Energy Changes

Question 1

Talk to me about endothermic reactions.

Answer 1

Takes in energy from the surroundings - shown by a fall in temperature
Reactions include
- thermal decompositions
- citric acid + sodiumhydrogencarbonate
uses:
- some sports injury packs

Question 2

talk to me about exothermic reactions.

Answer 2

transfers energy to the surroundings - shown by a rise in temperature
Reactions include
- combustion
- neutralisation
- most oxidation reactions
uses:
- self-heating cans
- hand warmers

Question 3

what is activation energy (Elittle a)?

Answer 3

the minimum amount of energy that reactants need to react

Question 4

describe the endothermic reaction profile diagram to me.

Answer 4

high activation energy needed, then where the products are, the distance between that and the reactants line is the amount of energy absorbed.

Question 5

describe the exothermic reaction profile diagram.

Answer 5

low activation energy needed and the reactants and higher than the products. The difference between the two is the amount of energy released.

Question 6

what is bond breaking in endothermic reactions?

Answer 6

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

Question 7

what is bond forming in exothermic reactions?

Answer 7

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

Question 8

what is the overall energy change equal to?

Answer 8

total energy needed to break bonds - total energy released by forming new bonds

Question 9

what is a battery?

Answer 9

two or more cells connected in series

Question 10

what happens in non-rechargable cells and batteries?

Answer 10

reaction in cell in irreversible - can’t be recharged once one reactant is used up

Question 11

what happens in rechargable cells and batteries?

Answer 11

reaction in cell is reversible - the cell can be recharged by connecting it to an external current, reversing the reaction

Question 12

what are advantages of hydrogen fuel cells compared with batteries?

Answer 12

• less pollution over a cell’s lifetime
• don’t need recharging/replacing as often
• cheaper to make

Question 13

what are disadvantages of hydrogen fuel cells compared with batteries?

Answer 13

• Storage of H2 gas takes up more space
• H2 production can emit greenhouse gases
• H2 is explosive - hard to store safely

Question 14

what is the anode reaction (oxidation) of a hydrogen fuel cell?

Answer 14

H2 > 2H+ + 2e-

Question 15

what is the cathode reaction (reduction) of a hydrogen fuel cell?

Answer 15

O2 + 4H+ + 4e- > 2H2O

Question 16

what is the overall equation for a hydrogen fuel cell?

Answer 16

2H2 + O2 > 2H2O