6.7 Reaction Enthalpy (ΔH) and Bond Energy

What does the greek symbol sigma signify in the formula below? ΔHrxn = Σ(bonds broken)−Σ(bonds formed)

• Sum of

• Difference of

• Product of

• Quotient of

What is the change in enthalpy when a C=O bond in CO₂ is broken or formed?

• The change in enthalpy is positive when a C=O bond is broken and negative when formed.

• The change in enthalpy is negative when a C=O bond is broken and positive when formed.

• The change in enthalpy is zero when a C=O bond is broken or formed.

• The change in enthalpy is always positive regardless of bond breaking or forming.

Using the bond energies listed in table, calculate ∆H° for the reaction of methane with chlorine and fluorine to give Freon-12 (CF₂Cl₂).

CH₄(g) + 2Cl₂(g) + 2F₂(g) → CF₂Cl₂(g) + 2HF(g) + 2HCl(g)

• ΔHrxn = -1194 kJ/mol

• ΔHrxn = -2220 kJ/mol

• ΔHrxn = -1987 kJ/mol

• ΔHrxn = -2100 kJ/mol

In the following reactions, bonds will be broken for which species?

CH₄(g) + Cl₂ → CCl₄(g) + HCl(g)

• CH₄(g) + Cl₂

• CCl₄(g) + HCl(g)

• no bonds will broken

The breaking of bonds in a chemical reaction is a(n) _______________ process.

• exothermic

• endothermic

• neither exothermic nor endothermic

_______________ change is the name given to the amount of heat evolved or absorbed in a reaction carried out at constant pressure. It is given the symbol _____.

• Entropy, ΔS

• Enthalpy, ΔH

• Temperature,ΔT

• Volume, ΔLiters

ΔH value in an endothermic reaction is a positive number.

• True
• False

The diagram shows that the reaction is ___________ and the enthalpy change is __________.

• endothermic, negative

• exothermic, negative

• endothermic, positive

• exothermic, positive

How would a catalyst affect the Maxwell Boltzmann curve below?

• The hill would become smaller.

• The activation energy line would shift to the left.

• The activiation energy line would disappear.

Which element was named after a star?

• Go Clippers!

• helium (He, after helios, the Sun)