3.1 - Energetics

Question 1

What is an exothermic reaction?

Answer 1

A reaction that releases heat energy to the surrondings eg. combustion

Question 2

How does energy change for the products during an exothermic reaction?

Answer 2

In an exothermic reaction products of reactants have less energy than reactants energy released to surrondings

Question 3

What are reactions that are exothermic?

Answer 3

Combustion:
Any reaction producing a flame is exothermic burning objects produces heat energy
Hydrogen burns in oxygen producing water and lots of heat

The reaction of metals with acids:
When magnesium reacts with dilute sulfuric acid the mixture gets war,

Neutralisation reaction:
Sodium hydroxide solution reacts with dilute hydrochloric acid is that the tempreture rises

Question 4

How can heat change be measured?

Answer 4

enthalpy change of the reaction given symbol delta H
enthalpy change = amount of heat energy taken in or out it is the difference between the energy if the products and the energy if the reactants

Question 5

How does an exothermic diagram show the changes in energy levels

Answer 5

In an exothermic reaction, reactants have more energy than the products the products re more stable than the reactants as the reaction happenes, energy given out in the form of heat. Energy warms up both the reaction and the surrondings

Question 6

What is an endothermic reaction?

Answer 6

A reaction that takes in heat energy from its surrondings

In an endothermic reaction, the products have more energy than the reactants. In order to supply the extra energy that is needed to convert the reactants into products.

Question 7

What are examples of endothermic reactions?

Answer 7

Thermal decompositions of metal carbonates

Question 8

Why is energy needed to break bonds?

Answer 8

To reach activation energy to break the bonds between the reactants. Allowing the products to start forming

Question 9

What type of reactions occour in endothermic and exothermic reaction?

Answer 9

Bond breaking is endothermic energy is required to break bonds
Bond making is exothermic energy is released when bonds are made

Question 10

Why are some reactions endothermic and some exothermic?

Answer 10

Depends on the magnitude of the energy for the bond breaking vs the energy for bond making

Question 11

What is the diagram and explanation for the endothermic change on an energy level diagram?

Answer 11

In an endothermic change, the products have more energy than the reactants so we say that the products are less stable than the reactants. That extra energy has to come from somewhere, and it is taken from surrondings.

Question 12

What is specific capacity?

Answer 12

The specific capacity heat capacity of a substance is the amount of heat needed to raise the tempreture of gram of a substance by 1oc

Question 13

How to calculate the heat energy change of a substance?

Answer 13

Mass (4.18) x SHC x temp change

Question 14

What is Calorimetry and how does it determine the enthalpy changes of reaction?

Answer 14

To measure the amount of heat absorbed or given out in different kinds of chemical reactions and physical changes heat transfer. If we use heat from a reaction to heat another substance heat energy change eq. can be used.

Question 15

Explain the practical: measuring enthalpy changes in combustion reactions?

Answer 15

Following procedure:
Measure 100cm3 of cold water using a measuring cylinder and transfer a copper can
Take initial temp of water
Wight spirit burner containing ethanol with its lid on. Lid should be kept on when wick is not lit to prevent the alcohol from evaporating
Arrange apparatus so spirit-burner can be used to heat the water in the copper can. The apparatus is shielded as far as possible to prevent draughts.
Light the wick to heat the water. Stop heating when reasonable temp of after flame can be extinguished
Stir water thoroughly and measure the maximum temp if the water
weigh the spirit burner again
Experiment repeated with same alcohol to check for reliability

Question 16

What sample data is used for this experiment?

Answer 16

Volume of water : 100cm3
Mass of burner + Ethanol before experiment : 137.36
Mass of burner + Ethanol after experiment : 136.58
Original temp of water : 21.5
Final temp of water 62.8

Combustion endothermic reaction so the temp of the water goes up. As ethanol is burned the total mass of the burner and ethanol goes down

Question 17

What is is the calculation for activity?

Answer 17

Mass/ Relative molecular mass and then Heat energy change/ Number of moles of ethanol burned.

Question 18

What is the evaluation of the experimental results?

Answer 18

Many errors in this experiment large amounts of heat loss.
The warm water gives out heat to the air, heat is lost from the flame, which goes straight into the air rather than into the water and heat is used to raise the temp and can the thermometer is well

Incomplete combustion of alcohol. Not enough oxygen present releasing less heat than complete. Flame is orange rather than blue and soot is produced at bottom of copper can.

This experiment can be repeated with other alcohols under similar conditions to find how heat evolved changes as the alcohol gets bigger

Question 19

Explain the practical: Measuring Enthalpy changes for displacement reactions.

Answer 19

This experiment can be used to measure molar enthalpy by changes in displacement reactions of the reaction of zinc and copper(II) sulfate

Place a polystyrene cup in a 250cm3 glass beaker
Transfer 50cm3 of 0.200 mol/dm3 copper(II) sulfate solution into the polystyrene cup using a measuring cylinder
Weight 1.2 g of zinc using a weighing boat on a balance
Record the initial tempreture of copper(II) sulfate solution
Add the zinc
Stir the solution as quickly as possible
Record the maximum tempreture reached

Initial temp of Copper(II) sulfate solution : 17 oc
Maximum temp of reaction mixture : 27 oc

Question 20

Explain the practical: Measuring Enthalpy changes when salts dissolve in water?

Answer 20

Calorimetry experiments can be used to work out the amount of heat given or taken out when salts dissolve in water.
Place a polystyrene cup in 250cm3 glass beaker
Transfer 100cm3 of water into the polystyrene cup using a measuring cylinder
Record the initial temp of the water
Weigh 5.2g of ammonium chloride using a weighing boat on a balance
Add the ammonium chloride to water and stir the solution vigorously until all the ammonium chloride has dissolved
Record the minimum tempreture

Inital temp of water: 18.3
Minimum tempreture of salt solution: 15.1

Question 21

Explain the practical: Measuring Enthalpy changes of neutralisation between an alkali and an acid.

Answer 21

The reaction between an alkali and an acid essentially between OH- and H+ ions to form water

In a reaction between potassium hydroxide and hydrochloric acid. Following method could be used to find out the conc of the acid and how much heat is released during the neutralisation reaction?

Place a polystyrene cup in a 250cm3 glass beaker
Transfer 25cm3 of 2.0 mol/dm3 pottasium hydroxide into the polystyrene cup using a measuring cylinder
Record initial tempreture
Fill a burette with 50.0cm3 of dilute hydrochloric acid
Use the burette to add 5.00cm3 of dilute hydrochloric acid to the potassium hydroxide
Stir vigorously and record the maximum tempreture reached.
Continue adding further 5.0cm of hydrochloric acid into the cup and keep stirring and recording the maximum tempreture reached each time till 50.0cm has been added

Question 22

Why do reactions either give out or absorb heat?

Answer 22

During Chemical reactants have to be broken and new ones formed to make products. breaking bonds needs energy (endothermic) and make bonds releases energy (exothermic).

Question 23

What is Bond energy?

Answer 23

The energy required to break the bond between two atoms

Question 24

What are the calculations of Enthalpy changes of reaction using bond energies?

Answer 24

Breaking chemical bonds requires energy. the stronger the bond the more energy required to break it. Bond energies are measured in kJ/mol.

Question 25

What are all the bonds and their bond energy?

Answer 25

Br-Br - 193
Cl-Cl - 242
C-Br - 276
C-C - 348
C-O - 360
C-H - 412
H-Cl - 431