C5 - Energy Changes

Question 1

Exothermic and Endothermic Reactions -
What is an exothermic reaction?

Answer 1

transfers energy to surroundings (usually by heat) - shown by a rise in temperature

Question 2

Exothermic and Endothermic Reactions -
What is an example of an exothermic reaction?

Answer 2

• Combustion - burning fuels (gives out a lot of energy)
• Neutralisation (acid+alkali)
  -many Oxidation reactions (eg sodium + water)

Question 3

Exothermic and Endothermic Reactions -
What are the every day uses of exothermic reactions?

Answer 3

• hand warmers (oxidation of iron in air with a salt solution catalyst)
• self heating cans (hot chocolate and coffee rely on chemical reactions between chemicals in their bases)

Question 4

Exothermic and Endothermic Reactions -
What is an endothermic reaction?

Answer 4

Takes in energy from surroundings (shown by fall in temperature)

Question 5

Exothermic and Endothermic Reactions -
Which reaction is more common?

Answer 5

Exothermic

Question 6

Exothermic and Endothermic Reactions -
What are examples of endothermic reactions?

Answer 6

• reaction between citric acid and sodium hydrocarbonate
• thermal decomposition

Question 7

Exothermic and Endothermic Reactions -
What is an example of thermal decomposition?

Answer 7

• calcium carbonate heated causing to decompose into calcium oxide (quicklime) and carbon dioxide

Question 8

Exothermic and Endothermic Reactions -
What are the everyday uses of endothermic reactions?

Answer 8

• sports injury packs (chemical reactions allow to become instantly cooler without being put in freezer)

Question 9

Exothermic and Endothermic Reactions -
What is an example of an endothermic process?

Answer 9

Melting

Question 10

Exothermic and Endothermic Reactions -
What is an example of an exothermic process?

Answer 10

Freezing

Question 11

Exothermic and Endothermic Reactions - RP
How can you measure the amount of energy released by a chemical reaction?

Answer 11

1. Taking temp of reagents
2. Mixing them in polystyrene cup
3. Measure temp of solution at end of reaction

Question 12

Exothermic and Endothermic Reactions - RP
What is the biggest problem with energy measurements?

Answer 12

Amount of energy lost to the surroundings

Question 13

Exothermic and Endothermic Reactions - RP
How can you reduce the amount of energy lost to the surroundings?

Answer 13

• Put polystyrene cup into beaker of cotton wool (more insulation)
• lid on sup (reduce energy lost by evaporation)

Question 14

Exothermic and Endothermic Reactions - RP
What reaction does using a polystyrene cup with a lid on work for?

Answer 14

Neutralisation reactions (metals + acids, carbonates + acids)

Question 15

Exothermic and Endothermic Reactions - RP
How do you test fro the effect of acid concentration on the energy released in a neutralisation reaction between HCl and NaOH?

Answer 15

1. Put 25 cm cubed of 0.25 mol/dm cubed of HCl and NaOH in separate beakers
2. Place beakers in water bath 25 degrees C until both at same temp
3. Add HCl then NaOH to polystyrene cup with lid
4. Take temp of mixture every 30s and record highest temp
5. repeat steps 1 to 4 using 0.5 mol/dm cubed and then 1 mol/ dm cubed of HCl

Question 16

Exothermic and Endothermic Reactions -
What are reaction profiles?

Answer 16

• relative energies of reactants and products in a reaction
• how energy changes over course of reaction

Question 17

Exothermic and Endothermic Reactions -
Where are the products in a exothermic reaction?

Answer 17

Lower than reactants

Question 18

Exothermic and Endothermic Reactions -
What does the difference in height on a reaction profile represent?

Answer 18

overall energy change in reaction per mole

Question 19

Exothermic and Endothermic Reactions -
What is the activation energy?

Answer 19

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

Question 20

Exothermic and Endothermic Reactions -
The greater the activation energy

Answer 20

the more energy needed to start reaction (supplied by heating reaction mixture)

Question 21

Exothermic and Endothermic Reactions -
What are reaction profiles sometimes called?

Answer 21

Energy level diagrams

Question 22

Bond Energies -
What happens during a chemical reaction to the bonds?

Answer 22

• old bonds broken
• new bonds formed

Question 23

Bond Energies -
What type of process is bond breaking?

Answer 23

Endothermic

Question 24

Bond Energies -
What type of process is bond formation?

Answer 24

Exothermic

Question 25

Bond Energies -
What happens in terms of energy to break and form bonds in exothermic reactions?

Answer 25

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

Question 26

Bond Energies -
What happens in terms of energy to break and form bonds in endothermic reactions?

Answer 26

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

Question 27

Bond Energies -
What does every chemical bond have?

Answer 27

Particular bond energy

Question 28

Bond Energies -
What can you use known bonds to calculate?

Answer 28

Overall energy change in a reaction

Question 29

Bond Energies -
What is the overall energy change in a reaction?

Answer 29

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