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The sign only depends on whether it's an exothermic or endothermic reaction. So no, it doesn't depend on phase change, and in some reactions, there is no phase change. But you will find correlation between the sign and the direction of the phase change because they will either be endothermic or exothermic. The enthalpy of fusion is positive because melting is an endothermic reaction (think - you put in heat to melt an ice cube). Endothermic reactions are represented by positive enthalpy. Conversely, freezing is an exothermic process (heat is taken out of the molecules in order to slow them down). We represent exothermic reactions with negative enthalpy.
Always be conscious of the sign when working with thermochemistry calculations. Unlike the usual math, the signs of these numbers are somewhat arbitrary. We say that the system is losing heat in an exothermic reaction; thus, the energy must be represented by a negative. However, there are a few areas (like electrical engineering) where an exothermic reaction is represented by a positive number (because usually the goal is to produce heat).
What else can I help you with?
True or false The molar enthalpy values and fusion stay the same if you were to reverse the process?
True. The molar enthalpy values for fusion (also known as the enthalpy of fusion) are independent of the direction of the process. This means that the enthalpy change for melting a substance is equal in magnitude, but opposite in sign, to the enthalpy change for freezing the substance.
If you need to reverse the following reaction in order for it to be an intermediate reaction in a Hess's law problem what would be the final value for the enthalpy of reaction you use for this interm?
To reverse a reaction in a Hess's Law problem, you must change the sign of the enthalpy change associated with that reaction. For example, if the original reaction has an enthalpy change of ΔH, the enthalpy change for the reversed reaction would be -ΔH. This means you would use the negative value of the original enthalpy change as the final value for the enthalpy of reaction for the intermediate.
How do you know if an enthalpy change diagram is endothermic or exothermic?
If you plot the reaction coordinate (what I think you mean by "enthalpy change diagram"), the reaction will be exothermic if the products are lower on the graph than the reactants. If they are higher than it is endothermic. For instance, if you go to the linked Wikipedia page (link to the left of this answer), the graph shown is of an exothermic reaction.
How does the presence of a catalyst affect the enthalpy change of a reaction?
The presence of a catalyst affect the enthalpy change of a reaction is that catalysts do not alter the enthalpy change of a reaction. Catalysts only change the activation energy which starts the reaction.
How do you calculate enthalpy change of formation from combustion?
To calculate the enthalpy change of formation from combustion, you can use Hess's law, which states that the total enthalpy change for a reaction is the sum of the enthalpy changes for individual steps. First, determine the enthalpy change for the combustion reaction using a calorimeter or from standard enthalpy values. Then, apply the equation: ΔH_f = ΔH_combustion + Σ(ΔH_f of products) - Σ(ΔH_f of reactants), where ΔH_f is the standard enthalpy of formation. This allows you to derive the enthalpy of formation for the desired compound based on its combustion data.
True or false The molar enthalpy values and fusion stay the same if you were to reverse the process?
True. The molar enthalpy values for fusion (also known as the enthalpy of fusion) are independent of the direction of the process. This means that the enthalpy change for melting a substance is equal in magnitude, but opposite in sign, to the enthalpy change for freezing the substance.
How do you calculate the enthalpy change of a reaction?
To calculate the enthalpy change of a reaction, subtract the total enthalpy of the reactants from the total enthalpy of the products. This difference represents the enthalpy change of the reaction.
If you need to reverse the following reaction in order for it to be an intermediate reaction in a Hess's law problem what would be the final value for the enthalpy of reaction you use for this interm?
To reverse a reaction in a Hess's Law problem, you must change the sign of the enthalpy change associated with that reaction. For example, if the original reaction has an enthalpy change of ΔH, the enthalpy change for the reversed reaction would be -ΔH. This means you would use the negative value of the original enthalpy change as the final value for the enthalpy of reaction for the intermediate.
What is the enthalpy change for the reverse reaction?
The enthalpy change for the reverse reaction is equal in magnitude but opposite in sign to the enthalpy change for the forward reaction.
What is molar enthalpy change?
Molar enthalpy change, also known as molar enthalpy of reaction, is the amount of heat energy released or absorbed during a chemical reaction per mole of a substance. It is usually expressed in units of kJ/mol. The molar enthalpy change can be positive (endothermic) if heat is absorbed or negative (exothermic) if heat is released during the reaction.
What is the value of the enthalpy change of the uncatalyzed reaction?
The enthalpy change of an uncatalyzed reaction is the heat energy absorbed or released during the reaction when no catalyst is present. This value can be determined by calculating the difference between the enthalpy of the products and the enthalpy of the reactants.
How do you know if an enthalpy change diagram is endothermic or exothermic?
If you plot the reaction coordinate (what I think you mean by "enthalpy change diagram"), the reaction will be exothermic if the products are lower on the graph than the reactants. If they are higher than it is endothermic. For instance, if you go to the linked Wikipedia page (link to the left of this answer), the graph shown is of an exothermic reaction.
What is the difference of enthalpy of formation and the enthalpy of combustion?
Enthalpy of combusion is energy change when reacting with oxygen. Enthalpy of formation is energy change when forming a compound. But some enthalpies can be equal.ex-Combusion of H2 and formation of H2O is equal
What is the difference between enthalpy change and enthalpy change per mole?
Enthalpy is the energy absorbed or lost from a reaction, but enthalpy change per mole is the amount of energy lost per mole, so in order to get the overall enthalpy from the change per mole, you must multiply that value by the amount of moles used in the reaction.
How do you calculate the change in enthalpy of solution?
To calculate the change in enthalpy of solution, subtract the enthalpy of the products from the enthalpy of the reactants. This difference represents the heat absorbed or released during the process of dissolving a solute in a solvent.
How does the presence of a catalyst affect the enthalpy change of a reaction?
The presence of a catalyst affect the enthalpy change of a reaction is that catalysts do not alter the enthalpy change of a reaction. Catalysts only change the activation energy which starts the reaction.
What is the equation for calculating the change in enthalpy of a system during a chemical reaction?
The equation for calculating the change in enthalpy of a system during a chemical reaction is H H(products) - H(reactants), where H represents the change in enthalpy, H(products) is the enthalpy of the products, and H(reactants) is the enthalpy of the reactants.
