The standard heat of reaction for the gas-phase dehydrogenation of propane (C3H8) to produce propene (C3H6) and H2 C3H8 → C3H6 + H2 is +124 kJ/mole. What would you predict for the heat of reaction of the following gas-phase process (in kJ/mole)? 2 C3H6 + 2 H2 → 2 C3H

Explanation:

The standard heat of reaction is the heat change when all the substances involved in a reaction have changed completely. it is the heat content of the reaction i.e the amount of heat involved in a chemical reaction.

For the reaction given ; C3H8 → C3H6 + H2 ; heat of reaction = +124 kJ/mole

2 C3H6 + 2 H2 → 2 C3H8, as such the heat of reaction is of backward reaction which will be negative, hence the prediction of the heat of reaction for the gas phase reaction will be -248KJ/mole.

The answer is -248.0 kJ/Mole

Explanation:

The heat of reaction is the total amount of energy either absorbed or released during a chemical reaction.

Heat of reaction, ΔH° rxn = ∑ nΔH°f products - ∑nΔH°f reactants

From heat of formation tables (standard reference materials)

Heat of formation ΔH°f of C3H8 = -103.85 kJ/mole

Heat of formation for all pure elements = 0

where n = no. of moles

We have two reactions in this experiment: a forward and backward reactions with different number of moles of reactants and products.

C3H8 → C3H6 + H2     (ΔH° = +124kJ/mole)  (1)

2C3H6 + 2 H2 → 2C3H8   (ΔH° = ?? = x)(2)

From eqn. (1),

ΔH°rxn = (ΔH°f C3H6 + ΔH°f H2) - ( ΔH°f C3H8)

124 = ( ΔH°f C3H6 + 0) - (-103.85)

124 = ΔH°f C3H6 + 103.85

⇒ΔH°f C3H6  = 124- 103.85

= 20.15 kJ/mole.

∴ solving ΔH°f C3H6   into eqn (2)

ΔH° =x = (2 × (-103.85)) - (2 × 20.15 + 2(0))

x= -207.7 - 40.3

= -248.0

∴ ΔH° = -248.0 kJ/mole.

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.”