complete combustion under standard conditions. All reactants and products are in their standard states
Features of standard enthalpy of combustion
- always one mole of what you are burning on the left hand side of the equation
- always exothermic
Standard enthalpy of reaction
The enthalpy change when molar quantities, as shown in an equation reacts in their standard states
Features of standard enthalpy of reaction
- enthalpy change is given
- equations must be balanced
Standard enthalpy of neutralisation
Th enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water
what is calorimetry?
practical determination of enthalpy changes. this usually involves heating/cooling known amounts of water
calculation for heat energy
q = m c T
q = heat energy joules
m = mass of object releasing or absorbing q (grams) usually of the surrounding water
c = specific heat capacity, water is 4.18 Jg-1K-1
t = change in temp (t final - t initial) K or degrees
calculation for enthalpy changes
change in H = -q/ n
q = heat energy
n = number of moles
mass and moles in a neutralisation reaction
m = mass of acid and alkali that react
n = moles of water that are formed in a reaction, from the balanced equation
what is Hess’s law?
enthalpy change is independent of the path taken providing that the starting and final conditions are the same for both pathways
what is an exothermic reaction?
enthalpy of the products are smaller than the enthalpy of reactions resulting in the heat loss to the surroundings
what is an endothermic reaction?
enthalpy of the products are greater than the enthalpy of the reactants, resulting in heat being taken in from the surroundings
definition of standard conditions
conditions required for all enthalpy calculations. all substances must be in their normal/ most stable physical state
why may calculated enthalpy values be different from experimental values?
- value of enthalpy change calculated may not be standard value .e.g. solids or liquids produced
- an average form several different compounds
