## Chem – Enthalpy and Enthalpy Change (ΔH) Part 2

WHAT IS ENTHALPY ΔH (DELTA H)?

Enthalpy is simply talking about energy in the form of heat. So ENTHALPY is HEAT ENERGY. Enthalpy is represented by Δ H (Delta H).

200kJ + N2(g) + 3 H2(g) —-> 2 NH3(g) Δ H = +200 kJ/mol

The above chemical reaction can be said to have a positive Δ H or is endothermic. All reactions with a positive Δ H are ENDOTHERMIC.

2 H2O(g) —-> 2 H2(g) + O2(g) + 147cal Δ H = -147 cal/mol

The above chemical reaction can said to have a negative Δ H or is exothermic. All reactions with a negative Δ H are EXOTHERMIC.

You should be able to use the information about Energy or Enthalpy to identify all representations of it in chemical reactions. Know what energy looks like in a reaction, how that relates to its Δ H, and how that relates to endothermic or exothermic.

This is a summary of what you have to know before you move on from this section:

 Endothermic Energy absorbed Energy appears on reactants Δ H is positive

AND

 Exothermic Energy released Energy appears on products Δ H is negative

PRACTICE PROBLEMS: Which side of the equation is the energy on? What is the Δ H? Is it endothermic or exothermic?

For the equation below, is the Δ H positive or negative? Is the reaction endothermic or exothermic?

C4(s) + 4 O2(g) —-> 4 CO2(g) + Energy

For the equation below, is the Δ H positive or negative? Is the reaction endothermic or exothermic?

330J + 2 Fe3+(aq) + 3 CO32-(aq) —-> Fe2(CO3)3(s)

For the equation below, which side of equation is the energy? Is the reaction endothermic or exothermic?

H2SO4(aq) —–> H2(aq) + S(s) + 2 O2(aq)Δ H = positive

Answer: energy is on left side (reactants), endothermic

For the equation below, which side of equation is the energy? Is the reaction endothermic or exothermic?

2 H2O(g) —-> 2 H2(g) + O2(g)Δ H = -14kJ

Answer: right side (products) 14kJ, exothermic

Fill in the missing column sections with the information you are given. The first two lines are finished for you as an example. Highlight missing sections to reveal the answer once you have attempted.

 Side of Equation Δ H Endo/Exo +45J in reactants +45J Endothermic products Negative Exothermic reactants Positive Endothermic + 500KJ in products -500KJ Exothermic + 300cal in reactants +300cal Endothermic products Negative Exothermic  