Energy Changes

Question 1

What do exothermic reactions do?

Answer 1

They transfer energy from the reacting molecules to the surroundings - surroundings get hotter

Question 2

Examples of exothermic reactions

Answer 2

• Neutralisation
• Combustion
• Oxidation
• Freezing

Question 3

What do energy profile diagrams show?

Answer 3

Scientists represent the energy changes that take place in reactions by drawing these

Question 4

What are endothermic reactions?

Answer 4

They take in energy from their surroundings - surroundings get colder

Question 5

Example of endothermic reactions

Answer 5

• Thermal decomposition
• Reaction between citric acid and sodium hydrogencarbonate
• Melting

Question 6

What is the activation energy?

Answer 6

Minimum amount of energy the reactants need to collide with each other and react

Question 7

What does a greater activation energy mean?

Answer 7

The greater the ae. the more energy needed to start the reaction - this has to be supplied

Question 8

What is the method to measure energy transfer?

Answer 8

1. Use the 50cm3 measuring cyclinder to put 30cm3 dilute hydrochloric acid into the polystyrene cup
2. Stand the cup in the beaker. This will make it more stable
3. Use the thermometer to measure the temperature of the acid. Record it
4. Pour5cm3 sodium hydroxide in the measuring cyclinder
5. Pour sodium hydroxide into the cup - gently stir with thermometer in the hole of lid
6. Repeat - pour more 5cm3 amounts to the cup - total of 40cm3 needs to be added
7. Repeat previous steps for second trial
8. Calculate mean of max temp

Question 9

Exothermic reactions in everday life examples

Answer 9

• Hand warmers - uses exothermic oxidation of iron in air to release energy
• Self heating cans

Question 10

Endothermic reactions in everday life examples

Answer 10

• Sports injury packs

Question 11

How can you reduce amount of energy lost to the surroundings?

Answer 11

• Putting polystrene cup into a beaker of cotton wool to give more insulation
• Putting a lid on the cup to reduce the energy lost by evaporation

Question 12

What does height represent in reaction profiles?

Answer 12

Overall energy change in the reaction per mole

Question 13

Is bonding breaking endothermic or exothermic, why?

Answer 13

Endo because energy must be supplied to break existing bonds

Question 14

Is bonding forming endothermic or exothermic, why?

Answer 14

Exo because energy is released when new bonds are formed

Question 15

How does the amount of energy related to bonds differ in exothermic reactions?

Answer 15

Energy released by forming bonds is greater than the energy used to break them

Question 16

How does the amount of energy related to bonds differ in endothermic reactions?

Answer 16

Energy used to break bonds is greater than energy released by forming them

Question 17

How do you work out the overall energy change?

Answer 17

Sum of energies needed to break bonds in the** reactants** - energy released when the new bonds are formed in the products

Question 18

What is an electromechanical cell?

Answer 18

Basic system made up of 2 diff electrodes in contact with ann electrolyte

Question 19

What causes the charge difference/ voltage of the cell?

Answer 19

Diff metals will react diff with the same electrolyte

Question 20

The ____ the ________ in ____ reactivity of the electrodes, the ____ the voltage of the cell

Answer 20

The bigger the difference in reactivity of the electrodes, the bigger the voltage of the cell

Question 21

What else would affect the size of voltage in fuel cells?

Answer 21

Electrolyte used in a cell - diff ions in solution will react diff with the metal electrodes used

Question 22

How is a battery formed?

Answer 22

2 or more cells together in series

Question 23

Example of non-rechargable batteries

Answer 23

Alkaline batteries contain cells which use irreversible reactions - once 1 reactant is used up, they produce no more charge and replacement is needed

Question 24

How do rechargable cells work?

Answer 24

The reaction can be reversedby connecting it to an external electric current

Question 25

Whats a fuel cell?

Answer 25

Electrical cell thats supplied with a fuel and oxygen/air and uses energy from the reaction between them to produce electrical energy effciently

Question 26

What happens to the fuel when it enters the cell?

Answer 26

Becomes oxidised and sets up a pd within the cell

Question 27

What does a hydrogen-oxygen fuel cell dp, type of reaction?

Answer 27

Combines O and H to produce water and release energy through a redox reaction

Question 28

What does a H-O fuel cell consist of?

Answer 28

• Electrolyte - usually an acid (phosphuric aicd(
• Electrodes - porous carbon with a catalyst
• Negative anode and positive cathode

Question 29

Process of H-O fuel cell

Answer 29

1. H goes into anode compartment
2. O goes into cathode
3. At anode, H loses electrons to produce H+ ions - oxidation
4. H+ ions in the electrolyte move to cathode
5. At cathode, O gains electrons and reacts with H+ ions from acidic eletrolyte to make water - reduction
6. Electrons flow through an external circuit from the anode to cathode - electric current

Question 30

Pros on fuel cells

Answer 30

• Not as many pollutants compared to other fuels - only by products are water and heat
• Electric cars dont produce many pollutants either - batteries are more polluting to dispose of than fuel cells because they’re made out of highly toxic compounds
• Limits to recharable electric vehicle batteries and they are more expensive than fuel cells
• Batteries store less energy than fuel cells so they need to be recharged more often - long time

Question 31

Cons on fuel cells

Answer 31

• H is a gas, takes up more space to store than a rechargable battery
• H is explosive when mixed with air - hard to store safely
• H fuel made from hydrocarbons (from fossil fuels) or electrolysis of water (electricity from ff)

Question 32

How do you know whether a reaction is mainly endo or exo thermic by using bond energies?

Answer 32

If the answer is negative, its exothermic as it has given that amount of energy to the surroundings
If its positive, its endothermic, has gained that muh energy from surroundings

Question 33

What does bond energy mean?

Answer 33

Amount of energy required to break one mole of a particular covalent bond.

Question 34

Fuel cell equation at negative electrode

Answer 34

2H₂ –> 4H⁺ + 4e⁻

Question 35

Fuel cell equation at positive electrode

Answer 35

O₂+ 4H⁺ + 4e⁻ –> 2H₂0

Question 36

Overall equation for fuel cells

Answer 36

2H₂ + O₂ –>2H₂0