Enthalpy change
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What symbol is used for the energy change of an exothermic reaction?
-
What symbol is used for the energy change of an endothermic reaction?
+
What is meant by enthalpy?
a measure of the heat energy in a chemical system
For an exothermic reaction what will happen to the temperature of the reaction mixture?
It will rise
For an endothermic reaction what will happen to the temperature of the reaction mixture?
It will fall
Why do most chemical reaction have an activation energy?
It is necessary to break bonds in reactant molecules
What is meant by a chemical system?
The atoms, molecules, or ions making up the chemicals
What is meant by the surroundings?
The apparatus, the laboratory (everything that isn’t the chemical system)
What directions can energy transfer occur in?
• From the chemical system to the surroundings (exothermic) - From the surroundings to the chemical system (endothermic)
What is the symbol for enthalpy?
H
What is the y axis on enthalpy profile diagrams?
enthalpy, H
What is the x axis on ethalpy profile diagrams?
Progress of the reaction
On an enthalpy profile diagram, for an exothermic reaction, what are the relative positions of the reactants and products?
The reactants are higher than the products
On an enthalpy profile diagram, for an endothermic reaction, what are the relative positions of the reactants and products?
The reactants are lower than the products
In terms of energy, how does bond breaking differ from bond making?
Bond breaking requires energy, whereas bond making releases energy
What is activation energy?
The minimum energy required for a reaction to take place
What must be included when drawing an enthalpy profile diagram?
The following must be labelled: • reactants • products • enthalpy change (delta H) • activation energy
What are the units for enthalpy change?
KJ per mol
What are standard conditions?
• 100KPa • 298k (25 degrees) • 1 mol per dm3 (applicable for solutions only)
What are standard states?
The state of a substance under standard conditions
What is the enthalpy change of formation?
The ENTHALPY change which accompanies the formation of one mole of a substance from its elements under standard conditions with all reactants and products in their standard states
What is the enthalpy change of combustion?
The enthalpy change which accompanies the complete combustion of one mole of a substance in oxygen, under standard conditions with all reactants and products in their standard states
What is the enthalpy of formation of elements?
0 KJ per mole
What is the enthalpy change of neutralisation?
The enthalpy change which accompanies the reaction of an acid BY a base to form one mole of water under standard conditions, with all reactants and products in their standard states.