Chapter 9 - Enthalpy

Question 1

What is enthalpy?

Answer 1

A measure of the heat energy in a chemical system

Question 2

What is enthalpy change?

Answer 2

The heat energy transferred in a reaction at constant pressure

Question 3

What is standard pressure?

Answer 3

100 kPa

Question 4

What is standard temperature?

Answer 4

298 K (25c)

Question 5

How to find ΔH?

Answer 5

H (products) - H (reactants)

Question 6

What is the law of the conservation of energy?

Answer 6

It states that energy can’t be created or destroyed in a chemical system

Question 7

What is an exothermic reaction?

Answer 7

Heat is transferred to the surroundings - ΔH is negative

Question 8

In exothermic reactions, what happens to the temperature of the chemicals?

Answer 8

It increases

Question 9

Give two examples of exothermic reactions

Answer 9

Combustion

Oxidation, e.g. of carbohydrates

Question 10

What is an endothermic reaction?

Answer 10

Heat is absorbed from the surroundings - ΔH is positive

Question 11

In an endothermic reaction, what happens to the temperature of the chemicals?

Answer 11

It decreases

Question 12

Give two examples of endothermic reactions

Answer 12

Thermal decomposition

Photosynthesis

Question 13

What is activation energy?

Answer 13

The minimum amount of energy needed to break the bonds in the reactants and start a chemical reaction

Question 14

What is standard concentration?

Answer 14

1 mol/dm3

Question 15

What is the standard state?

Answer 15

The state an element is in under standard conditions

Question 16

What is the standard enthalpy change of reaction?

Answer 16

The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions, with all substances in their standard states

Question 17

What is the standard enthalpy change of formation?

Answer 17

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

Question 18

What is the standard enthalpy change for the formation of an element?

Answer 18

0 kJ

Question 19

What is the standard enthalpy change of combustion?

Answer 19

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all substances in standard states

Question 20

What are the combustion products?

Answer 20

CO2 and H2O

Question 21

What is the standard enthalpy change of neutralisation/

Answer 21

The enthalpy change that occurs when an acid and a base react together, under standard conditions in their standard states, to produce one mole of water

Question 22

What is important to remember about Kelvin and Celcius?

Answer 22

A change in temperature will have the same value in kelvin or celcius

Question 23

What type of process is bond breaking?

Answer 23

Endothermic

Question 24

What type of process is bond making?

Answer 24

Exothermic

Question 25

What is bond dissociation enthalpy?

Answer 25

The amount of energy needed per mole of bonds to break

Question 26

What is the equation for enthalpy changes?

Answer 26

q = m x c x ΔT

Question 27

What does 'M' stand for?

Answer 27

The mass of the surroundings (g)

Question 28

What does 'C' stand for?

Answer 28

The specific heat capacity of the substance (J g-1 K-1)

Question 29

What is the specific heat capacity of water?

Answer 29

4.18

Question 30

What is specific heat capacity?

Answer 30

The energy required to raise the temperature of 1g of a substance by 1K

Question 31

What does 'ΔT' stand for?

Answer 31

Change in temperature (K)

Question 32

What are the units for 'q'?

Answer 32

kJ mol-1

Question 33

After using q=mcΔt, what are the units?

Answer 33

Joules, then divide by 1,000 to convert to kJ

Question 34

What is the definition for average bond enthalpy?

Answer 34

The energy needed to break one mole of bonds in the gas phase averaged over many different compounds

Question 35

How to measure the enthalpy change of combustion of a flammable liquid?

Answer 35

1) Pour 100 ml of water into a beaker, recording the initial temperature to the nearest 0.5 c 2) Add your liquid to a spirit burner and record the mass 3) Place the burner under the beaker and light 4) Stir the water frequently 5) After three minutes, extinguish the flame and record the new temperature 6) Reweigh the spirit burner 7) You can now use this data to perform calculations

Question 36

Reasons why your value of ΔH may be different to the textbook value

Answer 36

Heat loss to the surroundings Incomplete combustion of the flammable liquid In the time after you have extinguished the burner but not applied the cap, some of the liquid may evaporate Non-standard conditions

Question 37

How to calculate the enthalpy change of a reaction

Answer 37

Pippete 25 cm3 of copper sulfate into a polystyrene cup. Weigh out an excess of your zinc powder Start a stop clock and take the temperature of the solution every 30 seconds until it stays constant, then add the powder Record the temperature every 30 seconds and plot this data on a graph

Question 38

Is energy required to break bonds or make bonds?

Answer 38

Break bonds - it is endothermic

Question 39

How are enthalpy changes calculated using bond enthalpies?

Answer 39

Enthalpy change = bond enthalpy of reactants - bond enthalpies in products

Question 40

Is energy released when making or breaking bonds?

Answer 40

Making bonds - it is exothermic

Question 41

What is Hess' Law?

Answer 41

The total enthalpy change of a reaction is always the same, no matter what route is taken

Question 42

What is the standard enthalpy change of formation for all elements?

Answer 42

0

Question 43

Where do the two routes mentioned in Hess' Law start?

Answer 43

Where the lines diverge

Question 44

Where do the two routes mentioned in Hess' Law end?

Answer 44

Where the lines converge

Question 45

How to use standard enthalpy of formation to work out enthalpy changes?

Answer 45

ΔHf products - ΔHf reactants

Question 46

How to use standard enthalpy of combustion to work out enthalpy changes?

Answer 46

ΔcH reactants - ΔcH products