Topic 5 - Energy Changes

Question 1

why should you use a polystyrene cup and lid?(exo/endo required practical)

Answer 1

reduces heat energy lost to surroundings as it provides more insulation and lid reduces energy lost by evaporation

Question 2

what is the dependant variable for the temperature change RP

Answer 2

temperature

Question 3

what is the control variable for the temperature change RP

Answer 3

volume of acid

Question 4

what is the independent variable for the temperature change RP

Answer 4

volume of odium hydroxide

Question 5

why does the graph start to decrease at a certain volume (RP temperature change)

Answer 5

at a certain volume, the max temp decreases as sodium is in excess and cant react with enough HCl

Question 6

what is the method to find the temperature change for exo and endo reactions RP

Answer 6

1. measure 30 cm^3 of dilute hydrochloric acid using a measuring cylinder
2. pour the acid in a polystyrene up and place it in a beaker( stops cup from falling)
3. use a thermometer to measure temperature of acid- record in table
4. measure 5 cm^3 of sodium hydroxide with a measuring cylinder and pour it into the polystyrene cup
5. fit a plastic lid on cup and place thermometer through the hole- the bulb of thermometer must be in solution
6. use thermometer to gently stir solution
7. when temperature on thermometer stops changing, record the highest temp in the table
8. rinse out cup and repeat whole experiment increasing sodium- by 5cm^3 each time until reach 40cm^3
9. repeat whole experiment twice so you have sets of results and calculate mean

Question 7

what is a fuel cell

Answer 7

an electrical cell supplied with an external source of fuel and oxygen

Question 8

what happens to the fuel in the fuel cell

Answer 8

the fuel is oxidised electrochemically within the fuel cell to produce a potential difference

Question 9

what is the equation for the reaction in the hydrogen fuel cell

Answer 9

the hydrogen fuel cell involves the oxidation of hydrogen to produce water
hydrogen + oxygen –> water + electricl energy

Question 10

what is bad about using fuel cells

Answer 10

they are difficult to store

Question 11

what is good about fuel cell vehicles

Answer 11

they don’t produce as many pollutants(greenhouse gases etc)
the only product is water an heat

Question 12

what is the problem with electric vehicles

Answer 12

• the batteries are more polluting to dispose of than fuel cells as they are made from highly toxic metal compounds
• limit to how much they can be recharged
• more expensive than fuel cells
• store less energy so need to be recharged more often (time consuming)

Question 13

what happens in a hydrogen fuel cell

Answer 13

• hydrogen goes into the anode(-)
• oxygen goes into the cathode (+)

Question 14

what is formed at the anode(-) in a hydrogen fuel cell

Answer 14

hydrogen loses electrons to produce H+ ions (oxidation)

Question 15

what is formed at the cathode(-) in a hydrogen fuel cell

Answer 15

oxygen gains electrons, and reacts with H+ to make water (reducation)

Question 16

what is the electric current in a hydrogen fuel cell

Answer 16

the elctrons flow through an external circuit from the anode to the cathode this is the electric current

Question 17

what is the half equation for the reaction at the anode in a hydrogen fuel cell

Answer 17

H2 –> 2H+ + 2e-

Question 18

what is the half equation for the reaction at the cathode in a hydrogen fuel cell

Answer 18

O2 + 4H+ + 4e- —> 2H2O

Question 19

what is a battery

Answer 19

formed by connecting 2 or more cells in series to provide a greater voltage

Question 20

what are non rechargeable cells and batteries

Answer 20

made up of cells which use irreversible reactions

Question 21

what happens to the reaction in a non rechargeable cell and battery when one of the reactants is used up

Answer 21

reaction stops and dont produce any more charge and must be replaced

Question 22

what is an example of a non rechargeable cell and battery

Answer 22

alkaline batteries

Question 23

what are rechargeable cells and batteries

Answer 23

can be recharged because the reactions are reversed when an external electrical current is supplied

Question 24

what do cells contain

Answer 24

chemicals which react to produce electricity

Question 25

what is an electrochemical cell

Answer 25

a basic system made up of 2 different electrodes in contact with an electrolyte

Question 26

what are the electrodes made of

Answer 26

must be able to conduct electricity so are metals

Question 27

what does the difference in reactivity between electrodes mean

Answer 27

the greater the difference in reactivity, the greater the voltage

Question 28

what is the electrolyte

Answer 28

a liquid that contains ions which reacts with the electrodes

Question 29

what does the chemical reaction between electrodes and the electrolyte set up

Answer 29

a charge difference between the electrodes

Question 30

when is the charge in the electrodes able to flow

Answer 30

once the electrodes are connected by wire the charge is able to flow and electricity is produced

Question 31

how can the electrolyte affect the size of the voltage

Answer 31

as different ions react differently

Question 32

what are bond energies

Answer 32

energy required to break a particular covalent bond between 2 non metal atoms

Question 33

how do you find the overall energy change of reaction

Answer 33

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

Question 34

how much energy needed to break all bonds H-C=C-H [ [ H H

Answer 34

c-H x 4 = 413 x 4= 1652 c=c x1 = 612 x 1= 612 1652 + 612= 2264 kj/mol

Question 35

calculate the energy change for reaction H-H-C-H-H + 2 O=O ---> 2o-H-o + o=c=o

Answer 35

C-H x4 = 1652 O=O x2= 996 1652+996= 2648 0=H x 2x2= 1856 o=c x 2= 1610 1856+1610= 3466 2648-3466= -818 KJ/Mol

Question 36

what happens to bonds during chemical reactions

Answer 36

old bonds are broken and new bonds are formed

Question 37

what happens in an endothermic process (bonds)

Answer 37

energy must be supplied to break existing bonds

Question 38

what happens in an exothermic process (bonds)

Answer 38

energy is released when new bonds are formed

Question 39

what is the word to remember bonds making and breaking

Answer 39

Bendo Mex

Question 40

what does Bendo Mex stand for

Answer 40

bond Breaking is ENDOthermic, bond Making is EXOthermic

Question 41

in an exothermic reaction what happens to the energy when breaking and making bonds

Answer 41

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

Question 42

in an endothermic reaction what happens to the energy when breaking and making bonds

Answer 42

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

Question 43

what happens to amount of energy in chemical reactions

Answer 43

it is conserved energy before chemical reaction = energy after

Question 44

what happens if the products store less energy

Answer 44

the excess energy is transferred to the surroundings during the reaction

Question 45

what happens if the product store more energy than the original reactants

Answer 45

they must have taken the difference in energy from the surroundings during the reaction

Question 46

what is a catalyst

Answer 46

a chemical that speeds up the rate of reaction by providing a lower energy pathway

Question 47

what do reaction profiles show

Answer 47

show the relative energies of reactants and products in a reaction and how the energy changes in the reaction

Question 48

what does a reaction profile fore an exothermic reaction look like

Answer 48

- the reactants are higher than the products

Question 49

what does a reaction profile fore an endothermic reaction look like

Answer 49

the products are higher than the reactants

Question 50

how do you find the overall energy change on a reaction profile

Answer 50

the height in between the line for the products and the line for reactants

Question 51

how do you find the activation energy on a reaction profile

Answer 51

the height from the reactant line to the very top of the curve

Question 52

what is the activation energy

Answer 52

the minimum energy needed to stat a reaction

Question 53

what happens when the activation energy is large

Answer 53

the greater the activation energy, the more energy needed to start the reaction

Question 54

what is an exothermic reaction

Answer 54

transfers energy to the surroundings so temperature of surroundings increases

Question 55

what is examples of exothermic reactions

Answer 55

combustion, oxidation, neutralisation

Question 56

what are everyday uses of exothermic reactions

Answer 56

- hand warmers use exothermic oxidation of iron in air to release energy - self heating cans of coffee rely on exothermic reactions between chemicals in the bases

Question 57

what are endothermic reactions

Answer 57

takes energy from the surroundings so the temperature decreases

Question 58

what are examples of endothermic reactions

Answer 58

- reactions between citric acid and sodium hydrogen carbonate - thermal decompositions

Question 59

what is an everyday use of endothermic reactions

Answer 59

- used in some sport injury packs- the chemical reaction allows pack to become instantly cooler with no freezer