Energy changes

Question 1

exothermic reactions……

Answer 1

transfers heat energy from the chemicals to the surroundings

Question 2

Examples of exothermic reactions

Answer 2

Combustion

Neutralisation

Many oxidation reactions

Question 3

examples of endothermic reactions

Answer 3

citric acid and sodium hydrogen carbonate

thermal decomposition

self-cooling ice packs

Question 4

endothermic reaction

Answer 4

takes in energy from the surroundings

Question 5

what happens in chemical reactions

Answer 5

atoms are rearranged

old bonds are broken and new bonds are formed

Question 6

what must happen for bonds to be broken

Answer 6

Reacting particles must collide with sufficient energy

this is why bond breaking is an endothermic reaction because it requires energy

Question 7

exothermic energy profile

Answer 7

The energy of products is less than the energy of reactants because energy has been lost to surroundings in the form of heat. SEE GRAPH

Question 8

Bond breaking is an ……….. reaction

Answer 8

endothermic because it requires energy to break bonds

Question 9

Bond formation is an ………. reaction

Answer 9

exothermic - because when a new chemical bond is formed, energy is given out

Question 10

endothermic energy profile

Answer 10

The energy of products is greater than the energy of reactants as energy has been absorbed from the surroundings.

Question 11

in an exothermic reaction …..

* to do with formation of bonds

Answer 11

the energy released from forming new bonds is greater than the energy needed to break existing bonds.

Question 12

in an endothermic reaction……

Answer 12

the energy needed to break existing bonds is greater than the energy released from forming new bonds.

Question 13

how do you measure energy changes in a solution?

Answer 13

mix reactants in an insulated container which enables the temperature change to be measured before heat is lost to its surroundings.

suitable for neutralisation and reactions involving solids

Question 14

the chemical reactions between the electrodes and the electrolyte …….

Answer 14

set up the charge difference between the reactivity

Question 15

what factors affect voltage in cells

Answer 15

• the type of electrode as different metals react differently to different electrolytes
• the bigger the difference in the reactivity of electrodes the bigger the voltage of the cell
• the electrolyte used as different ions in solution will react differently with metal electrodes used

Question 16

How does hydrogen fuel cells generate electricity?

Answer 16

A hydrogen fuel cell generates electricity by reacting hydrogen with oxygen.

• hydrogen and oxygen are supplied to the fuel cell - the fuel is oxidised and produces a potential difference
• the hydrogen is oxidised

Question 17

the process of a hydrogen fuel cell

Answer 17

1) hydrogen is added at the anode
2) the hydrogen molecules lose electrons to form hydrogen ions
3) oxygen is added at the cathode
4) the hydrogen ions formed at the anode travel through the electrolyte to the cathode
5) oxygen and hydrogen react to form water

Question 18

advantages if fuel cells

Answer 18

hydrogen fuel cells produce water,

not polluting

they are lightweight and small

they have no moving parts so they are unlikely to break down

Question 19

disadvantages of hydrogen fuel cell

Answer 19

• hydrogen is highly flammable
• hydrogen is difficult to compress or store
• sometimes produced from non- renewable sources

Question 20

Hydrogen is added at the anode. the hydrogen molecules lose electrons to form hydrogen ions

What is the equation at the anode?

Answer 20

2H_{2 =} 4H⁺ + 4e^-

Question 21

Oxygen is added at the cathode and the hydrogen ions formed at the anode travel through the electrolyte to the cathode, where they react with oxygen.

What is the equation of the cathode?

Answer 21

O₂ + 4H⁺ + 4e^- = 2H₂O

Question 22

what is the overall equation of a hydrogen fuel cell?

Answer 22

2H₂ + O₂ = 2H₂O