Energetics 5.1/5.2

Question 1

Enthalpy

Answer 1

1) Measure of the amount of heat energy contained in a substance.
2) Stored in chemical bonds and inter molecular forces as potential energy.

Question 2

Enthalpy of products for Endothermic reactions

Answer 2

Products have more enthalpy than reactants

Question 3

Standard conditions for enthalpy change

Answer 3

1) Pressure of 100kPa
2) Concentrations of 1 mol dm-3 for all solutions
3) all substances in their standard states.
4) 298K Temperature

Question 4

What is Temperature?

Answer 4

1) Average K.E of particles

2) absolute temperature is directly proportional to the average kinectic energy

Question 5

Factors determining the increase in temp

Answer 5

1) mass of the object
2) heat added
3) nature of the substance

Question 6

What is Specific heat capacity

Answer 6

property of substance which gives the heat needed to increase the temperature of unit mass by 1 K.

Question 7

What is Heat capacity

Answer 7

heat needed to increase the temperature of an object by 1 K

Question 8

What is the Direction of energy change

Answer 8

In the direction of lower stored energy (Generally)

Question 9

Enthalpy for products of exothermic reactions

Answer 9

Products of exothermic reactions are much more stable.Lower energy

Question 10

When do Endothermic reactions occur ?

Answer 10

1) Less common

2) Occur when there is an increase in the disorder of the system.

Question 11

Standard enthalpy change of combustion definition

Answer 11

Enthalpy change for the complete combustion of 1 mole of a substance in its standard state in excess Oxygen under standard conditions.

Question 12

Unit of enthalpy changes

Answer 12

kJmol-1

Question 13

No of moles matter for combustion

Answer 13

1) Enthalpy change of combustion is given for 1 mole

2) If more than 1 mole is give in question then divide the value of enthalpy by given moles.

Question 14

Errors in the experiment conducted in a polystyrene cup

Answer 14

Heat loss to the environment

Question 15

Assumption made for standard enthalpy of solution

Answer 15

1) No heat loss from the system
2) All heat goes from the reaction to the water
3) Volume of solvent = Volume of water
4) water has the density of 1.00gcm-3.

Question 16

Enthalpy of solution calculation

Answer 16

Divide enthalpy value by no of moles of limiting reactant.

Question 17

heat energy

Answer 17

heat is the total energy (potential + kinetic) of a system,

Question 18

Average K.E

Answer 18

total kinetic energy divided by number of

molecules.

Question 19

Standard Enthalpy of formation

Answer 19

1) 1 mole of a compound is formed from its elements in their standard states under standard condition.
2) Moles of reactant do not matter but they should be in standard states

Question 20

Formula for change in enthalpy calculations (FORMATION REACTIONS)

Answer 20

▲H(products)-▲H(Reactants)= Change

Question 21

Formula for change in enthalpy calculations (COMBUSTION REACTIONS)

Answer 21

▲H(reactants)-▲H(products)= Change

Question 22

What is Hess’s Law

Answer 22

Enthalpy change for any chemical reaction is independent of the route, provided the starting and final conditions and reactants and products are the same.

Question 23

▲H for Stable elements

Answer 23

The standard enthalpy change of formation of an element in its most stable form is 0

Question 24

Why ΔHf° is useful ?

Answer 24

1) how stable this substance is compared to the elements. If the
ΔHf° is negative, it is more stable! If positive, it is more unstable.