Standard enthalpy changes + Hess' law

Question 1

What are the two main fundamental reactions?

Answer 1

1. Formation reactions.
2. Combustion reactions.

Question 2

What is a standard condition?

Answer 2

set of conditions in which all reactions should take place in

Question 3

what are the standard conditions in terms of pressure and temperature.

Answer 3

Pressure: 100,000 Pa, and 100 KPa
Temp: 298 K.

Question 4

what is a formation reaction?

Answer 4

A formation reaction occurs when one mole of a substance , is produced from its constituent elements.

Question 5

what is a constituent element?

Answer 5

The elements that bond together, to form a substance.

Question 6

What is standard enthalpy of formation?

Answer 6

The enthalpy change of a substance, when one mole of a substance is formed from its constituent elements, under standard conditions, with all the elements in their standard states.

Question 7

What is the symbol for “ standard enthalpy change of formation”

Answer 7

ΔH f.

Question 8

explain why single uncombined elements have no standard enthalpy changes of formation.

Answer 8

Because under standard conditions, it takes no energy for elements to form into their standard states.

Ex; O2, H2

Question 9

What is a combustion reaction?

Answer 9

When a substance reacts with oxygen, and releases heat energy.

Question 10

explain the difference between complete and incomplete combustions.

Answer 10

complete: When a substance with carbon as a constituent, completely burns in oxygen, in order to form carbon dioxide and water,

Incomplete: When a substance with carbon as a constituent, doesnt completely burn in oxygenn and doesnt form CO2. instead it forms C(s), or CO(g).

Question 11

what are the by products of complete combustion.

Answer 11

Carbon dioxide (CO2), and water (H2O).

Question 12

what are the by products of incomplete combustion.

Answer 12

slid carbon (sooth) - C(s), and carbon monoxide - CO(g)

Question 13

What is the standard enthalpy of combustion?

Answer 13

When one mole of a substance is completely burnt in oxygen , under standard conditions with all the reactants and products in its standard state.

Question 14

what is the symbol for standard enthalpy of combustion?

Answer 14

ΔH∘C

Question 15

What is an alternative route?

Answer 15

It only contains elements that make up the reactants , and the products in the direct reaction route.

Question 16

What is a thermochemical cycle?

Answer 16

It shows all of the different possible routes for a reaction, from starting reactants to final products.

Question 17

Define Hess’ law.

Answer 17

the enthalpy change of a reaction at a constant pressure, is independant of the route taken.

Question 18

What are the two types of data book reactions?

Answer 18

Enthalpy of formation data.

Enthalpy of combustion data.

Question 19

How do you find the total enthalpy change of a reaction when given the enthalpy formation data?

Answer 19

1.Write an equation for the direct reaction route.
2.draw a thermochemical cycle and note which arrow was reversed.
3.calculate the total enthalpy of formation for the reactants or products.
4.Add formation values to the thermochemical cycle , checking the signs.
5.Finally, add together the reactions on the alternative route, in order to find the total enthalpy change.

Question 20

How do you find the total enthalpy change of a reaction when given the enthalpy combustion data?

Answer 20

SAME METHOD AS ENTHALPY OF FORMATION.