5: Energy Change

Question 1

what happens in exothermic reactions?

Answer 1

energy is given out to the surroundings so the temperature of the surroundings increases

Question 2

give examples of where exothermic reactions are used in everyday life.

Answer 2

hand warmers; self-heating cans of food and drinks

Question 3

give examples of typical exothermic reactions.

Answer 3

combustion, oxidation and neutralisation

Question 4

what happens in endothermic reactions?

Answer 4

energy is taken in from the surroundings so the temperature of the surroundings decreases

Question 5

give examples of typical endothermic reactions.

Answer 5

electrolysis, thermal decomposition reactions and the first stages of photosynthesis

Question 6

give examples of where endothermic reactions are used in everyday life.

Answer 6

cold packs used in sports injury treatment

Question 7

describe the practical to perform a calorimetric study of the reaction between HCl and NaOH.

Answer 7

1. using a measuring cylinder, place 25 cm3 of the NaOH into the calorimeter.
2. measure and record the temperature of the solution.
3. add 5cm3 of the dilute HCl and stir, lid should be placed on the calorimeter as soon as the acid is added to minimise heat loss to the surroundings
4. measure and record the highest temperature reached by the mixture.
5. repeat steps 1 – 4 increasing the amount of acid added by 5cm3 each time.
   conc. the larger the difference in the temperature the more energy is absorbed or released.

Question 8

what is activation energy?

Answer 8

the minimum quantity of energy required to start a reaction - the amount of energy needed to break the bonds of the reactants

Question 9

what does a reaction profile for an exothermic reaction look like?

Answer 9

energy increases when bond breaking then decreases by more than it increased when bond making - reactants are at a higher energy than products
more energy is released when new bonds are formed than energy required to break the bonds in the reactants

Question 10

what is enthalpy change?

Answer 10

the difference in energy between reactants and products

Question 11

what is the enthalpy change like in an exothermic reaction?

Answer 11

always negative bc heat/energy is given out to surroundings

Question 12

what does a reaction profile for an endothermic reaction look like?

Answer 12

energy rises as bonds break, energy decrease when making bonds, but the decrease is lower than the increase, so the energy of the products is higher than the energy of the reactants
more energy is required to break the bonds than that gained from making the new bonds

Question 13

what is the enthalpy change like in an endothermic reaction?

Answer 13

always positive bc energy is taken in from surroundings; this increase in energy is also known as the heat input

Question 14

what kind of reaction is bond breaking?

Answer 14

endothermic - energy is needed so is absorbed from the surroundings

Question 15

what kind of reaction is bond making?

Answer 15

exothermic - energy is transferred back to the surroundings

Question 16

how do you calculate energy change?

Answer 16

energy change = energy taken in (add together all the bond energies for all the bonds in the reactants) - energy given out (add together the bond energies for all the bonds in the products)

Question 17

what is bond energy?

Answer 17

the amount of energy required to break the bond or the amount of energy given out when the bond is formed

Question 18

what is the structure of a simple cell?

Answer 18

take two different metals (electrodes) with different reactivities, place them in an electrolyte (solution that can conduct electricity) and connect them to an external voltmeter by wire, creating a complete circuit.

Question 19

why can a cell only produce electricity for a certain period of time?

Answer 19

eventually, the chemicals in the cell will run out and reaction will stop - ions in the electrolyte and the metal ions at the electrodes are converted into products

Question 20

how do simple cells work?

Answer 20

the more reactive electrode will form ions for easily and release electrons; the electrons give the more reactive electrode a negative charge and sets up a charge difference between the electrodes; electrons then flow around the circuit to the other electrode which is now the more positive electrode; difference in ability of electrodes to release electrons causes a voltage to be produced; the greater the difference in the metals reactivity, greater the voltage produced; the electrolyte used also affects the voltage - different ions react with electrodes in different ways

Question 21

what are batteries?

Answer 21

contains 2 or more cells connected in series to produce a greater voltage

Question 22

how do rechargeable batteries work?

Answer 22

the reactions are reversed by connecting the cells to an external electrical supply, recharges the battery which can then be used again

Question 23

what are the adv. and dis-adv. of a rechargeable battery?

Answer 23

adv. : saves money; decreases waste, lasts longer

dis-adv. : preparation required; not versatile; classified as hazardous waste

Question 24

what are the adv. and dis-adv. of a non-rechargeable battery?

Answer 24

adv. : convenient; cost less per battery; ready to use; high energy density; relatively versatile
dis-adv. : waste and pollution

Question 25

what is a fuel cell?

Answer 25

an cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode

Question 26

how does a fuel cell work?

Answer 26

the fuel enters the cell becoming oxidised which sets up a potential difference or voltage within the cell

Question 27

what is produced in a hydrogen-oxygen fuel cell?

Answer 27

energy, water

Question 28

at the anode in a fuel cell, what reaction takes place (in acidic conditions)?

Answer 28

4H+ + O2 + 4e– -> 2H2O

Question 29

at the cathode in a fuel cell, what reaction takes place (in acidic conditions)?

Answer 29

H2 → 2H+ -> 4e–

Question 30

at the anode in a fuel cell, what reaction takes place (in alkaline conditions)?

Answer 30

H2 + 2OH -> 2H2O + 2e-

Question 31

at the cathode in a fuel cell, what reaction takes place (in alkaline conditions)?

Answer 31

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

Question 32

what are the adv. of a hydrogen fuel cell?

Answer 32

do not produce pollution; easy to maintain as there are no moving parts; small size; water is the only chemical product

Question 33

what are the dis-adv. of a hydrogen fuel cell?

Answer 33

very expensive to manufacture; need a constant supply of hydrogen fuel, which is a flammable gas; high pressure tanks are needed to store the oxygen and hydrogen in sufficient amounts which are dangerous and difficult to handle