Enthalpy + Hess's Law + Enthalpy and Hess's Law + Energy Cycles and Hess's Law Flashcards
What is Hess’s Law?
The enthalpy changes for a reaction is independent of the reaction pathway
Explain Hess’s Law
If a reaction is carried out in a series of steps or mini-reactions, the overall change in enthalpy from the main reaction
(Reac. A → Reac. B) is the same as the sum of the enthalpy changes of all the separate steps mini-reactions
(Reac. A → Reac. C→ Reac. B)
What are some ways you can manipulate an different equations to find the enthalpy change?
1) Add different equations
2) Subtract different equations
3) Multiply an equation by a number (and the enthalpy of it)
3) Reverse or Flip the equation (and the sign of the enthalpy)
What are the two steps involved in a chemical reaction?
1) The breaking of chemical bonds
2) the formation of chemical bonds
Does making bonds use or release energy?
it releases energy
Does breaking bonds use or release energy?
it uses energy
What events causes a difference in the chemical energy of the reactants and the products mean?
the transformation of energy and the transfer of energy between the system and the surroundings
What is enthalpy?
the measure of heat absorbed or released.
What is an enthalpy change?
the energy required (the energy required to break the reactant bonds in reactant molecules) - (the energy released when the bonds of the new product molecules forms)
What is the formula for enthalpy?
▲H (Rxn) = Sum of ▲H (Products) - Sum of ▲H (reactants_
What is the official enthaply formula?
▲H (rxn) = ∑▲H(products) - ∑▲(reactants)
What are standard conditions?
25 degrees Celsius or 298 Kelvin , 100 kPa (standard atmospheric pressure)
What is bond energy/bond enthalpy?
the energy required to break 1 mol of a specific bond into its gaseous atoms at standard conditions
Why are bond enthalpies unreliable for calculating the enthalpy of reactions?
1)
When is bond energy negative.
Never, it is always positive
