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
what are potential sources of enthalpy practical errors
- fuel may not have burned completely (incomplete combustion)
- conditions aren’t standard
- evaporation of alcohol
- heat capacity of the calorimeter is not included
- heat transferred to the air not water
- experiment takes time, not all of the heat energy transferred from the water too surroundings were compensated for
how would you improve accuracy in an enthalpy practical?
- use a lid
- use a bomb calorimeter, its designed to ensure that the sample burns completely and energy losses are minimised
What is enthalpy change
Heat change at constant pressure
What has a higher enthalpy a liquid or a solid
Liquid
What are the standard condition
Pressure 100kPa
Temp 298k
Why is graphite more thermodynamically stable than diamond
The change of graphite into diamond is endothermic/ change tends to go in the direction of lower energy level
Disadvantages of using a glass beaker on a tripod and gauze
- glass beaker is a poorer conductors than copper
- Tripod and gauze would reduce heat transfer
How could you reduce percentage uncertainty in the temperature change during a calorimetry experiment?
Increase the concentration of a solutionS
Why is the heat change Calculated from the bomb colorimeter experiment Not an Enthalpy change
The pressure is not constant in a bomb calorimeter
How can you determine an accurate value for the enthalpy of reaction in a calorimetry experiment?
- place a lid on a beaker to reduce heat loss
- Use a calorimeter instead of a beaker
- Extrapolate back to the addition, so you can find a temperature change
- Record temperature in set intervals, so you can plot a graph
- Measure the initial temperature of a solution before addition
- Measure the temperature after addition at regular intervals
- We have a glass after adding a solid
