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Chemistry Paper 1. > Energy Changes. > Flashcards

Energy Changes. Flashcards

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1
Q

What do chemicals store?

A

Certain amount if energy, and different chemicals store different amounts.

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2
Q

If products of a reaction store more energy than the original reactants what does this show?

A

They have taken in the difference in energy between the products and reactants from the surroundings during the reaction.

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3
Q

What does it mean if the products of a reaction store less energy than the original reactants what does this show?

A

The excess energy was transferred to surroundings during the reaction.

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4
Q

Why doesn’t the overall amount of energy change ?

A

Energy is conserved in reactions it cannot be created or destroyed so the amount of energy in the universe always stays the same.

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5
Q

Define the term exothermic.

A

Transfers energy to the surroundings Usually by heating, this is shown by a rise in temperature.

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6
Q

Best example of an exothermic reaction?

A
  • Burning fuels also known as combustion.
  • Gives alot of energy so very exothermic.
  • Neutralisation reactions (acid+alkali)
  • Oxidation reaction. EX: adding sodium to water releases energy so it is exothermic. The reaction releases energy and the sodioum mves about the surface of water when oxidised.
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7
Q

Give some examples of exothermic every day uses.

A
  • hand warmers use exothermic oxidation of iron in air with a salt solution catalyst to release the energy.
  • self heating cans of hot chocolate and coffee also rely on exothemic reactions between chemicals and bases.
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8
Q

Define endothermic reaction.

A
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9
Q

Give some examples of an endothermic reaction.

A
  • Reaction between citric acid and sodium hydrogencarbonate.
  • Thermal decomposition, heating calcium carbonate causes it to decompose into calcium oxide and carbon dioxide. CaCO3(+heat)=Co2 + CaO
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10
Q

Give endothermic reactions for everyday uses.

A

-sports injusry packs, the chemical reaction allows the pack to become instantly cooler without having to tput it into the freezer.

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11
Q

Practical for testing the effect of acid concentration on energy released in an neutralisation reaction between hydrochloric acid and sodium hydroxide.

A

Put 25cm^3 of 0.25mol/dm^3 of hydrochloric acid and sodium hydroxide in separate beakers.
place beakers in a water bath of 25 degrees,until they are both at the same temperature of 25 degrees.
add the HCL followed by the NaOH to a polystyrene cup with a lid.
Take the temperature of the mixture every 30 seconds, the highest temperature.
Repeat steps 1-4 using 0.5 mol/dm^3 then 1 mol/dm^3 of hydrochloric acid.

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12
Q

Summary of how to measure the amount of energy being transferred?

A

Take the temperature of the reagents mixing them in a polystyrene cup and measuring the temperature of the reagents mixing them in a polystyrene cup and measuring the temperature of solution at the end of the reaction.

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13
Q

Problem with measuring energy transfer?

A

The amount of energy lost to its surroundings.

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14
Q

How to reduce the problem of energy transfer experiment?

A

Put the polystyrene cup in a beaker of cotton wool to give more insulation
Lid on cup to reduce energy lost by evaporation.

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15
Q

What does the experiment for energy transfer also be used for?

A
  • Neutralisation reactions or reactions between metals and acids or carbonates and acids.
  • -investigate what effect different variables have on amount of energy transferred eg: mass or concentration of reactants used.
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16
Q

What are reaction profiles?

A

Diagrams that show the relative energies of the reactants and products in an reaction and how the energy changes over the course of a reaction.

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17
Q

Describe an exothermic reaction profile.

A

Products are at a lower energy than the reactants.
The difference in height represents the overall energy change in the reaction per mole.
The initial rise in energy represents the energy needed to start the reaction this is called the activation energy.

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18
Q

Define the term activation energy.

A

The activation energy is the minimum amount of energy the reactants need to collide with each other and react.

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19
Q

What does it mean if a reaction needs a greater amount of energy?

A

The greater the activation energy the more energy is needed to start the reaction.
This has to be supplied by heating the reaction mixture.

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20
Q

Describe the reaction profile for an endothermic reaction.

A

Products are at a higher energy than the reactants.

Difference in height represents the overall energy change during the reaction (the energy taken in) per mole.

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21
Q

What happens during a chemical reaction?

A

Old bonds are broken and new bonds are formed.

22
Q

How to break existing bonds?

A

Energy must be supplied to break these existing bonds so bond breaking is exothermic.

23
Q

How to form a bond?

A

Energy is released when new bonds are formed so bond formation is exothermic

24
Q

What does every chemical bond have?

A

Particular bond energy associated with it this bond energy varies slightly depending on the compound the bond occurs in.

25
Q

How to find the overall energy change?

A

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

26
Q

Why can you not compare the overall energy changes of reactions?

A

You can’t unless you know the numerical differences on the bond energies.

27
Q

What is an electrochemical cell?

A

basic system made up of two different electrodes in contact with an electrolyte.

28
Q

The two electrodes must be able to conduct electricity so what should they be made from?

A

Metal

29
Q

Define electrolyte.

A

Liquid that contains ions which react with the electrodes.

30
Q

In an electrochemical cell what does it mean if the electrodes are connected by a wire?

A

Charge is able to flow and electricity is produced.

A voltmeter can also be connected to circuit to measure the voltage of the cell.

31
Q

What causes the charge difference?

A

Different metals react differently with the same electrolyte, so the type of electrode used will affect the voltage of the cell.

32
Q

What does the electrolyte used in a cell affect?

A

The size of voltage since different ions in solutions will react differently with the metal electrodes used.

33
Q

How is a battery formed?

A

Connecting two or more cells together in a series

The voltage of the cells in the battery are combined so there is a bigger overall voltage.

34
Q

What reactions happen in the electrodes?

A

They are irreversible.

35
Q

In an non rechargeable battery what happens to the reacting particles?

A

The ions in the electrolyte and the metal ions on the electrode get used up and turn into the products of the reaction.

36
Q

What happens in a non rechargeable battery if one reactant gets used up?

A

The reaction cannot happen so no electricity is produced.

The products cannot be turned back into reactants so the cell can’t be recharged.

37
Q

Define the term non rechargeable battery.

A

For example: Alkaline batteries contains cells which use irreversible reactions.
Once one of the reactants is used up they don’t produce anymore charge and you have to replace it.

38
Q

What happens to the reaction in a rechargeable cell?

A

The reaction can be reversed by connecting it to an external electric current.

39
Q

Define the term fuel cell.

A

Electric cell that’s supplied with a fuel and oxygen and uses energy from the reaction between the to produce electrical energy efficiently.

40
Q

What happens when fuel enters the fuel cell?

A

It becomes oxidised and sets up a potential difference within the cell.

41
Q

What are the different types of fuel cells?

A

They use different fuels and different electrolytes.

One important one is the hydrogen-oxygen fuel cell.

42
Q

What is they hydrogen fuel cell?

A

Combines hydrogen and oxygen to produce nice clean water and release energy.

43
Q

Describe how the hydrogen-oxygen fuel cell works?

A

Electrolyte often a solution of potassium hydroxide.Electrodes are often porous carbon with catalyst.
Hydrogen goes into anode compartment and oxygen goes into the cathode compartment.
At the positive electrode the anode, hydrogen loses electrons to produce h+ ions. This is oxidation.
H+ ions in the electrolyte move to the cathode +ve
At the positive electrode the cathode oxygen gains electrons from the cathode and reacts with H+ ions from electrolyte to make water.
This is reduction.
Electrons flow through an external circuit from anode to cathode this is the electric current.
Overall reaction is hydrogen plus oxygen which gives water.

44
Q

Give the half equation for oxidation at the anode.

A

H2 = 2H+ + 2e-

45
Q

Half equation for reduction at the cathode.

A

02 + 4H+ + 4e- = 2H20

46
Q

Give the overall reaction equation in a fuel cell.

A

2H2+02=2H20

47
Q

What are conventional fuels?

A

For vehicles like petrol have a finite supply.

Won’t last forever, very polluting.

48
Q

What type of vehicle is becoming more popular?

A

Vehicles that use electrical energy.

49
Q

Advantages of hydrogen oxygen fuel cells?

A

Fuel cell vehicles don’t produce many pollutants as other fuels. -no greenhouse gases, nitrogen oxides, sulfur dioxide, carbon monoxide.
Only by products are water and heat.
Electric vehicles don’t produce many pollutants but their batteries make pollutants when disposed of than fuel cells as they are made form highly toxic metal compounds.
Batteries in electric vehicles are rechargeable but there’s a limit to how many times it can recharge before replacing. Batteries are more expensive to make than fuel cells.
Batteries store less energy than fuels cells so would need to recharge more often which takes longer .

50
Q

Disadvantages of hydrogen oxygen fuel cell.

A

Hydrogen is a gas so takes up more space to store than a rechargeable battery.
Hydrogen is explosive when mixed with air so hard to store safely.
Hydrogen fuel is often made from hydrocarbons from fossil fuels or by electrolysis of water, which uses electricity and thus is also generated from fossil fuels.

Chemistry Paper 1. (5 decks)

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