5a Chemical Energetics I Flashcards
Define the enthalpy of a system. What does it imply?
The enthalpy of a system is a measure of the energy content of the system and has the symbol H. It implies the stability of a system as the higher the energy content of a system, the more unstable it is.
Define enthalpy change in two ways. What are the units of enthalpy change?
Enthalpy change is defined as the change in energy content (energy absorbed or released) of a process in a system at constant pressure.
It is also defined as the difference between the quantity of heat absorbed to break the bonds in the reactants and that released during the formation of new bonds in the products at a constant temperature. Units are KJ mol^-1
Why does enthalpy change?
During a physical or chemical process, the new system is likely to have different energy because new substances may have fired and/or the intermolecular forces/bonding may have changed. It appears as absorption or release of heat.
What are the endothermic and exothermic processes that take place during a chemical reaction?
During a chemical reaction, bonds are broken and new bonds are formed. The breaking of bonds is an endothermic process as it absorbs energy while the formation of bonds is an exothermic process as it releases energy.
Define an exothermic reaction.
A reaction in which energy is released to the surroundings.
Define an endothermic reaction.
A reaction in which energy is absorbed from the surroundings.
Describe the temperature changes and enthalpy changes that take place during an exothermic and endothermic reaction.
During an exothermic reaction, the temperature of the surroundings increases. The enthalpy change is negative as more energy is released than absorbed. Thus, the energy level of the products is less than that of the products and the products are energetically more stable than the reactants. However, during an endothermic reaction, the temperature of the surroundings decreases. The enthalpy change is positive as more energy is absorbed than released. The energy level of the products is more than that of the products and the products are energetically less stable than the reactants.
Sketch the energy level diagram and energy profile diagram of both an exothermic and endothermic reaction.
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Are exothermic or endothermic reactions more likely to occur and why?
Exothermic reactions have more energetically stable products hence they are more energetically feasible and are more likely to occur once initiated.
Define activation energy, Ea of a reaction.
The activation energy of a reaction is the minimum amount of energy that the reactant particles must possess before they can collide successfully to form products.
Can we predict with certainty whether a reaction will occur by looking at its enthalpy change?
No, the enthalpy change of a reaction is an indication of the energetic stability, not its kinetic stability (which indications the rate of a reaction), of the products with respect to the reactant. Many of the reactions are energetically feasible because they are exothermic reactions but they may occur very slowly due to very high activation energies.
What is a thermochemical equation and what does the enthalpy change only apply to?
It is a balanced equation that includes the enthalpy change of reaction and state symbols. It only applies to the number of substances in their specified physical states for that given balanced equation. If the reaction is reversed, the sign of enthalpy change is also reversed. If the chemical equation is multiplied by a factor, the enthalpy change would also be multiplied by the same factor.
What are standard conditions?
Pressure at 1 bar (10^5 Pa)
Temperature at 298K (25 degree Celcius)
All substances involved are in their standard states (most stable form at 1 bar and 298K).
What are standard enthalpy changes?
They are enthalpy changes determined under standard conditions.
What are allotropes?
Different forms of the element in the same physical state.
What is the standard state of carbon?
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Define the standard enthalpy change of reaction.
The standard enthalpy change of reaction is the energy change in a chemical reaction when the molar quantities of reactants stated in a chemical equation react under standard conditions (i.e. 1 bar and 298K).
Define the standard enthalpy change of formation. What can it be used for?
The standard enthalpy change of formation is the energy change in a chemical reaction when one mole of the pure substance is formed from its constituent elements in their standard states under standard conditions (i.e. 1 bar and 298K). (Name the individual elements in their standard state when asked to explain standard enthalpy change of formation of e.g. The standard enthalpy change of formation of gaseous nitrogen dioxide is the energy change in a chemical reaction when one mole of NO2 (g) is formed from N2 (g) and O2 (g) under standard conditions.) It can be used to predict the stability of a compound relative to its constituent elements. The more negative the standard enthalpy change of formation, the more stable the compound is relative to its constituent elements, and the less likely the decomposition of the compound back into its constituent elements.
Define the standard enthalpy change of formation of an element.
The standard enthalpy change of formation of an element in its standard state standard under standard conditions (i.e. 1 bar and 298K) is zero.
Define the standard enthalpy change of combustion. What can it be used for?
The standard enthalpy change of combustion of a substance is the energy released when one mole of the substance is completely burnt in excess oxygen under standard conditions (i.e. 1 bar and 298K). It can be used to indicate the energy values of fuels. The more heat is liberated upon complete combustion, the better the fuel is.
