Energetics - Enthalpy

Question 1

Describe an enthalpy diagram for an Exothermic change

Answer 1

Question 2

Describe an enthalpy diagram for an Endothermic change

Answer 2

Question 3

What are key features to remember for an exothermic reaction?

Answer 3

• Heat energy loss to environment
• Reacting chemicals lose energy
• Increase in temperature
• Bond making
• Enthalpy = -kJmol^-1

Question 4

What is important to remember about the enthalpy of an exothermic reaction?

Answer 4

The enthalpy of the products is less than the enthalpy of the reactants so energy has been released to the surroundings

Question 5

Examples of exothermic reactions

Answer 5

• Combustion
• Neutralisation
• Respiration
• Displacement

Question 6

What are key features to remember for an endothermic reaction?

Answer 6

• Heat energy gained from the environment
• Reacting chemicals absorb energy
• Decrease in temperature
• Bond breaking

Question 7

What is important to remember about the enthalpy of an endothermic reaction?

Answer 7

The enthalpy of the products is greater than the reactants so less energy has been absorbed from the surroundings

Question 8

Examples of endothermic reactions

Answer 8

• Thermal decomposition
• Photosynthesis
• Dehydration of hydrated copper sulfate

Question 9

What is activation energy?

Answer 9

The minimum amount of energy which the reactants must have in order to react

Question 10

What is enthalpy?

Answer 10

Enthalpy relates to the energy of the bonds broken and made during a chemical reaction.

Endo = bond breaking

Exo = bond making

Question 11

What are the Standard conditions / states?

Answer 11

Standard pressure : 100kPa (1atm)

Standard temperature : 298K

Standard concentration : 1.00mol dm^-3

Question 12

What is Mean Bond Enthalpy?

Answer 12

The energy required to break 1 mol of a gaseous covalent bond averaged across many compounds containing the bond.

Units : kJmol^-1

H = Σ MBE bonds broken (reactants) - Σ MBE bonds made (products)

Question 13

What is the Standard enthalpy of reaction?

Answer 13

This is the enthalpy change when substances react under standard conditions in quantities given by the equation for the reaction.

E.g. CaO(s) + H₂0 → Ca(OH)₂(s) Change_rH° = -63.7kJ mol^-1

This means that when 1 mole of calcium oxide reacts with 1 mole of water to form 1 mole of calcium hydroxide, 63.7kJ of heat would be released.

Question 14

What are calorimetery experiments?

Answer 14

Experiments that measure the heat given out by reactions. These reactions take place in solutions, they can be placed into a polystyrene container and the temperature change recorded with a thermometer.

Question 15

Why are polystyrene cups used?

Answer 15

They are good insulators and have a low heat capacity. This reduces heat loss through the sides of the container.

Question 16

What are the issues involving caloirmetry?

Answer 16

• Some heat will be absorbed by the container, rather than going towards heating up the water.
• Some heat is always lost to the surroundings during the experiment (depending on how well the container is insulated)

Question 17

What calculations are used for direct enthalpy change?

Answer 17

Enthalpy change = -q (J) / n

q = mcAT

Density = mass/volume

Question 18

What is the density of water?

Answer 18

1 g/cm³

It can be used in the calculation to replace the mass in q = mcAT

This can be used for example when using the mass as the volume of water making up a solution in the reaction.

Question 19

What is the enthalpy change of formation?

Answer 19

This is the enthalpy change when one mole of compound is formed from its elements under standard conditions, all reactants and products being in their standard states.

Question 20

What is the Enthalpy change for all elements and why?

Answer 20

Zero, because there is no change in enthalpy

Question 21

What are the equations for the element formations of methane, water, sulfur dioxide, hydrogen sulfide and nitrogen monoxide?

Answer 21

C(s) + 2H₂(g) → CH₄(g)

H₂(g) + 1/2O₂(g) → H₂O(l)

S(s) + O₂(g) → SO₂(g) - Sulfur is a yellow solid

H₂(g) + S(s) → H₂S

1/2N₂ + 1/2O₂(g) → NO(g)

Question 22

What is Hess’s Law?

Answer 22

Hess’s Law states that the enthalpy change for a chemical reaction is the same, what ever route is taken from reactants to products.

Question 23

What is the diagram for Hess’s Law, which is used to calculate the enthalpy for a reaction?

Answer 23

Question 24

What can this diagram be arranged to?

Answer 24

A_rH° = A_fH° products - A_fH° reactants

Question 25

What is the standard enthalpy change of combustion?

Answer 25

A_cH° is the enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions with all reactants and products in their standard states

Question 26

What is the formula for calculating the enthalpy of formation using the enthalpy of combustion?

Answer 26

A_fH° = A_cH° reactants - A_cH° products

Question 27

Define enthalpy change (DELTA H)

Answer 27

Change in heat energy (of a reactant) at a constant pressure

Question 28

What are the units for enthalpy change?

Answer 28

kJmol-1

Question 29

Name the standard conditions

Answer 29

• 1 atm (100kPA)
• 298 K