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chemistry > C7 energy changes > Flashcards

C7 energy changes Flashcards

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1
Q

what are exothermic reactions

A

these transfer energy from the reacting chemicals to the surroundings this usually causes a temperature rise to the surroundings

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2
Q

what are endo thermic reactions

A

these transfer energy to the reacting chemicals from the surroundings this usually causes a temperature decrease to the surroundings

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3
Q

example of exothermic reactions

A
  • combustion
  • neutralisation of acids and alkalis
  • oxidation reactions
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4
Q

energy content of exothermic reactions

A

the products have a lower energy content than the reactants

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5
Q

examples of endothermic reactions

A
  • thermal decompositions
  • the reaction of citric acid and sodium hydrogen carbonate
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6
Q

energy content of endothermic reactions

A
  • products have a higher energy content than reactants
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7
Q

what does the exothermic reaction profile look like

A

——-(not apart of the prof
ㅤㅤㅤㅤㅤㅤ——–
ㅤㅤㅤㅤ ——|ㅤㅤ|
ㅤreactantsㅤㅤㅤㅤ|
ㅤㅤㅤㅤㅤㅤㅤㅤㅤ|——
ㅤㅤㅤㅤㅤㅤㅤㅤㅤproducts

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8
Q

what does the endothermic reaction profile look like

A

—– (not apart of the profile)
ㅤㅤㅤㅤㅤㅤ——–
ㅤㅤㅤㅤㅤㅤ|ㅤㅤ|——–
ㅤㅤㅤㅤㅤㅤ|ㅤㅤㅤproducts
ㅤㅤㅤ—— -|
ㅤreactants

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9
Q

how to calculate the energy change

A

the difference between the energy levels of the reactants and products

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10
Q

what happens when the reactants form products in exothermic reactions

A

energy is released

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11
Q

what happens when the reactants form products in endothermic reactions

A

energy is taken in from the surroundings

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12
Q

what is the activation energy

A

the minimum energy required for the reactant particles to collide and react

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13
Q

what happens if the reactant particles collide with less energy than the activation

A

they will bounce off each other an no reaction would occur

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14
Q

what type of reaction is breaking bonds

A

an endothermic reaction as energy has to be supplied to break bonds

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15
Q

why type of reaction is bond making

A

an exothermic reaction as energy is released when forming new bond

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16
Q

what does comparing the amount of energy to break and make new bonds allow us to find

A

allows us to find the overall energy change and discover whether the reaction is exothermic or endothermic

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17
Q

REQUIRED PRACTICAL temperature changes

A

1) use a measuring cylinder to measure 30cm3 of dilute hydrochloric acid
2) transfer the acid into a polystyrene cup and put the cup inside a beaker (this stops it from falling over)
3) use a thermometer to measure the temp of the acid and record it
4) use a measuring cylinder to measure 5cm3 of sodium hydroxide and transfer it to the polystyrene cup then place a lid on the cup and place a thermometer through the lid
5) gently stir the solution using the thermometer
6) record the highest temperature reach when the readings of the thermometer stop changing
7) rinse out the cup and repeat the experiment several times but increase the sodium hydroxide solution by 5cm3 until we reach 40cm3 of sodium hydroxide solution
5

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18
Q

why do you stir the solution in the REQUIRED PRACTICAL

A

to ensure a complete reaction

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19
Q

why does the temperature start to decrease in the REQUIRED PRACTICAL

A

as we are adding so much sodium hydroxide that there is not enough hydrochloric acid so some of the sodium hydroxide is unable to react so the energy released by the experiment has reached a maximum

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20
Q

anomalies of REQUIRED PRACTICAL

A
  • heat loss (can be prevented with polystyrene cup as its a good insulator so reduces heat loss to the air)
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21
Q

dependent independent and control variable of REQUIRED PRACTICAL

A

I - volume of sodium hydroxide
D - maximum temperature reached
C - volume of hydrochloric acid and and concentration of HCl and sodium hydroxide

22
Q

what type of reaction are handwarmers

A

exothermic reactions

23
Q

how do hand warmers using the oxidation of iron work

A
  • sodium chloride act as a catalyst, speeding up the reactions and the iron oxide turns into hydrate iron oxide through an exothermic reaction this transfers energy to the surroundings but can only be used once and last a long time
24
Q

how do hand warmers using the salts work

A

the main salt, sodium ethanoate is used a super saturated solution dissolves salt in hot water this makes an exothermic reaction the solution then cools down and can be used again

25
Q

how do hand warmers using the crystallisation work

A

a small metal disk is found in the hand warmer when it is pressed metal particles are released this sets of crystallisation and the crystals spread through the solution transferring energy to the surroundings this only last for 30 mins

26
Q

how do self heating cans work

A
  • self heating cans heat drinks without the need of any external heat warmers it does this by a person pressing a button which breaks a seal and releases calcium oxide and water this causes an exothermic reaction
27
Q

how do cold packets work

A

when the separate packs of ammonium nitrate and water are squeezed so the ammonium nitrate dissolves in the water causing an endothermic reaction

28
Q

how to work out the overall energy change

A

energy required to break bonds - energy required to make bonds OR bond of reactants - bond of products

29
Q

how to calculate heat transferred

A

mass of water x specific heat capacity of water x change in temperature

30
Q

what is the specific heat capacity f water

A

4.2 J/g/degrees

31
Q

what are batteries

A

two or more cells joined together t increase the voltage available

32
Q

what are batteries

A

two or more cells joined together t increase the voltage available

33
Q

how do you make batteries and electrical cells

A

by using the difference in reactivity

34
Q

what is the relationship between voltage and reactivity

A

the lower the voltage the larger the difference between reactivity between two metals

35
Q

what is the relationship between the tendency of a metal and its reactivity

A

the greater the tendency the more reactive a metal is

36
Q

why are some batteries non rechargeable

A

the chemical reactions stop when one of the reactants has been used up and has to be disposed to landfill

37
Q

examples of non rechargeable batteries

A
  • alkaline batteries
  • primary batteries
38
Q

what affect the voltage produced by a cell

A
  • the type of electrode
  • electrolyte
39
Q

how are some batteries rechargeable

A
  • they reverse the reaction that occur at each electrode when the electrons are discharged this happens when the battery is connected to a power supply
40
Q

what is hydrogen fuel used for

A
  • in cars
  • in fuel cells
41
Q

advantages of using hydrogen fuel in cars

A
  • would help the human impact of global warming as CO2 would not be released
42
Q

disadvantages of hydrogen fuel

A
  • making hydrogen through electrolysis requires electricity from non renewable fossil fuels
  • power stations produce CO2 and use our limited supply of resources
43
Q

what do scientist have to consider about before replacing petrol and diesel cars with fuel cells

A
  • price
  • convenience
  • the performance of hydrogen fuel cells in cars
44
Q

how do hydrogen fuel cells work

A
  • the hydrogen is supplied to the negative electrode (anode) and then diffuses through the graphite and reacts with hydroxide to form water, the hydrogen is oxidised and provides electrons to the circuit, this sets up a potential difference across the cell
  • the oxygen is supplied to the positive electrode (cathode) the diffuses through the graphite and reacts with hydroxide and uses the electrons provided in the circuit and produces electricity
45
Q

what is the half equation at the anode in hydrogen fuel cells

A

2H2 —-> 4H+ + 4e-

46
Q

what is the half equation at the cathode in hydrogen fuel cells

A

O2 + 4H+ + 4e- =====> 2H20

47
Q

what is the overall reaction for hydrogen fuel cells

A

2H2 + O2 ======> 2H20

48
Q

is the anode positive or negative in hydrogen fuel cells

A

negative

49
Q

is the cathode positive or negative in hydrogen fuel cells

A

positive

50
Q

advantages of fuel cells

A
  • does not produce pollutants
  • last longer than batteries
  • less polluting to dispose
  • hydrogen and oxygen are renewable
51
Q

disadvantages of fuel cells

A
  • hydrogen is highly flammable
  • hydrogen is difficult to store
  • explosive when mixed with air
  • hydrogen can be produced by non renewable resources
52
Q

what does a fuel cell do

A

it converts energy between chemical and electrical forms and does this by reacting a fuel with oxygen to convert the chemical energy to electrical energy

chemistry (13 decks)

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