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Energetics

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Energetics

Energy changes in chemistry.

Enthalpy

- What is enthalpy?
- Enthalpy (H) is the total energy of system. It is in the molecules, the bonds, and stored chemically.
- Enthalpy change (Δ H) is the measure of the change in the potential energy of the bonds.

Endothermic vs exothermic

- What is the difference between the two?
- Endothermic reactions have more energy stored in the new bonds (products) than were in the old bonds (reactants) which takes in energy.
- Exothermic reactions have more energy in the old bonds (reactants) than in the new bonds (products).

Enthalpy change

- What is the value of enthalpy change for exothermic reactions?
- Exothermic reaction enthalpy change is negative due to Δ H = enthalpy products – enthalpy reactants.
- Endothermic reaction would be the opposite because of the same equation.

Enthalpy change

- How do we show enthalpy change of a reaction?
- It is usually written with the equation.
- + represents endothermic, - represents exothermic
- It is given in the units of kJ/mol (or just kJ) because it changes with the amount of reactants and limiting reactants.
- This is at thermochemicalstandard conditions which are 25 degrees C, 1 atm and solution concentrations of 1 mol/dm3.

Hess’s Law

- What does Hess’s Law say?
- It states that if you can add 2 or more different equations to produce the final equation, you can add the individual enthalpy changes to find the total.
- This means that if you break down the steps involved, the overall change will be the same.

Hess’s Law

- 2H2O2(l) 2H2O(l) + O2(g)
- 2H2(g) + O2(g) 2 H2O(l)ΔH= -572 kJ
- H2(g) + O2(g) H2O2(l)ΔH= -188 kJ
- Where do we start?