Important

Question 1

How do you know if reactants or products are more stable?

Answer 1

Lower enthalpy implies stronger bonds between the compounds and energetically more stable

Question 2

What is the speed of endothermic reaction and exothermic reaction?

Answer 2

endo: usually slow
exo: u dont know. It could be fast or slow

Question 3

What is the relationship of the forward reacton and backward reaction?

Answer 3

forward= exo, backward= endo (vice versa)

Question 4

Do we know the actual enthalpies of the reactants and products?

Answer 4

No. They cannot be measured directly.

Question 5

Definition of enthalphy change. What is the condition

Answer 5

Heat change at constant pressure
condition: at constant pressure

Question 6

Standard conditions

Answer 6

Temperature: 298K/ 25 degree Celsius °C
1atm

298K
Gas:1 atm
Solutions: 1.00 mol dm-³
All substances involved = in their normal physical state/ standard state( most stable state at 25 °C and 1 atm)

Question 7

Definition of standard enthalpy change

Answer 7

Standard enthalpy change of a reaction ( triangle H ^ 0 ) is the enthalpy (heat) change when the no. of moles of reactants as stated in the balanced equation completely react to give the products under standard conditions (298 K, 1 atm).

Question 8

What will be the enthalphy change of neutralization that involves weak acid or weak alkali?

Answer 8

Less heat energy will be released if the neutralization reaction involves a weak acid or a weak alkali
because some energy is absorbed for ionization of the weak acid or weak alkali

Question 9

What is the physical state of an acid when it is under combustion?

Answer 9

acid under 10 Carbon: in liquid form!
acid that is 10 C or above = solid

Question 10

What is the physical state of methanol?

Answer 10

liquid

Question 11

What is the physical state of formed products that are oxides?

Answer 11

solid beacause they are ionic compounds = solid in room temp

Question 12

What is the most stable form of carbon?

Answer 12

C(s, graphite)

Question 13

Special case where the standard enthalpy change of formation of a substance is not combustion

Answer 13

1/2 N2(g) + O2(g) -> NO2(g)
because nitrogen gas cannot be combusted

Question 14

What are the relationship of oxygen gas and ozone?

Answer 14

they are allotropes of O

Question 15

Why the standad enthalpy change of formation of an element is defined as zero?

Answer 15

Because the element does not have allotrope

Question 16

Who is used in defining standard enthalpy change of formation

Answer 16

For elements with allotropes, the most energetically stable allotrope is used in defining standard enthalpy change of formation

Question 17

I dont get

Answer 17

Standard enthalpy change of formation of a compound is a measure of energetic stability of this compound with respect to the elements from which it is formed

⇒ more-ve triangle H°f ⇒ more energetically stable with respect to the elements

Question 18

When can we use bond enthalpies to determine enthalphy change

Answer 18

Only for gaseius reactions that involve gaseous reactants and products only.
no solid, no liquid

Question 19

What types of reactions that its enthalpy changes can be determined directly

Answer 19

1. (aq) + (aq)
2. (s) + (aq)
3. standard enthalpy change of liquid

-> other can be determined indirectly ( Hess’s law)

Question 20

ΔΗf [MgO(s)] *(s) +(g)

Answer 20

ΔΗf [MgO(s)] cannot be determined directly by calorimetric experiments because

(1) the extent of reaction cannot be controlled
so it is hard to make sure all Mg(s) reacted

(2) the direct combustion of magnesium can be violent

Question 21

Hand-warmer

Answer 21

4Fe(s) + 3O2(g) -> 2Fe2O3(s) + heat
increase rate becausw larger surface area
-> more heat per unit time