energy changes Flashcards

1
Q

what is an exothermic reaction?

A

a reaction in which heat energy is given off- more energy is released when new bonds are formed than energy required to break the bonds in the reactants. has a negative enthalpy value.

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2
Q

what is an endothermic reaction?

A

a reaction in which heat energy is taken in from the surroundings- more energy is needed to break bonds than is released in making bonds. has a positive enthalpy value

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3
Q

enthalpy change (ΔH) = …?

A

energy taken in - energy given out

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4
Q

what are some uses of endo and exothermic reactions?

A

endothermic- sports injury packs
exothermic- self heating food, hand warmers

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5
Q

what are some examples of exothermic reactions?

A

combustion, neutralisation, oxidation, displacement

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6
Q

what are some examples of endothermic reactions?

A

therman decomposition, alkane cracking, sodium hydrogencarbonate + citric acid

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7
Q

what is the link between reactivity and energy changes?

A

the bigger the difference in reactivity in a displacement reaction, the more exothermic the reaction

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8
Q

what is activation energy?

A

the minimum amount of energy required to start a chemical reaction.

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9
Q

describe an energy profile diagram for an exothermic reaction

A

reactants are at a higher energy level than products

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10
Q

describe an energy profile diagram for an endothermic reaction

A

products are at a higher energy level than reactants.

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11
Q

what is the definition of bond energy?

A

the average amount of energy that will break/make a bond between two atoms

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12
Q

what is a catalyst + how do they work?

A

a substance that speeds up the rate of a chemical reaction without being used up- provides an alternate reaction pathway with lower activation energy

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13
Q

what is an electrochemical cell?

A

a device containing chemicals that react to produce electricity- 2 or more cells together in series form a battery

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14
Q

what factors determine a cell’s voltage?

A

type of electrode, concentration of electrolyte, type of electrolyte

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15
Q

how are some cells rechargeable?

A

the chemical reaction is reversed when a current is applied

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16
Q

what is a fuel cell?

A

an electrochemical cell that can be refilled with fresh chemicals

17
Q

how do hydrogen fuel cells generate electricity?

A

hydrogen and oxygen react and produce electricity, water and heat

18
Q

what are some advantages and disadvantages of hydrogen fuel cells?

A

advantages: only product is water, can be refilled quickly
disadvantages: hydrogen obtained using fossil fuels (a finite resource), it is flammable and difficult to store

19
Q

what is the half equation at the negative terminal?

A

2H2 + 4OH- -> 4H2O + 4e-

20
Q

what is the half equation at the positive terminal?

A

O2 + 2H2O + 4e- -> 4OH-