C7. Energy Changes (Y11 - Autumn 1)

Question 1

🟢 What is the 1st Law Of Thermodynamics?

Answer 1

Energy cannot be created or destroyed

In chemical reactions, energy is coserved. The total amount of energy in universe at the end of a chemical reaction is the same as before the reaction takes place

Question 2

🟢 What are Exothermic Reactions and How Does Energy Change

Answer 2

An exothermic reaction is one that gives out heat energy to the surroundings so the temperature of the surroundings so the temperature of the surroundings increases.

In an Exothermic reaction more energy is released making new bonds, than is required to break the reactant bonds.

Energy Change: Chemical energy → Heat energy
products have less chemical energy than the reactants, so the temperature is hotter

E.g of an Exothermic is HCl (aq) + Mg (s)

Question 3

🟢 What Reactions are Exothermic Reactions, and What Exothermic Reactions happen in Everyday Products

Answer 3

What Types of Reactions are Exothermic:

• Neutralisation
• Oxidisation, e.g Respiration
• Displacement Reaction
• Combustion

Everyday Products:

• Hot Cans/Tins (Tinned Soup)
• A Cooker
• Handwarmers

Question 4

🟢 What are Endothermic Reactions and How Does Energy Change

Answer 4

An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases.

In an Endothermic reaction less energy is released making new bonds, than is required to break the reactant bonds.

Energy Change: Heat energy → Chemical energy
products have more chemical energy than the reactants, so the temperature is colder

E.g of an Endothermic reaction is Ba(OH)2 (s) + NH4Cl (s)

Question 5

🟢 What Reactions are Endothermic Reactions, and What Endothermic Reactions happen in Everyday Products

Answer 5

What Types of Reactions are Exothermic:

• Thermal Decomposition (of a carbonate)
• Electrical Decomposition
• Acid + Hydrogencarbonate

Everyday Products:

• Sherbert Sticks
• Photosynthesis
• Ice Packs (ones that snaps, then releases two chemicals that produce ammonia and water, making it cold)

Question 6

🟢 What Reactions are Endothermic Reactions, and What Endothermic Reactions happen in Everyday Products

Answer 6

The Y-Axis of thegraph will be Chemical Energy, while the X-Axis will be the time (although the x-axis isn’t always drawn on).

For Exothermic Reactions, there will be a horizontal line on the graph showing the level of chemical energy the reactants have. Once the reaction begins, the activation energy causes the chemcial energy to spike, before the heat energy is transferred to the surroundings, meaning it comes back down lower than the intial line of the reactions, to show the decreased level of chemical emergy in the products.

Question 7

🟢 How to draw a Graph of an Endothermic Reaction

Answer 7

The Y-Axis of the graph will always be Chemical Energy, while the X-Axis will be the time (although the x-axis isn’t always drawn on).

For Endothermic Reactions, there will be a horizontal line on the graph showing the level of chemical energy the reactants have. Once the reaction begins, the activation energy causes the chemcial energy to spike, before the heat energy is taken in from the surroundings, meaning it comes back higher than the intial line of the reactions, to show the increased level of chemical emergy in the products.

Question 8

🟢 What is Activation Energy?

Answer 8

(Activation energy = minimum energy particles need to react)

There is a minimum amount of energy needed before colliding particles of reactants have sufficient energy to cause a reaction. This energy needed to start a reaction is called the activation energy. This activation energy is shown on reaction profiles.

If colliding particles of reactants collide with less energy than the activation energy, they will just bounce off each other.

Question 9

🟢 Which reaction is Exothermic or Endothermic:

1. Burning petrol
2. Reaction of hydrochloric acid with sodium hydrogencarbonate
3. Reaction of sulfuric acid with sodium hydroxide
4. Reaction of sulfuric acid with magnesium
5. Ammonium chloride dissolving in water in an ice pack
6. Thermal decomposition of zinc carbonate
7. Electrical decomposition of sodium chloride

Answer 9

Exothermic Reactions:

1. Burning petrol
2. Reaction of hydrochloric acid with sodium hydrogencarbonate
3. Reaction of sulfuric acid with sodium hydroxide
4. Reaction of sulfuric acid with magnesium

Endothermic Reactions:

5. Ammonium chloride dissolving in water in an ice pack
6. Thermal decomposition of zinc carbonate
7. Electrical decomposition of sodium chloride

Question 10

🟢 For this reaction, use the bond energy data to:

• Calculate the energy required to break the bonds
• Calculate the energy released when the bonds in products are made
• Calculate the energy change for the reaction
• State wether the reaction is exothermic or endothermic (+why?)

2 H—H + O===O —> 2 H—O—H

(H-H = 436
O=O = 498
O-H = 463)

Answer 10

Energy To Break The Bonds:
2 x H—H
2 x 436 = 872

1 x O===O
1 x 498 = 498

872 + 498 = 1370

Energy To Make The Bonds:
4 x O—H
4 x 463 = 1852

Energy Change:
1370 - 1852 = - 482kJ/mol

Exothermic or Endothermic:
It’s exothermic because more energy is released making new bonds, than is required to break the reactant bonds.

This is shown through the fact that -482kJ/mol of energy was given out in this reaction

(Exothermic is a negative energy change)

Question 11

🟢 For this reaction, use the bond energy data to:

• Calculate the energy required to break the bonds
• Calculate the energy released when the bonds in products are made
• Calculate the energy change for the reaction
• State wether the reaction is exothermic or endothermic (+why?)

. H H H H
. | | | |
H—C—C—O—H —> C===C + H—O—H
. | | | |
. H H H H

(C-H = 412
 C-O = 360
 O-H = 463
 C-C = 348
 C=C = 612)

Answer 11

Energy To Break The Bonds:
5 x C-H
5 x 412 = 2060

1 x C-O
1 x 360 = 360

1 x C-C
1 x 348 = 348

1 x O-H
1 x 463

2060 + 360 + 348 + 463 = 3231

Energy To Make The Bonds:
1 x C=C
1 x 612 = 612

4 x C-H
4 x 412 = 1648

2 x O-H
2 x 926

612 + 1648 + 926 = 3186

Energy Change:
3231 - 3186 = + 45kJ/mol

Exothermic or Endothermic:
It’s endothermic because less energy is released making new bonds, than is required to break the reactant bonds.

This is shown through the fact that 45kJ/mol of energy was given out in this reaction

(Endothermic is a positive energy change)

Question 12

🟢 How are Chemical Bonds Made and Broken

+What happens when more/less energy is used to break bonds than what it takes to form

Answer 12

There is always a balance between the energy needed to break bonds and the energy released when new bonds are made in a reaction. This is what decides wether the reaction is endothermic or exothermic.

• In some reactions, the energy released when new bonds afe formed (as the products are made) is more than the energy needed to break the bonds in the reactants. These reactions transfer energy to the surroundings. They are exothermic.
• In other reactions, the energy needed to break the bonds in the reactants is more than the energy released when new bonds in the reactants js more than the energy released when new bonds are formed in the products. These reactions transfer energy from the surroundings to the reacting chemicals. They are endothermic.

Question 13

🟢 What is Bond Energy?

Answer 13

The energy needed to break the bond between two atoms is called the bond energy for that bond.

Bond energies are measured in kJ/mol. You can used bond energies to work out the energy change for many chemical reactions.

Question 14

🟢 How Do You Work Out the Energy Change for a Chemical Reaction

Answer 14

To calculate the energy change for a chemical reaction, you need to work out:

1. How much energy is needed to break the chemical bonds in the reactants.
2. How much energy is released when the new bonds are formed in the products.

Question 15

🟢 How Does the difference between reactivity of metals affect the voltage?

Answer 15

The greater the difference in reactivity between the two metals used, the higher the volatge produced.

Question 16

🟠 What are Non-Rechargable Batteries made up of and how do they work?

Answer 16

• Chemicals inside produce electricity
• Chemicals get used up the battery goes flat (the voltage drops to zero) when the reactions inside stop because one of the reactants has been used up

Question 17

🟠 What are Rechargable Batteries made up of and how do they work?

Answer 17

• Chemicals inside produce electricity
• Chemicals get used up but can be reproduced when recharged
• When recharged the chemical reactions can be reversed when connected to an external electric current
• The cell can be used over and over again

Question 18

🟠 What is put into Hydrogen Fuel Cells and what do you get out (+ Chemical Equation, Half Equations, and Ionic Equation)

Answer 18

• Needs continuous supply of hydrogen and oxygen into cell
• Inside chemical processes convert hydrogen and oxygen into water
• These have a continuous fuel supply so do not go flat or need recharging
• You get Hydrogen from doing electrolysis of water which uses fossil fuels

Equation for the fuel cell: 2H2 + O2 –> 2H2O

Half Equation: H2 –> 2H+ + 2e-

Ionic Equation: O2 + 4H+ + 4e- –> 2H2O

Question 19

🟠 How do Hydrogen Fuel Cells Work (What happens inside of them)

Answer 19

Hydrogen gas is supplied as fuel to the negatively charged electrode. It diffuses through the graphite electrodes amd reacts with hydroxide ions to form water and provides a source of electrons to an external circuit

Oxygen gas is supplied to the positvely charged electrode. It diffuses through the graphite amd reacts to form hydroxide ions, accepting electrons from the external circuit.

If you add the two electrode reactions together, the electrons and the OH^- ipns on either side of the half equations cancel out. So you are left with the overall change in the hydrogen fuel cell, that is the oxidisation of hydrogen (the fuel).

The only waste product from this is water.

Question 20

🟠 Advantages and Disadvantages of Hydrogen Fuel Cells

Answer 20

Pros:

• Not not need to be electrically recharged
• The only waste product is water
• Can be a range of size for different uses
• Continuous supply of electricity (as long as there is a continuous supply of fuel)

Cons:

• Hydrogen is highly flammable
• Hydrogen is sometimes produced for the cell by non-renewable sources
• Hydrogen is difficult to store
• Very expensive
• Catalyst used in cell eventually poisoned

Question 21

🟠 Pros and Cons of Non-Rechargable Batteries

Answer 21

Pros:

• The initial costs are cheaper
• A single life lasts longer/It’s more powerful
• Don’t have to waste time recharging - plug new ones in
• Can be used many times

Cons:

• They are single use (Has finite life)
• They are prone to leaking
• They cost more in the long term
• Disposal of lots of used cells (which contain toxic chemicals) (Generates more long term waste)

Question 22

🟠 Pros and Cons of Rechargable Batteries

Answer 22

Pros:

• Can be used many times (You are able to recharge them)
• Cheaper than fuel cells
• No emissions
• Charging points more available
• A better longer term investment
• Generate less long-term waste

Cons:

• A single life lasts shorter/It’s less powerful
• You can spend a long time recharging them
• High upfront cost
• They become weaker with age
• Has finite life

Question 23

🟠 Example Uses of Non-Rechargable Batteries

Answer 23

-Alkaline cells (e.g. used in remote controls)

Question 24

🟠 Example Uses of Rechargable Batteries

Answer 24

• Cells in remote controls, clocks, etc.

- Lithium-ion cells (e.g. in mobile phones, tablets, laptops)

Question 25

🟠 Example Uses of Hydrogen Fuel Cells

Answer 25

-Hydrogen fuel cell (e.g. cars, spacecraft)

Question 26

🟠 What are the two half equations that take place for Hydrogen Fuel Cells

Answer 26

At the negative electrode (cathode): 2H2 + 4OH- → 4H2O + 4e- At the positive electrode (anode): O2 + 2H2O + 4e- → 4OH-

Question 27

🟢 What are Electrolytes

Answer 27

Electrolytes are ionic compounds that are: - In the molten state (heated so they become liquids), or dissolved in water - Under these conditions, the ions in electrolytes are free to move within the liquid or solution.