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21 Cards in this Set
- Front
- Back
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State Hess' Law
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If a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route.
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What is meant by 'standard conditions'?
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A pressure of 100 kPa (equivalent to 1 atmosphere)
A stated temperature, usually 298 K (equivalent to 25 degrees C) A concentration of 1 mole per decimetre (for reactions in an aqueous solution) |
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What is 'enthalpy'?
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H, the heat content that is stored in a chemical system.
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What is 'enthalpy change'?
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An enthalpy change is the heat energy exchanged with the surroundings during a chemical reaction, at constant pressure.
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The standard enthalpy change of a reaction...
ΔH°r |
Is the enthalpy change which occurs when equation quantities of materials react under standard conditions, and with everything in its standard state.
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The standard enthalpy change of combustion...
ΔH°c |
Is the enthalpy change that takes place when one mole of a substance completely reacts with oxygen under standard conditions.
When a substance is completely combusted. |
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The standard enthalpy change of formation...
ΔH°f |
Is the enthalpy change that takes place when one mole of a compound is formed from its constituent elements under standard conditions.
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The first law of thermodynamics states that...
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Energy cannot be created or destroyed, although it can be changed from one form into another.
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In an exothermic reaction...
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-There is heat loss from the chemical system (to the surroundings)
-The temperature of the surroundings rises -The enthalpy of the products is smaller than the enthalpy of the reactants -enthalpy change is NEGATIVE |
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Oxidation reactions are...
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Important exothermic reactions.
e.g. combustion of fuels, oxidation of carbohydrates such as glucose in respiration. Explosions are essentially rapid exothermic reactions. |
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Enthalpy change =
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Final enthalpy - initial enthalpy
ΔH = H2 - H1 |
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Endothermic processes require an input of heat energy. The energy comes from the surroundings, resulting in a lowering of the temperature of the surroundings.
In an endothermic reaction... |
-The chemical system gains heat (from the surroundings)
-The temperature of the surroundings falls -The enthalpy of the products is greater than the enthalpy of the reactants -enthalpy change is POSITIVE |
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Endo
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A prefix from greek ἔνδον meaning "within, inner, absorbing, or containing".
In an endothermic reaction, the chemical system 'absorbs' heat from the surroundings. |
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Exo
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A prefix from Ancient Greek ἔξω, meaning "outer, external”.
In an exothermic reaction, heat is lost to the outside environment - the surroundings. Like exoskeleton - a skeleton on the outside. In an exothermic reaction, the energy moves outside of the chemical system. |
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What is meant by the term 'enthalpy change of combustion'?
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The enthalpy change when one mole of a substance is completely combusted/burns in excess oxygen.
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In terms of bond breaking and forming, why might a reaction be endothermic?
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Bond breaking absorbs energy. Bond forming releases energy. In an endothermic reaction, more energy is absorbed than is released.
Therefore, the enthalpy of the products is greater than the enthalpy of the reactant. |
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C6H12O6 + 6 O2 = 6 CO2 + 6 H2O
Glucose completely combusts to form carbon dioxide and water. Explain, in terms of bond breaking and bond forming, why this reaction is exothermic. |
Bond breaking absorbs energy. Bond forming releases energy. In an exothermic reaction, more energy is released than is absorbed.
Therefore, the enthalpy of the reactants is greater than the enthalpy of the products. |
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Suggest why the enthalpy change of formation of C6H12O6 cannot be determined directly.
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Would make carbon dioxide and water instead.
Activation energy too high. Rate is too slow. Wouldn't react together. |
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The energy change, Q, = ...
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Q = Mass of the immediate surroundings (multiplied by) the specific heat capacity of the surroundings (multiplied by) the change temperature.
Q =MCΔT |
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The enthalpy change, H, = ...
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The energy chance divided by the amount in moles.
ΔH = Q/n |
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Suggest the reasons why standard enthalpy changes of combustion determined experimentally are less exothermic than the calculated theoretical values.
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Heat released to the surroundings during an experiment / 'heat loss'.
Incomplete combustion/incomplete reaction/not everything burns. Non-standard conditions. |
