16 — chemical energetics Flashcards
Exothermic
System (reactants and products) releases thermal energy to the surroundings (reaction mixture, reaction flask, surrounding air). Temperature of the reaction mixture increases. When the CR is complete, tempt of the reaction mixture gradually decreases until it reaches room tempt
Examples of exothermic changes
Physical processes
1. Freezing
2. Condensation
3. Dissolving acids and alkalis in water
Chemical reactions
1. Combustion
2. Oxidation of metals
3. Neutralisation
4. Displacement of metals
5. Respiration
Endothermic
When [system] (reactants and products) reacts, energy is absorbed from the surroundings. The temperature of the
surroundings decreases.
When the dissolution is complete, the temperature of the reaction mixture gradually increases until it returns to room temperature.
Examples of endothermic changes
Physical:
Melting
Boiling
Sublimation
Dissolution of some ionic compounds in water
Chemical:
Photosynthesis
Thermal decomposition
Electrolysis of water
Photography
Enthalpy change
Enthalpy change, [triangle] H, measures the difference in energy content of the reactants and products
Activation energy
Activation energy Ea is the minimum amount of energy that colliding reactant particles must possess to react w each other
Bond energy
Bond energy is the amt of energy absorbed to break one mole of a chemical bond. It is also the amount of energy released when 1 mole of that bond is formed.
Overall Enthalpy change (after calculation)
The energy absorbed for bond breaking in [reactants] is less (exo)/more (endo) than the energy released when bond making in [product]. Hence, the reaction is exothermic/endothermic.
Using ideas about breaking and forming bonds to explain why all of the values in the table are negative
A negative enthalpy change value means that the reaction is exothermic. Combustion is an exothermic reaction because the total energy absorbed for breaking the bonds in the alcohol molecules is less than the energy released during the combustion of alcohols.
The enthalpy change of combustion in kJ/mol increases from methanol to butanol. Suggest a reason why.
The number of Carbon and hydrogen atoms increases from methanol to butanol. Thus, more bonds are formed and a larger amount of energy is released during the combustion of the alcohols.
Suggest why the enthalpy changes of combustion for the 2 isomers are different
The amount of energy needed to break the bonds in ethanol is different from that in dimethylether. In ethanol, energy is needed to break 1 C—C bond, 5 C—H bonds, 1 C—O bond and 1 O—H bond. In dimethylether, energy is needed to beak 6 C—H bonds and 2 C—O bonds.
C—F bond has a lesser bond energy than C—Cl bond. Explain why the ozone layer contains many more chlorine atoms than fluorine atom.
C—Cl bond requires lesser energy to break than C—F bond (quote data). Thus more C—Cl bonds are broken and more free Cl atoms can be formed.
Lesser energy is needed to maintain the temperature for a chemical equation which has a lesser enthalpy change, B, than A.suggest 2 reasons why reaction B requires less energy.
The activation energy for reaction B is lower than the activation energy needed for A.
Temperature condition needed for reaction B is lower than A (quote data) thus less energy is needed for B’s reaction.
Describe the differences in the energy changes and temperature changes that happen as each compound dissolves (1 endo 1 exo)
When X dissolves, the enthalpy change is negative and exothermic but when Y dissolves, the enthalpy change is positive and endothermic.
Temperature of the X solution increases until the highest temperature is reached and decreases to room temperature but temperature of Y solution decreases until the lowest tempt is reached and increases to room temperature.
Explain why the calculated energy change includes a plus sign
A plus sign indicates that the energy change is positive. Heat energy is absorbed from the surroundings as the temperature decreases, thus it is endothermic.
