10 - Enthalpy
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Define enthalpy
How much heat is stored in a system symbol: H unit : kJ / mol or kJ mol^-1
Define system
The atoms, ions and particles in the chemical involved in the reaction
Define exothermic and endothermic
An exothermic reaction is one which releases heat energy from the system into the surroundings An endothermic reaction is one which takes in heat energy from the surroundings into the system
Give an example of an exothermic reaction
Combustion reactions
Give an example of an endothermic reaction
Thermal decomposition of calcium carbonate
What is the difference in enthalpy change for exothermic or endothermic reactions?
Exothermic reactions give a negative enthalpy change Endothermic reactions give a positive enthalpy change
What is the enthalpy change of the thermal decomposition of calcium carbonate?
+ 178 kJ mol^-1
Define activation energy
Minimum energy required to start a reaction by breaking bonds symbol: E~a
What equation do we use to find out how much energy is released by / stored in a given amount of substance?
E = nH Energy = moles * enthalpy
What 3 things is enthalpy affected by?
Temperature Pressure Concentration
What are the standard conditions for measuring enthalpy change?
298K temperature 101kPa pressure 1 mol dm^-3 concentration
Define standard enthalpy change of formation
Enthalpy change when 1 mole of a compound is formed from its elements under standard conditions with substances in standard states
How do you calculate the equations of standard formation?
You write the product (like H~2 0) on the right of the reaction arrow You then decide how many moles of the elements are needed to make that 1 mole of product
What is the standard formation equation of ammonia
0.5 N~2 (g) \+ 1.5 H~2 (g) --> NH~3 (g) You would then probably have to write the standard enthalpy change of formation afterwards
What is the enthalpy of formation of just an element?
Always 0 kJ/mol for just elements
What is the first way of measuring enthalpy change?
Transferring heat to and from water or surrounding air You then measure the temperature of the water / air
Define specific heat capacity
The amount of energy needed to heat 1g of a substance by 1K
What is the specific heat capacity of water?
4.18 J g^-1 K^-1
How do you calculate the heat energy a fuel has released?
Q = mc delta T Heat energy = mass of surroundings specific heat capacity temperature change in surroundings
What is the density of water / a solution?
1 g cm^-3 Remember that it is grams and centimetres
How do you calculate enthalpy change from heat energy?
Delta H = Q / n Enthalpy change = heat energy released / number of moles which released that energy
What is direct determination?
When enthalpy change is calculated from one experiment
What are the 4 steps for calculating enthalpy change from experiment results?
Calculate Q using Q = mc Delta T Calculate n of the surroundings Do Delta H = Q / n Make enthalpy change +ve / -ve if reaction is endothermic or exothermic respectively
Define standard enthalpy change of combustion
Enthalpy change when 1 mole of a substance reacts completely with oxygen under standard conditions with substances all in standard states