Chemical Energetics (Energy of reactions)
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What is associated with all types of reactions?
Change is a macro-concept that is associated with all types of reactions.
When one substance is transformed to another, _______ is almost always associated with the process.
Energy change
What are the three important types of energy conversions?
(a) Chemical reactions that involve heat energy change or light energy change (exothermic and endothermic reactions） (B) chemical reactions that involve an electrical change (in cells and batteries, in electrolysis); and (C)* nuclear reactions (nuclear fission) The focus for this chapter on chemical energetics is on heat energy change.
What is the definition of exothermic reaction?
An exothermic reaction is one where heat energy is given out or released to the surroundings, causing a temperature increase in the surroundings.
The energy content of the products is _______ than that for the reactants for an exothermic reaction. Thus, the enthalpy change or heat energy change of the reaction, triangle H, will have a _______ value.
Lower Negative
What does Er (r is a subscript) represent?
Er = energy content of reactants
What does Ep (where p is a subscript) represent?
Ep = energy content of the products
What does triangle H mean?
Enthalpy change = Ep - Er
What is an endothermic reaction?
An endothermic reaction is one where heat energy is taken in or absorbed from the surroundings, causing a temperature decrease in the surroundings.
The products have a _____ energy content than the reactants for an endothermic reaction.
higher
The enthalpy change (triangle H) will be _____ in an endothermic reaction.
Positive
How does an energy level diagram look like for an exothermic reaction?
Vertical axis labelled as energy and at the bottom, there is a marking for Ep (energy content of products) and there is a marking above Ep which is Er which is energy content of reactants. The reactants (with underline) is found at the right side of the Er marking and the products (with underline) is found at the right side of the Ep marking and—> further to the right An arrow pointing downwards, only one arrow head, connects reactants to the product word Beside the arrow, you will write triangle H = - ve
How does an energy level diagram look like for an endothermic reaction?
Vertical axis labelled as energy and at the bottom, there is a marking for Er (energy content of products) and there is a marking above which is Ep which is energy content of reactants. The reactants (with underline) is found at the right side of the Er marking and the products (with underline) is found at the right side of the Ep marking and—> further to the right An arrow pointing upwards, only one arrow head, connects reactants to the product word Beside the arrow, you will write triangle H = +ve
The direction of the _______ in the energy level diagram are in the direction of the ________.
The direction of the arrows in the energy level diagrams are in the direction of the energy change.
What is the definition of the enthalpy change of a reaction?
The enthalpy change of a reaction, triangle H is the heat energy change when reactants from products according to the quantities in moles for a specified equation.
Is it true that the heat energy change will depend on the quantities of reactants reacting? What does this mean?
Yes. The heat energy change will depend on the quantities of reactants reacting. It is therefore important to specify the reaction equation when referring to a triangle H value.
Consider methane, CH4, burning in oxygen, O2 CH4(g) + 2O2 (g) —> CO2 (g) + 2H2O (l) Triangle H is= -890 kJ What does the above imply?
The above implies when 1 mol CH4 and 2 mol O2 react to produce 1 mol CO2 and 2 mol H2O, 890 kJ of heat energy will be released.