energetics Flashcards
What is enthalpy?
It is a measure of the heat content of a substance
not the same as energy and a more accurate way to measure energy changes during a reaction
What is Enthalpy change (∆H)
change in heat content at constant pressure
What are the Standard conditions (∆H°)
100 kPa and a stated temperature (usually 298K)
show the reaction profile for an exothermic reaction
the energy of the substance is in the bonds
x axis is time
show reaction profile for an endothermic reaction
the energy of the substance is in the bonds
x axis is time
explain ethalpy change in reaction
(∆rH°) (“enthalpy of reaction”)
This is the enthalpy change for a reaction with the quantities shown in the chemical equation. This means that the value should always be quoted along with the equation.
explain and define ethalpy of formation (∆fH°)
Enthalpy change when 1 mole of a substance is formed from its constituent elements with all reactants and products in standard states under standard conditions
give some examples of enthalpy change of formation
H2 + ½O2 → H2O (has to only be one mole so must balance like this)
½N2 + 1½H2 → NH3 (g)
What is Standard enthalpy change of combustion (∆cH°) (“enthalpy of combustion”)
Enthalpy change when 1 mole of a substance is completely burned in oxygen with all reactants and products in standard states under standard conditions.
show examples of enthalpy of combustion
CH4 + 2O2 → CO2 + 2H2O
Na + ¼O2 → ½Na2O
must only be one mole of the beginning substance
What is Standard enthalpy change of neutralisation (∆neutH°) (“enthalpy of neutralisation”)
Enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard conditions.
show examples of standard enthalpy change of neutralisation
when balancing, balance normally then half or whatever to give one mole of water
½H2SO4 + ½Ba(OH)2 → ½BaSO4 + H2O
explain the q = m c ∆T equation
enthalpy change for a reaction can be found by measuring the temperature change in a reaction.
The heat energy given out (or taken in) is used to heat (or cool) a known mass of water.
We know that it takes 4.18 J of energy to raise the temperature of 1 g of water by 1°C (i.e. 1 K).
The amount of energy needed to make 1 g of a substance 1°C (1 K) hotter is called the specific heat capacity (measured in J g-1 K-1).
The following equation is then used to find the amount of heat energy give out (or absorbed).
explain the enthalpy change (per mole)
To find the enthalpy change in terms of J (or kJ) per mole, the following expression is needed
THINK kJ per mole!
explain one issue with calorimetry
Heat loss is a major problem with calorimetry and can lead to errors in the results. The techniques used in calorimetry are designed to reduce heat loss (one way to reduce errors from heat loss is to measure the heat capacity of the calorimeter as a whole (see flame calorimeters and bomb calorimeters)
explain calorimetery with reactions in soultion
Reactions taking place in solution
The reaction is carried out in an insulated beaker (polysyrene) and the temperature change measured.
The reaction must be fast so that the maximum temperature is reached quickly (before it starts to cool).
The specific heat capacity of the solution must be taken as being the 4.18 J g-1 K-1 (the same as water) and we usually use the mass of the water (not the solution) in the calculation.
explain and describe flame calorimeters
Flame calorimeters are usually used to find the enthalpy of combustion of substance (usually a fuel).
- The fuel, which is in a bottle with a wick, is burned so that the heat is passed to water which it heats.
- Sometimes the specific heat capacity of water is used, but in more accurate flame calorimeters, the actual heat capacity of the flame calorimeter can be determined and used (by finding the temperature rise when a known amount of a substance with an accurately known enthalpy of combustion is tested).
