Enthalpy is the total heat content of a system. Watch the following video to learn a little bit more about enthalpy: youtu.be/ZVhJ4TO8a-M

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- enthalpy of fusion is also called latent heat of fusion or heat of fusion. As you continue to add heat to the block of ice at the melting point, the ice melts, but the temperature remains the same. The heat energy that is applied during this phase change is called the latent enthalpy of fusion.
- enthalpy of vaporization is also called latent heat of vaporization or heat of vaporization. As you continue to add heat to the liquid water, water vaporizes, but the temperature remains constant. The heat energy during the liquid-gas phase change is called the latent enthalpy of vaporization.

- ∆H = (Hproducts - Hreactants)
- ∆H = q = mCΔT
- m = mass in grams
- C = specific heat of the substance
- ΔT = change in temperature

- Step 1: What are you given?
- m= 100 grams
- c = 4.18 J/ (g x degrees C)
- ΔT = (0-100) = -100 degrees Celsius

- Step 2: What are you trying to find?
- Δ H =?

- Step 3: What equation will you use?
- ΔH = q = mCΔT

- Step 4: Plug in your numbers and solve.
- ΔH = 100 x 4.18 x -100 = -41,800 J OR -41.8 kJ

- Note: The ΔH is negative, so this reaction is exothermic. This makes sense because the water is releasing heat as it cools.
- Note: Endothermic reactions have a positive ∆H because the products have more enthalpy than the reactants.

**Writing Thermochemical Equations**

All thermochemical equations must have the following information:

- Must be balanced
- States of all reactants and products are in parentheses
- Liquids (l)
- Gases (g)
- Solids (s)

- Change in enthalpy (ΔH) is written on the right (units of kJ)

The Reverse of a Thermochemical equation has the SAME Value but Opposite Sign of ΔH.

- H2(g) + Cl2(g) → 2HCl(g) ΔH = -185 kJ
- So this equation is exothermic

- 2HCl(g) → H2(g) + Cl2(g) ΔH = 185 kJ
- So this equation is endothermic