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GCSE Chemistry > Topic 5 - Energy Changes > Flashcards

Topic 5 - Energy Changes Flashcards

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1
Q

What can changes in heat be determined and measured by?

A

The changes in heat content can be determined and measured with a thermometer.

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2
Q

What is the law of conservation of energy? (3)

A
  • The overall amount of energy does not change as energy is conserved in reactions.
  • This means that it cannot be created or destroyed but it can be transferred.
  • So, if energy is transferred to the surroundings during a chemical reaction, then the products formed must have less energy than the reactants by the same amount as that transferred.
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3
Q

What is an exothermic reaction?

A

An exothermic reaction releases heat energy. This means that the temperature increases.

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4
Q

What is an exothermic reaction in summary?

A

In exothermic reactions, the temperature of the surroundings increases and the heat content of the system falls.

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5
Q

What are 2 examples of exothermic reaction?

A
  • Neutralisation
  • Combustion
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6
Q

What is an endothermic reaction?

A

An endothermic reaction takes heat energy in. This means that the temperature decreases.

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7
Q

What is an endothermic reaction in summary?

A

In endothermic reactions, the temperature of the surroundings falls and the heat content of the system increases.

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8
Q

What are some typical endothermic reactions?

A
  • Electrolysis
  • Thermal decomposition reactions
  • The first stages of photosynthesis
  • The reaction between citric acid and sodium hydrogencarbonate.
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9
Q

How do sports injury treatments use endothermic reactions?

A

Sports injury treatment often use cold packs based on endothermic reactions to take heat away from a recently injured area to prevent swelling.

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10
Q

What happens to energy in displacement reactions?

A

These can either take energy in or give it out.

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11
Q

What materials are needed for Required practical 4: Investigating temperature changes? (5)

A
  • Dilute hydrochloric acid
  • Dilute sodium hydroxide solution
  • Styrofoam (polystyrene) calorimeter & lid
  • 25 cm3 measuring cylinder
  • Thermometer & stirrer
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12
Q

What is the 6 step method for Required practical 4: Investigating temperature changes?

A
  1. Using a measuring cylinder, place 25 cm3 of the NaOH solution into the calorimeter.
  2. Measure and record the temperature of the solution.
  3. Add 5 cm3 of the dilute HCl and stir.
  4. Measure and record the highest temperature reached by the mixture.
  5. Repeat steps 1 – 4 increasing the amount of acid added by 5 cm3 each time.
  6. Record results in a suitable table => one column for volume of acid, one for temperature.
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13
Q

What conclusion can be made from Required practical 4: Investigating temperature changes?

A

The larger the difference in the temperature the more energy is absorbed or released.

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14
Q

What must happen for atoms/ particles to react?

A

For atoms or particles to react with each other in a chemical system they must first come into contact with each other in a collision.

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15
Q

What 3 factors come into play when analysing collisions?

A
  • Energy
  • Orientation
  • Number of collisions per second.
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16
Q

What is activation energy?

A
  • In terms of the energy of the collision, there is a minimum amount of energy required for the collision to be successful (for the particles to react together).
  • This minimum amount of energy is called the activation energy.
  • It is given the symbol E(a) => the a is in subscript.
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17
Q

What does the activation energy tell us (high or low)?

A

Reactions which have higher activation energies require more energy to start than those with lower activation energies.

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18
Q

Why do all reactions have an activation energy?

A

All reactions have an activation energy as the chemical bonds in the reactant molecules have to be broken first.

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19
Q

What is an energy profile?

A

Energy profiles are graphical representations of the relative energies of the reactants and products in chemical reactions.

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20
Q

How are energy profiles set out?

A

The energy of the reactants and products are displayed on the y-axis and the progress of the reaction is shown on the x-axis.

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21
Q

What does the difference in height between the energy of reactants and products represent?

A

The difference in height between the energy of reactants and products represents the overall energy change of a reaction.

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22
Q

What does an energy profile look like for exothermic reactions? (2)

A
  • The energy of the products will be lower than the energy of the reactants, so the change in energy is negative.
  • This is represented on the energy profile with a downwards arrow as the energy of the products is lower than the reactants.
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23
Q

What does an energy profile look like for endothermic reactions? (2)

A
  • The energy of the products will be higher than the energy of the reactants, so the change in energy is positive.
  • This is represented on the energy profile with an upwards arrow as the energy of the products is higher than the reactants.
24
Q

What is a reaction being endothermic vs exothermic dependent on?

A

Whether a reaction is endothermic or exothermic depends on the difference between the energy needed to break existing bonds and the energy released when the new bonds are formed.

25
Q

What kind of reaction is bond breaking? Why?

A

Bond breaking is always an endothermic process as energy needs to be taken in from the surroundings to break the chemical bonds.

26
Q

What kind of reaction is bond making? Why?

A

Bond making is always an exothermic process as energy is transferred to the surroundings as the new bond is formed.

27
Q

What kind of change in energy does an endothermic reaction have? Why? (4)

A
  • If more energy is absorbed than is released, this reaction is endothermic.
  • More energy is required to break the bonds than is released from making the new bonds.
  • The change in energy is positive since the products have more energy than the reactants.
  • Therefore an endothermic reaction has a positive change in energy.
28
Q

What kind of change in energy does an exothermic reaction have? Why? (4)

A
  • If more energy is released than is absorbed, then the reaction is exothermic.
  • More energy is released when new bonds are formed than energy required to break the bonds in the reactants.
  • The change in energy is negative since the products have less energy than the reactants.
  • Therefore an exothermic reaction has a negative change in energy.
29
Q

What is a way to remember bond breaking vs bond making and the different types of reactions?

A

Remember bond breaking is ENDothermic and results in the END of the bond.

30
Q

What does each chemical bond have?

A

Each chemical bond has a specific bond energy associated with it.

31
Q

What is bond energy?

A

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

32
Q

What can bond energy be used to calculate? What must you know in order to do this?

A

This energy can be used to calculate how much heat would be released or absorbed in a reaction. To do this it is necessary to know the bonds present in both the reactants and products.

33
Q

What must you know to calculate the total change in energy for a reaction?

A

We can calculate the total change in energy for a reaction if we know the bond energies of all the species involved.

34
Q

How do you calculate the total change in energy for a reaction? (3)

A
  • Add together all the bond energies for all the bonds in the reactants – this is the ‘energy in’.
  • Add together the bond energies for all the bonds in the products – this is the ‘energy out’.
  • Calculate the energy change using the equation:

Energy change = Energy taken in - Energy given out

35
Q

What are some key things to remember when doing bond energy questions? (3)

A
  • It is helpful to write down a displayed formula equation for the reaction before identifying the type and number of bonds, to avoid making mistakes.
  • Don’t forget to take into account the balancing numbers when working out how many of each type of bond is being broken/ formed.
36
Q

What are chemical cells?

A

Chemical cells or simple cells are a source of electrical energy.

37
Q

What do simple chemical cells look like?

A

The simplest design consists of two electrodes made from metals of different reactivity immersed in an electrolyte and connected to an external voltmeter by wire, creating a complete circuit.

38
Q

What is a common example of metals used in simple chemical cells?

A

A common example is zinc and copper.

39
Q

How does a chemical cell work using the example of zinc and copper? (6)

A
  • Zinc is the more reactive metal and forms ions more easily, readily releasing electrons.
  • The electrons give the more reactive electrode a negative charge and sets up a charge difference between the electrodes.
  • The electrons then flow around the circuit to the copper electrode which is now the more positive electrode.
  • The difference in the ability of the electrodes to release electrons causes a voltage to be produced.
  • The greater the difference in the metals reactivity then the greater the voltage produced.
  • The electrolyte used also affects the voltage as different ions react with the electrodes in different ways.
40
Q

What are batteries?

A

Batteries consist of two or more cells connected together in series to provide a greater voltage.

41
Q

What happens in non-rechargeable cells and batteries?

A

In non-rechargeable cells and batteries the chemical reactions stop when one of the reactants has been used up.

42
Q

What is an example of a non-rechargeable cell/ battery?

A

Alkaline batteries.

43
Q

Why can rechargeable cells and batteries be recharged?

A

Rechargeable cells and batteries can be recharged because the chemical reactions are reversed when an external electrical current is supplied.

44
Q

What is a fuel cell?

A

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

45
Q

Why/where are fuel cells popular?

A

These cells are becoming more common in the automotive industry to replace petrol or diesel engines.

46
Q

How does the fuel cell work? (2)

A
  • As the fuel enters the cell it becomes oxidised which sets up a potential difference or voltage within the cell.
  • Different electrolytes and fuels can be used to set up different types of fuel cells.
47
Q

What is an important fuel cell?

A

An important cell is the hydrogen-oxygen fuel cell which involves the oxidation of
hydrogen to produce water.

48
Q

What is important to remember about fuel cells?

A

In fuel cells the polarity on the electrodes are the other way around: the anode is the negative electrode and the cathode is the positive electrode.

49
Q

What do hydrogen fuel cells offer a potential alternative to?

A

Hydrogen fuel cells offer a potential alternative to rechargeable cells and batteries.

50
Q

What are the advantages of hydrogen fuel cells? (5)

A
  • They do not produce any pollution.
  • They produce more energy per kilogram than either petrol or diesel.
  • No power is lost in transmission as there are no moving parts, unlike an internal combustion engine.
  • No batteries to dispose of which is better for the environment.
  • Continuous process and will keep producing energy as long as fuel is supplied.
51
Q

What are the disadvantages of hydrogen fuel cells? (4)

A
  • Materials used in producing fuel cells are expensive.
  • High pressure tanks are needed to store the oxygen and hydrogen in sufficient amounts which are dangerous and difficult to handle.
  • Fuel cells are affected by low temperatures, becoming less efficient.
  • Hydrogen is expensive to produce and store.
52
Q

Where do hydrogen and oxygen enter the fuel cell?

A

Hydrogen enters at the anode where it is oxidised and oxygen enters at the cathode where it is reduced.

53
Q

What reaction occurs at the anode? (half equation)

A

2H(2) → 4H+ + 4e–

54
Q

What reaction occurs at the cathode? (half equation)

A

4H+ + O(2) + 4e– → 2H(2)O

55
Q

What is the overall, full ionic reaction/ equation for the fuel cell?

A

2H(2) + O(2)→ 2H(2)O

56
Q

How do electrons move within the fuel cell? How is this used?

A
  • The electrons move around the external circuit from the anode to the cathode.
  • This movement of electrons is used to drive an electric motor.

GCSE Chemistry (9 decks)

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