HEAT ENERGY AND ENTHALPY

Question 1

What does exothermic mean?

Answer 1

When heat is transferred from the system to the surroundings.

Question 2

What does endothermic mean?

Answer 2

When heat energy is transferred from the surroundings to the system.

Question 3

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

Answer 3

The enthalpy change measured at standard conditions, when the number of moles of substances in the equation as written react.

Question 4

Chemical energy is made up of two conditions…

Answer 4

• Kinetic energy (measure of motion of particles in a substance)
• Potential energy (measure of how strongly these particles interact with each other, e.g. attract/repel.)

Question 5

What is heat energy?

Answer 5

The portion of potential energy and the kinetic energy of a substance that is responsible for the temperature of the substance. Heat energy is directly proportional to absolute temperature.

Question 6

What is enthalpy?

Answer 6

The measure of total energy of a system.

Question 7

The enthalpy change of a process is…

Answer 7

the heat energy that is transferred between the system and the surroundings at constant pressure.

Question 8

You can’t measure enthalpy directly, but…

Answer 8

you can measure the enthalpy change.

Question 9

What are standard conditions?

Answer 9

100kPa pressure and 298K temperature.

Question 10

Under standard conditions, the enthalpy change measured is called…

Answer 10

the standard enthalpy change.

Question 11

Exothermic reactions have what value?

Answer 11

-

Question 12

Endothermic reactions have what value?

Answer 12

+

Question 13

Enthalpy has the symbol…

Answer 13

H

Question 14

What is the equation to show the relationship between enthalpy and internal energy?

Answer 14

H = U + PV
(P = pressure, V = volume)

Question 15

Give some examples of exothermic reactions?

Answer 15

Freezing water, condensing water vapour, combustion of petrol, dissolving sodium hydroxide in water.

Question 16

Give some examples of endothermic reactions?

Answer 16

Melting ice, evaporating water, dissolving ammonium nitrate in water, photosynthesis.

Question 17

The change in enthalpy is given by …

Answer 17

products - reactants

Question 18

What does an enthalpy level diagram look like for an exothermic reaction?

Answer 18

Reactants ↓ Products

Question 19

What does an enthalpy level diagram look like for an endothermic reaction?

Answer 19

Reactants ↑ Products

Question 20

What points do you need to remember when constructing enthalpy level diagrams?

Answer 20

• You only need to label the vertical axis (Enthalpy, H).
• The formulae for both reactants and products should be giving, including state symbols.
• The value for ΔH, correct sign included.
• Not essential to show activation energy.

Question 21

What is the definition of standard enthalpy change of combustion?

Answer 21

The enthalpy change measured at standard conditions when one mole of a substance is completely burned in oxygen.

Question 22

What are the two common ways of writing the combustion of hydrogen?

Answer 22

H₂(g) + ½O₂ → H₂O(l) and 2H₂(g) + O₂ → 2H₂O(l)

Question 23

What are the steps of calculating enthalpy change of combustion?

Answer 23

1. Calculate heat energy, Q, transferred to the water using the equation: Q = mcΔT
2. Calculate the amount of n (moles) of ethanol burned.
3. Calculate the Δ𝒸H, using the equation: ΔH=-Q/n

Question 24

What is the experimental determination of enthalpy change of combustion of a liquid?

Answer 24

• A spirit burner containing the liquid under test is weighed.
• A known volume of water is added to a copper can.
• The temperature of the water is measured.
• The burner is lit.
• The mixture is constantly stirred with the thermometer.
• When the temperature of the water has reached approximately 20°C about its initial temperature, the flame is extinguished and the burner is immediately reweighed.
• The final temperature is measured.

Question 25

What are some errors and assumptions made in the experiment determining the enthalpy change of combustion?

Answer 25

- Some of the heat energy produced in burning is transferred to the air and not the water. - Some ethanol may not have burned completely to form carbon dioxide and water. - Some heat produced is transferred to the copper can and not the water. - The conditions are not standard, e.g. water vapour produced not liquid water. - Not all of the heat energy transferred from the water to the surroundings is compensated for.

Question 26

What is the standard enthalpy change of neutralisation?

Answer 26

The enthalpy change measure at standard conditions when one mole of water is produced by the neutralisation of an acid with an alkali.

Question 27

What is the experimental determination of enthalpy change of neutralisation?

Answer 27

- Use a pipette fitted with a safety filter, place 25.0cm³ of 1.00 mol dm⁻³ acid into a polystyrene cup. - Measure the temp. of the acid. - Using a pipette, place 25.0cm³ of the alkali into the beaker. - Measure the temp. of alkali. - Add the alkali to the acid, stir with the thermometer and measure the maximum temp. reached.

Question 28

What are the sources of error and assumptions in the neutralisation reaction?

Answer 28

- Uncertainties of measurements involved with the use of the pipette and the thermometer. - Some heat energy will be transferred to the polystyrene cup and the thermometer.

Question 29

How do you calculate standard enthalpy change of neutralisation?

Answer 29

- Find mean starting temp. - Calculate temperature change. - Calculate the volume of solution heated. - Use Q = mcΔT - Amount of acid neutralised = amount of water formed - Then, ΔneutH=-Q/n - Value should be negative.

Question 30

What is the standard enthalpy change of formation?

Answer 30

The enthalpy change when one mole of a substance is formed from its elements in their standard states.

Question 31

What is Hess's Law?

Answer 31

States that the enthalpy change of a reaction is independent of the path taken in converting reactants into products, provided the initial and final conditions are the same in each case.

Question 32

How do you calculate the standard enthalpy change of formation using Hess's Law?

Answer 32

ΔfH = sum of ΔcH of elements - ΔcH of compound

Question 33

How do you calculate enthalpy change of formation with two equations?

Answer 33

-Add theΔH of the two equations.