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GCSE AQA Chemistry 9-1 > C7 Energy Changes > Flashcards

C7 Energy Changes Flashcards

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1
Q

what is an exothermic reaction?

A

a reaction that transfers energy from the reacting molecules to the surroundings
this means the temperature of the surroundings increases

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2
Q

what are some examples of exothermic reactions?

A

combustion
certain oxidation reactions
neutralisation

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3
Q

what is an energy profile diagram?

A

a way to represent the energy changes that take place in reactions

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4
Q

where do you find exothermic reactions?

A

hand warmers
self heating cans

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5
Q

what is the key fact for an energy profile diagram of an exothermic reaction?

A

the products have less energy than the reactants
the difference in energy between the products and the reactions is the amount of energy transferred to the surroundings

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6
Q

what is an endothermic reaction?

A

a reaction that takes energy from the surroundings
this means the temperature of the surroundings decreases

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7
Q

what is an example of an endothermic reaction?

A

thermal decomposition

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8
Q

what is the key fact about the energy profile diagram for an endothermic reaction?

A

the products have more energy than the reactants as it has taken in energy from the surroundings

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9
Q

what is the peak on an energy profile diagram? (for bot exothermic and endothermic reactions)

A

the activation energy

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10
Q

what is the activation energy?

A

the minimum amount of energy the particles must have in order to react

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11
Q

how do you represent the activation energy on an energy profile diagram?

A

it is the energy from the reactants to the peak of the curve

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12
Q

are an exothermic reaction’s energy changes positive or negative?

A

negative - energy has been transferred from the chemicals

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13
Q

are an endothermic reaction’s energy changed positive of negative?

A

positive - energy has been gained

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14
Q

what type of reaction is breaking a bond?

A

endothermic - it requires energy

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15
Q

what type of reaction is making a bond?

A

exothermic - it releases energy

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16
Q

what is the energy value of a chemical bond?

A

the energy required to break the bond

17
Q

what is required to calculate the energy change of a reaction?

A

the structures of the molecules involved
the energy value for each bond

18
Q

how do you calculate the energy change for a reaction?

A

1) put the energy value for each bond under the structures of the molecules on the left
2) if the bonds are being broken, put a + in front. then add them together
3) put the energy value for the bond on the right
4) if the bonds are being made, put a - in front
5) put the right value under the left value and carry out the sum

19
Q

how do you investigate the temperature change in the neutralisation reaction of hydrochloric acid and sodium hydroxide?

A

add increasing volumes of sodium hydroxide solution to hydrochloric acid and measure the maximum temperature reached

20
Q

what are the variables of the investigation of the neutralisation reaction of hydrochloric acid and sodium hydroxide solution?

A

independent - volume of sodium hydroxide solution added
dependent - maximum temperature reached
control - volume of HCl, concentrations of HCl and sodium hydroxide solution

21
Q

what is the trend for the neutralisation reaction of HCl and sodium hydroxide?

A

as the volume of sodium hydroxide increases, the maximum temperature reached increases
this is because as more particles of sodium hydroxide are added, they react with the HCl
at a certain point, the maximum temperature begins to decrease.
this is because there is not enough HCl for the amount of sodium hydroxide being added so some is left unreacted. the maximum temperature has reached a maximum

22
Q

why is a polystyrene cup and plastic lid used in the investigation of the neutralisation of HCl and sodium hydroxide?

A

the experiment is measuring temperature so the amount of heat loss needs to be minimised.
polystyrene is a good insulator and prevents heat lost from the sides and bottom
the lid reduces heat lost from the air

23
Q

what happens if two different metals are put into an electrolyte?

A

they can produce electricity

24
Q

what are two key facts about cells?

A

they can only produce electricity for a certain period of time. the reaction will eventually stop when the chemicals run out
cells only produce electricity if metals with different reactivities are used

25
Q

What affects the potential difference of a cell?

A

the greater the difference between the reactivity of the metals, the greater the potential difference produced by the cell
the electrolyte used also affects the p.d

26
Q

what is a battery?

A

two or more cells connected in series to produce a greater p.d.

27
Q

can alkaline batteries be recharged? why? why not?

A

no
at some point the reactants will run out and no electricity is produced
there is no way to reverse the reaction so it is non-rechargeable

28
Q

why can rechargeable batteries be recharged?

A

the chemical reaction can be reversed when an electrical current is applied

29
Q

what happens inside a fuel cell?

A

a fuel such as hydrogen is reacted with oxygen to produce electricity and water

30
Q

what is the half equation for the reaction at the negative electrode in a fuel cell?

A

2H2 –> 4H+ + 4e-
or
2H2 + 4OH- –> 4H2O + 4e-

31
Q

what is the half equation for the reaction at the positive electrode in a fuel cell?

A

O2 + 4H+ + 4e- –> 2H2O
or
O2 + 2H2O + 4e- –> 4OH-

32
Q

what is the overall equation for the reaction in a fuel cell?

A

O2 + 2H2 –> 2H2O

33
Q

what are the advantages of hydrogen fuel cells over rechargeable batteries?

A

will provide energy as long as you provide hydrogen
do not get less efficient the longer they run
can be a source of drinking water

34
Q

what are the disadvantages of hydrogen fuel cells?

A

hydrogen gas is very explosive and difficult to store safely
produce a relatively low potential difference so several are needed together

GCSE AQA Chemistry 9-1 (16 decks)

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