Chapter 9
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Define Enthalpy Change, ΔH/ Kjmol-1
A change in heat energy between a system and its surroundings, under standard conditions.
What do enthalpy diagrams show?
The enthalpy change during a reaction.
Do exothermic reactions have a positive or negative enthalpy value?
NEGATIVE
Give two examples of a exothermic reaction.
Combustion or Acid/Base neutralisation reactions.
What is an exothermic reaction?
A reaction when energy if given out, so the system looses heat energy.
What labels do you need to include on an enthalpy diagram?
.X axis as 'progress of reaction' .y axis as 'energy' .Add enthalpy value if given .Reactants and Products .Activation energy
What is an Endothermic Reaction?
A reaction when energy is put into the system.
Name two examples of an endothermic reaction
Decomposition and Photosynthesis
Define Activation Energy, Ea
Activation energy is the minimum energy required to start a chemical reaction by breaking bonds.
Is Activation Energy, Ea a positive or negative value?
Positive
What is the rate of reaction for a small activation energy?
Fast
What is the reaction rate for a large activation energy?
Slow
What are the four Standard Conditions for enthalpy change?
• 100 Kpa (1 atm) • 298 K (25`) • Solutions a 1 moldm-3 • All reactants and products in their standard states.
Define Standard Enthalpy Change of Reaction, ΔrH*
The enthalpy change when molar quantities shown in a chemical equation, react under standard conditions, with all the reactants and products in their standard states.
Define Enthalpy Change of Formation, ΔfH*
The enthalpy change when 1 mole of product is formed from its elements under standard conditions, with all he reactants and products in their standard states.
What is the usual ΔfH* value for elements ?
Zero
Define the standard enthalpy change of Nuetralisation, ΔneutH*
The enthalpy change when an acid and base react to produce 1 mole of H20, under standard conditions, when all products and reactants are in their standard state.
Define the standard enthalpy change of combustion,ΔcH*
The enthalpy change when 1 mole of reactant burns completely with oxygen, under standard conditions with all reactants and products in their standard state.
What is the approximate value of ΔneutH*?
-57 kjmol-1
What is measuring enthalpy changes by experiment called?
Calorirmetry
What are the two common experiments for calculating enthalpy change?
Coffee Cup calorimeter Flame Calorimeter
What experiment can calculate enthalpy change that cannot be used in a college environment?
Bomb Calorimeter
What is the equation for calculating Enthalpy Change from experimental results?
q=mcΔT
What does the ‘q’ in q=mcΔt mean?
Energy released/ absorbed by reaction