Topic 5 - Chemical Energetics

Question 1

What is the enthalpy change of a reaction?

Answer 1

The heat change in a reaction at constant pressure.

Question 2

What are the units for enthalpy change?

Answer 2

kJmol^-1

Question 3

What are Standard Conditions?

Answer 3

100kPa
298K

Question 4

What are endothermic reactions?

Answer 4

Reactions that absorb energy from the surroundings.

Question 5

What are exothermic reactions?

Answer 5

Reactions that release energy into the surroundings.

Question 6

What is activation energy?

Answer 6

The minimum amount of energy required for a reaction to occur.

Question 7

What is delta H for endothermic reactions?

Answer 7

Positive

Question 8

What is delta H for exothermic reactions?

Answer 8

Negative

Question 9

What is delta Hr?

Answer 9

The standard enthalpy change of a reaction.

Question 10

What is delta Hneut?

Answer 10

The standard enthalpy change of neutralization.

Question 11

What is delta Hf?

Answer 11

The standard enthalpy change of formation.

Question 12

What is delta Hc?

Answer 12

The standard enthalpy change of combustion.

Question 13

What does Standard Enthalpy change of Neutralization mean?

Answer 13

The enthalpy change when an acid and alkali react to form 1 mole of water, under standard conditions.

Question 14

What does Standard Enthalpy change of Formation mean?

Answer 14

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions.

Question 15

What does Standard Enthalpy change of Combustion mean?

Answer 15

The enthalpy change when 1 mole of a substance is completely burned in oxygen to make CO2 and H2O, under standard conditions.

Question 16

Is bond breaking endothermic or exothermic?

Answer 16

Endothermic. +ve

Question 17

Is bond making endothermic or exothermic?

Answer 17

Exothermic. -ve

Question 18

What is bond enthalpy?

Answer 18

The amount of energy needed to break 1 mole of a bond type in a molecule in the gaseous state.

Question 19

What is calorimetry used for?

Answer 19

To work out the enthalpy change of combustion.

Question 20

What is the formula for energy change?

Answer 20

q = mc^T

Question 21

What do all the letters stand for in the energy change equation?

Answer 21

q = energy lost or gained
m = mass of water
c = specific heat capacity of water
T = change in temperature

Question 22

How do you turn an energy change into an enthalpy change?

Answer 22

Divide the energy change by the no. of moles.

Question 23

What is the formula for enthalpy change?

Answer 23

enthalpy change = q/moles

Question 24

What is Hess’s Law?

Answer 24

The total enthalpy change of a reaction is independent of the route taken.

Question 25

What are the two Hess Cycles?

Answer 25

Formation and Combustion.

Question 26

Do arrows in a Hess Formation Cycle point towards the reactants and products or away from the reactants and products?

Answer 26

Towards.

Question 27

Do arrows in a Hess Combustion Cycle point towards the reactants and products or away from the reactants and products?

Answer 27

Away.

Question 28

What are the rules for +/- signs and the direction of the arrow in Hess Cycles?

Answer 28

When the arrow goes with the cycle, the sign stays the same. When the arrow goes against the cycle, the sign flips.

Question 29

What can also be used to calculate enthalpy changes using a Hess Cycle?

Answer 29

Mean bond enthalpies.