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Flashcards in Enthalpy Changes Deck (17)
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Measure of heat energy in a chemical system
Can be thought of as the energy stored within bonds
Only changes can be measured


Enthalpy change

Delta H, difference in enthalpies
Can be positive or negative


Law of conservation of energy

Energy cannot be made or destroyed, it can only be transferred from 1 to another
Chemical reactions occur between systems and surroundings
System, chemicals
Surroundings, apparatus


Chemical system

Atoms, molecules, ions making up chemicals


exothermic reactions

Energy from system to surroundings
Surroundings increase in energy
-ve delta H


Endothermic reactions

Energy from surroundings to system
Surroundings decrease in energy
+ve delta H


Activation energy

Energy input required to break bonds which act as a barrier to reactions
Minimum energy required for a reaction to take place


Standard conditions for standard enthalpy changes

Standard pressure, 101kPa, 1 atm
Standard temperature, 298K, 25C
Standard concentration, 1 mol dm-3 (solutions only)
Standard state, physical state under standard conditions
kJ mol-1


Standard enthalpy change of reaction

Standard enthalpy change of reaction that accompanies a reaction in molar quantities in a chemical reaction
Depends on balancing numbers


Standard enthalpy change of formation

Enthalpy change that takes place when 1 mole of a compound is formed from its elements under standard conditions


Standard enthalpy change of combustion

Enthalpy change that takes place when 1 mole of a substance reacts completely with oxygen under standard conditions


Standard enthalpy change of neutralisation

Enthalpy change that accompanies the reaction of an acid by a base to form 1 mole of water under standard conditions

Same for all neutralisation reactions


Measuring energy changes

Q=mc delta t

Calculated from mass, specific heat capacity and temperature changes.
C=4.18J g-1 K-1


Determination of delta c H

Weigh spirit burner
Place under colorimeter filled with 150 cm3 water
Measure initial temperature
Burn fuel
Measure temperature of water, weigh spirit burner

Find the moles burnt
Use the mass of water and use q=mc delta t
Q/moles=enthalpy change


How accurate is the experiment for delta c H

Heat loss to surroundings other than water
Incomplete combustion of methanol
Evaporation of methanol from wick
Non standard conditions

Use fraught screens and input of oxygen gas, minimise errors from heat loss, incomplete combustion


Determination of delta r H

Reactions between 2 solutions, solid and solution
Polystyrene cups, offer insulation against heat loss to surroudings
When carrying out reactions between aqueous solutions, the solution itself is the immediate surrounding
Chemical particles within solution react when they collide, energy transferred between chemical particles and water molecules

Same method as before, use mass of solution only in first calculation


Determination of delta neut H

2 solutions for water and and a salt

Same method as before, the total volume is the mass for the first calculation