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Chemistry > C5- Energy Changes > Flashcards

C5- Energy Changes Flashcards

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1
Q

What is an exothermic reaction?

A

A reaction where energy is transferred to the

surroundings.

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2
Q

Give examples of exothermic reactions.

A

Combustion, respiration and many neutralisation reactions

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3
Q

What happens to the temperature of the

surroundings during an exothermic reaction?

A

They increase

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4
Q

What is an endothermic reaction?

A

A reaction where energy is transferred from the

surroundings.

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5
Q

Give examples of endothermic reactions.

A

Thermal decomposition reactions, the reaction between

citric acid and sodium hydrogencarbonate and respiration

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6
Q

What happens to the temperature of the

surroundings during an endothermic reaction?

A

They decrease

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7
Q

State two uses of exothermic reactions

A

Self-heating cans, hand warmers

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8
Q

State two uses of exothermic reactions

A

Some cooling sports injury packs

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9
Q

What is a reaction profile?

A

A diagram which shows whether the reactants have more
or less energy than the products and the overall energy
change of a reaction.

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10
Q

What is the activation energy?

A

The energy needed to break bonds to start a reaction

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11
Q

Why does the reaction profile always go up at the start?

A

A reaction starts with bonds being broken

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12
Q

Why does the reaction profile always go down at the end?

A

A reaction always ends with bonds being made

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13
Q

If the reactants have more energy than the products,

what kind of a reaction must have taken place?

A

An exothermic one. Energy has been transferred to the surroundings.

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14
Q

If the reactants have less energy than the products,

what kind of a reaction must have taken place?

A

An endothermic one. Energy has been taken in from the surroundings.

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15
Q

Is breaking bonds endothermic or exothermic?

A

Exothermic. Energy is released when chemical bonds are formed (like how two
magnets move together when close and generate kinetic energy)

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16
Q

How do we work out the overall energy change of a

reaction?

A

Work out the difference between the energy needed to break all the bonds in
the reactants and the energy released to form all the bonds in the products.

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17
Q

How do we work out the energy change from a reaction profile?

A

The difference between the energy of the reactants and the energy of the
products

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18
Q

How do we work out the activation energy from a reaction

profile?

A

The difference between the energy of the reactants and the highest point on
the profile

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19
Q

What is meant by bond energy?

A

The amount of energy required to break a particular type of bond.

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20
Q

What is a chemical cell?

A

A unit which contains chemicals which produce electricity

through a reaction

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21
Q

What are the main components of a chemical cell?

A

An anode, a cathode (made of 2 different metals) and an

electrolyte

22
Q

What is an electrolyte?

A

A solution containing ions

23
Q

What does the voltage of a cell depend on?

A

The type of electrode and the electrolyte.

24
Q

What is a battery?

A

Two or more cells connected in series

25
Q

What happens to the electrodes in a chemical cell?

A

The more reactive metal is used up and the less reactive

one increases in size.

26
Q

In non-rechargeable cells, why do the chemical

reactions stop over time?

A

One of the reactants becomes used up.

27
Q

How can certain cells be recharged?

A

Applying an external electric current

28
Q

How does the reactivity of the metal electrodes

affect the size of the potential difference?

A

The greater the difference in reactivity, the greater the

voltage

29
Q

Which way do electrons move in a cell?

A

From the more reactive metal to the less reactive metal

30
Q

What is a fuel cell?

A

A cell which uses a fuel and oxygen (or

air) to generate electricity.

31
Q

What are the products in a hydrogen

fuel cell?

A

Water

32
Q

State advantages of hydrogen fuel

cells.

A

Do not need to be recharged, no

pollutants are produced

33
Q

State three disadvantages of

hydrogen fuel cells.

A

Hydrogen is highly flammable,
hydrogen is often made from fossil
fuels, hydrogen is difficult to store

34
Q

What reaction occurs at the negative

electrode of a hydrogen fuel cell?

A

H2 → 2H + 2e⁻

35
Q

Which reaction occurs at the positive

electrode of a hydrogen fuel cell?

A

O2 + 4H+ + 4e- → 2H2O

36
Q

What must be supplied for a bond to break

A

Energy

37
Q

What is the conservation of energy principle?

A

Energy is conserved in chemical reactions. The amount of energy in the universe
at the end of a chemical reaction is the same as before the reaction takes place

38
Q

What is an exothermic reaction? Give examples

A

A reaction where energy is transferred to the surroundings so that the
surroundings temperature increases – combustion, oxidation reactions and
neutralisation (acid + alkali) reactions. Negative sign of energy change.

39
Q

What is an endothermic reaction? Give examples

A

A reaction where energy is taken in from the surroundings so the surroundings
temperature decreases – thermal decomposition, reaction of citric acid and
sodium hydrogencarbonate. Negative sign of energy change.

40
Q

What is activation energy?

A

Minimum amount of energy that particles need to react

41
Q

What is a reaction profile?

A

Reaction profile is a graph which shows the relative energies of reactants and
product, as well as activation energy of the reaction.

42
Q

What occurs in a chemical reaction in terms of
bond energies? Describe exothermic and
endothermic reactions in terms of bond
breaking/forming.

A

Energy is supplied to break bonds and energy is released when bonds are made;
exothermic – energy released from forming bonds is greater than that needed to
break the bonds; endothermic – energy needed to break bonds is greater than
energy released making them

43
Q

What is the equation to find enthalpy change in terms of bond energies?

A

Energy of reaction = sum of bonds broken – sum of bonds made

44
Q

What is a cell?

A

A cell is composed of two electrodes dipped in an electrolyte solution. It produces
electricity from a chemical reaction.

45
Q

What is a battery

A

A battery consists of two or more cells connected in series.

46
Q

What determines the voltage obtained from a

cell?

A

Identities of metals used as electrodes and the identity and concentration of an
electrolyte.

47
Q

State the advantages and disadvantages of

using cells and batteries

A

(+) more or less cheap, some are rechargeable, a convenient source of electrical
energy
(-) harmful chemicals

48
Q

Describe rechargeable and non-rechargeable cells

A

Rechargeable – chemical reactions are reversed when an external current is
supplied
Non-rechargeable – reactants are used up, cannot be recharged

49
Q

What is a fuel cell? What is the
overall reaction in a hydrogen
fuel cell? What are the half
equations?

A

Fuel cells are supplied by fuel and oxygen to oxidise the fuel to generate
electricity.

50
Q

What is the overall reaction in a hydrogen fuel cell? What are
the half equations?

A

Cathode: 2 H2 → 4 H+ + 4 e−
Anode: O2 + 4 H+ + 4 e−→ 2 H2O
Overall: → 2 H2 + O2 → 2 H2O

51
Q

What are the advantages and disadvantages of hydrogen fuel

cells?

A 
Advantages: no pollutants, no recharging
 Disadvantages: flammable, H2
 difficult to store, fossil fuel production, toxic
chemicals, expensive production of H2
 by electrolysis

Chemistry (10 decks)

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