Use the standard reaction enthalpies given below to determine ΔH°rxn for the following reaction:

2 S(s) + 3 O2(g) → 2 SO3(g) ΔH°rxn = ?
Given:
SO2(g) → S(s) + O2(g) ΔH°rxn = +296.8 kJ
2 SO2(g) + O2(g) → 2 SO3(g) ΔH°rxn = -197.8 kJ

a. -494.6 kJ
b. -692.4 kJ
c. -791.4 kJ
d. 1583 kJ

The answer is -791.5 kJ (option c)

Explanation:

You know:

SO₂ (g) → S (s) + O₂ (g) ΔH°rxn = +296.8 kJ

2 SO₂ (g) + O₂ (g) → 2 SO₃ (g) ΔH°rxn = -197.8 kJ

You must add them to obtain the desired equation:

2 S(s) + 3 O₂(g) → 2 SO₃(g) ΔH°rxn = ?

You want to calculate the ∆H (heat of reaction) of the combustion reaction, that is, the heat that accompanies the entire reaction. The calculation is made using Hess's law. This law states: when the reactants are converted to products, the enthalpy change is the same, regardless of whether the reaction is carried out in one step or in a series of steps.

In Hess's law he explains that the enthalpy changes are additive, ΔHneta = ΣΔHr and contains three rules:

The sum of the fitted equations should give the problem equation. In this case:

2*( S(s) +  O₂(g)  → SO₂ )     To obtain the desired reaction, this equation must be inverted, so the enthalpy value is also inverted.  It must also be multiplied by 2, then the whole equation is multiplied, both reactants and products and the value of the enthalpy.  So ΔH°rxn = (-296.8 kJ)*2= -593.6 kJ

2 SO₂(g) + O₂(g) → 2 SO₃ (g)               ΔH°rxn = -197.8 kJ

2 S(s) + 3 O₂(g) → 2 SO₃(g)    ΔH°rxn =  -791.4 kJ ( Enthalpies are added algebraically)

The answer is -791.5 kJ (option c)

pre-sliced bread is a relatively recent development in baking technology. previous to that, it was always sold in a bakery in daily-made contiguous loaves with no wrapping, and was moderately perishable, usually being used with a day or two (which is why every bakery had a discounted “day-old” bread table.) therefore, pre-sliced bread would grow stale quickly. the hard crust provided a protective layer, and once the exposed dry slice left from the previous cut was removed, the interior was still soft.

what keeps bread soft is the moisture retained in the crumb (the light spongy part that’s not crust). the reason the crust develops is the surface of the dough gets exposed to hot dry air during baking which browns the outer layer and seals it, preventing all the moisture from escaping the dough. longer exposure to air will also eventually cause your bread to lose flavor and become stale.

people were used to cutting slices as they needed them and storing bread in a specially-made wooden or metal breadbox or pail with a lid to preserve it longer, but the first automatically sliced commercial loaves were produced on july 6, 1928, in chillicothe, missouri, using a machine invented by otto rohwedder, an iowa-born, missouri-based jeweler.[1] pre-sliced bread would indeed dry out, were it not for another 20th century technology: the plastic bag. bread could now be pre-sliced, and if the enclosing bag remains sealed as much as possible, the bread will remain soft and fresh for longer than if sitting unwrapped on a shelf or enclosed in a bread box that is not air-tight. sealed packaging meant bread could also be sold on store shelves and eliminate frequent trips to a bakery.

sealing bread in a plastic bag had another effect: the moisture in the loaf will re-distribute into the crust and soften it, which is sometimes more pleasing in a sandwich - especially for children whose teeth might not yet be strong enough to tear through a hard crust. a homemade loaf of bread can be sealed in a bag overnight and the crust will become soft like store-bought sandwich bread.

these were novel changes in how bread was produced and sold, which led to the coining of the phrase “the greatest thing since sliced bread.”