When a chemical reaction occurs, the free energy of the products is always lower than the free energy of the reactants. This is due to the fact that chemical reactions are always spontaneous and occur in the downward direction of the graph of free energy versus reaction coordinate.
When a chemical reaction occurs, the decrease in free energy is called the Gibbs free energy change, denoted by G. The larger this value is, the stronger the reaction is and the more likely it is to occur.
The value of ΔG° refers to either the reactant or product phase only, and does not take into account any other phase that may exist. For example, if a reaction occurred in two phases (reactant and product), then ΔG° only refers to changes in Gibbs free energy in the reactant phase compared to the product phase.
This quiz will test your knowledge on which one of several reactions has the same change in Gibbs free energy as that of its product.
CO(g) + H2O(l) → CO(g) + H2O
In this reaction, the value of Δh°rxn is equal to Δh°f for the product when CO(g) and H2O(l) react to form CO(g) and H2O.
This is referred to as an equilibrium reaction, because the concentrations of the products and reactants are equal, so the reaction ceases. At this point, the molecules are in a stable state where they will not continue to react any further.
The overall reaction is written as: CO(g) + H2O(l) ↔ CO(g) + H2O, where each molecule on the left side of the arrow represents one molecule of CO(g) and one molecule of H2O, respectively.
The equation can also be written as:CO(g) + H2O↔CO(g) + H2O.
NaCl(s) → Na+Cl-
In this reaction, the value of Δh°rxn for the products is equal to Δh°f for the reactants. This is due to the fact that the number of particles is not changed in this reaction and that there are no external forces acting on the particles.
In this case, the value of Δh°rxn is 0 kJ mol-1, as noted by the equation above. This is because there is no heat absorbed or released during this reaction- it simply occurs.
This can be seen in images of crystals of sodium chloride, which are very stable compounds. They do not melt or break down easily, which shows that no other compounds were formed during the NaCl(s) → Na+Cl- reaction.
This was a simple reaction, and one that can be easily reversed. Reversing this chemical reaction requires energy, proving that it was an exothermic reaction.
CaCO3(s) → Ca+2CO3-
When calcium carbonate reacts with a strong acid, it produces calcium ions and carbonate ions. The reaction also results in the formation of a product molecule and empty space.
The value of the enthalpy change for the reaction is equal to the enthalpy change for the product molecule plus the enthalpy change for the empty space that is formed.
For this reaction, the value of ΔH°rxn is not equal to ΔH°f for the product (CaCO3(s) → Ca+2CO3-). The value of ΔH°f for CaCO3(s) is very small, so it is easier to identify which step has a larger enthalpy change.
Looking at the second step in the reaction, we can see that there is an exothermic reaction with a ΔH°rxn of −890 kJ/mol. This is higher than ΔH°f for CaCO3(s), which is −816 kJ/mol.
C6H6(l)+H2O→C6H12+OH-
In this reaction, the hydrogenolysis reaction, hydrochloric acid (HCl) is used to break down benzene into hydrogen and oxygen. The value of the enthalpy change for the reaction is equal to the enthalpy of formation for the product, C6H12.
Enthalpy of formation is a measurement of energy used to determine how much energy it takes to create one molecule of a substance from its constituent parts.
The equation for enthalpy of formation is H=ΔHf, where H represents enthalpy and f represents formation.
There are three main types of reactions for which the value of ΔH°rf is not equal to ΔH°rxn: endothermic reactions, spontaneous reactions, and chemical equilibrium reactions.
NH4Cl(s)+NaNO3(aq)+KNO3(aq)=NH4NO3+(aq)+NaCl+KNO3
In this reaction, the value of Δh°rxn is equal to Δh°f for the product. This is because the overall reaction is a neutralization reaction.
In a neutralization reaction, the products and reagents are both acids and bases. When these two compounds are put together, they either form a salt or water.
The compound that forms a salt in this case is NH4NO3. When it reacts with water, it forms NH4+NO3- which are both salts.
The compound that forms water in this case is NH4Cl. When it reacts with water, it forms NH4Cl(aq) which is water. Both of these compounds are dissolving in the solution, so there is no remaining solid material.
FeS-> Fe+ S^{27} -> Fe^{27}
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The value of the enthalpy of reaction is the same for the product and the reactant. For this reaction, the enthalpy of formation is zero, which means that this reaction is a decomposition reaction.
This question should be asked when there is a difference in energy between the reactant and product. In those cases, it is important to know that there are other ways to determine whether a chemical reaction is spontaneous or not.
Reactions that have a negative enthalpy of reaction are spontaneous, while reactions with a positive enthalpy of reaction are nonspontaneous.
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For Which One of the Following Reactions Is ΔH°rxn Equal to ΔH°f for The Reactant?
- What Is “ΔH°f”?
The term “ΔH°f” stands for “enthalpy of formation.” It refers to how much energy it would take to form 1 mole (6.022 \times 1023 atoms) of a given substance under standard conditions (25 °C and 1 atm). This value can be determined by experimentally determining how much energy it takes to combine its constituent atoms or molecules into its fully formed state.
In general, substances in their pure state have lower ΔH°f than when they are in their typically observed compounds (e.g., water in its liquid state has higher ΔH°f than when it is formed from HO molecules). This is due to intermolecular forces such as Van der Waals forces and covalent bonding.
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“ΔH f” stands for “enthalpy offormation”,itrefers tonhowmuchenergyitwouldtake toproduceonemole(602223atoms)ofaspecificsubstanceunderstandardconditions(25Canda atm).Thisvaluecanbe determinedbyexperimentallydetermining how much energy it takes to combine its constituent atoms or molecules into its fully formed state.
“ΔH f” stands for “enthalpy offormation”,itrefers tonhowmuchenergyitwouldtake toproduceonemole(602223atoms)ofaspecificsubstanceunderstandardconditions(25Canda atm).Thisvaluecanbe determined by experimentally determining how much energy it takes to combine its constituent atoms or molecules into its fully formed state.. In general, substances in their pure state have lower ΔH f than when they are in their typically observed compounds (e.g., water in its liquid state has higher Δ H f than when it is formed from HO molecules). This is due to intermolecular forces such as Van der Waals forces and covalent bonding.
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