Chapter 9 - Enthalpy

Question 1

What is enthalpy?

Answer 1

the heat content stored in a chemical system

Question 2

What is a system?

Answer 2

the reactants and products in a reaction

Question 3

What are the surroundings?

Answer 3

the area around the system (e.g beaker, air)

Question 4

What is the enthalpy change? ΔH

Answer 4

the energy change at a constant pressure

Question 5

What is the activation energy?

Answer 5

the minimum energy required to start a reaction

Question 6

What is the definition of an EXOthermic reaction?

Answer 6

EXOthermic chemical is where the products have a lower enthalpy content than the reactants.
Energy is given out to the surroundings (temp goes up)n
ΔH = NEGATIVE

Question 7

What is the definition of an ENDOthermic reaction?

Answer 7

ENDOthermic chemical reaction is where the products have a higher enthalpy content than the reactants.
Energy is taken in from the surroundings (temp goes down)
ΔH = POSITIVE

Question 8

What is an enthalpy change?

Answer 8

an energy change at constant pressure

Question 9

What are the standard conditions?

Answer 9

100kPa - Pressure
298K (25C) - Temperature
1.0M - Concentration

Question 10

What is the definition of the standard enthalpy change of reaction ΔHr⦵?

Answer 10

the enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation.
(If in standard conditions) under standard conditions, all reactants and products being in their standard states

Question 11

What is the definition of enthalpy change of neutralization ΔHn?

Answer 11

the standard enthalpy change when an aqueous acid is neutralized by an aqueous base to form ONE MOLE OF WATER under standard conditions

Question 12

Why is the value of enthalpy of neutralization always the same?

Answer 12

because the reaction between any strong acid and alkali involves the reaction between H+ ions and OH- ions

Question 13

What is the value of the enthalpy of neutralization?

Answer 13

-57 kJ mol -1

Question 14

Why is the enthalpy of neutralization for weak acids LESS EXOTHERMIC than that for strong acids?

Answer 14

Weak acids are partially dissociated into H+ solution.

Energy must, therefore, be put in to break the O-H bonds before neutralization can take place

Question 15

What is the equation used for enthalpy of neutralization?

Answer 15

Q = mc∆T

Question 16

Describe the experiment for Colourimetry (Neutralisation)?

Answer 16

1) Put a clean polystyrene cup into a beaker
2) Pipette 25cm of 1.0 mol of HCL into cup and place lid over
3) Put thermometer through lid within a tripod
4) Pipette 25cm of 1.0 mol of sodium hydroxide into another polystyrene cup
5) Wait until both temperatures settle and then record
6) Pour alkali into acid and place lid onto cup
7) Stir well and record results once its reached maximum temp
8) Put results into table

Question 17

What is the definition of the enthalpy of combustion?

Answer 17

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states

Question 18

When energy is taken from the surroundings when bonds are broken it is…..

Answer 18

endothermic (+)

Question 19

When energy is given to the surroundings when bonds are formed it is…..

Answer 19

exothermic (-)

Question 20

Reaction which is…………more energy is released when new bonds are formed in the products than is taken into break bonds in the reactants

Answer 20

Exothermic

Question 21

Reaction which is…………less energy is released when new bonds are formed in the products than is taken into break bonds in the reactants

Answer 21

Endothermic

Question 22

What is the definition of bond enthalpy?

Answer 22

The amount of energy required to break a particular bond in a gaseous molecule

Question 23

What is the definition of the average bond enthalpy?

Answer 23

Change that takes place when breaking 1 mole of a given type of bond in the molecules of a gaseous species