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Chemistry > Energy changes > Flashcards

Energy changes Flashcards

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1
Q

What is an exothermic reaction?

A

A reaction that transfers energy to the
surroundings (meaning the temperature of the surroundings increases).

-usually by heating-

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2
Q

Name an exothermic reaction:

A

-Combustion (the reaction of a fuel with oxygen)

-Neutralisation reactions

-Many oxidation reactions

-(Respiration)

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3
Q

Everyday uses of exothermic reactions:

A

-Hand warmers (use the energy released by iron oxidation or from the crystallisation of salt solution catalysts)

-Self-heating cans (another reaction between chemicals and their bases)

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4
Q

What is an endothermic reaction?

A

A reaction that takes in energy from the surroundings (so the temperature of the surroundings decreases).

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5
Q

Name an endothermic reaction:

A

-Thermal decomposition

-The reaction of citric acid and sodium hydrogencarbonate (to form sodium bicarbonate)

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6
Q

Everyday uses of endothermic reactions:

A

-Sports injury packs (ice packs)

-Where the fast chemical reaction between ammonium nitrate and water allows the pack to become instantly cooler without having to put it in the freezer

-When forcefully squeezed

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7
Q

What happens to energy in chemical reactions?

A

It is conserved.

This is because it can’t be created or destroyed, only moved around (transferred).

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8
Q

If a reaction transfers energy to the
surroundings, the product molecules…

A

…must have less energy than the
reactants, by the amount transferred.

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9
Q

If the products of a reaction store more energy than than the reactants, then…

A

…they must have taken in the difference in energy between the products and reactants from the surroundings during the reaction.

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10
Q

When can chemical reactions occur?

A

Only when reacting particles collide
with each other and with sufficient energy.

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11
Q

What is activation energy?

A

The minimum amount of energy that particles must have to react, so therefore the energy needed for a reaction to occur.

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12
Q

What are reaction profiles used to show?

A

The relative energies of reactants and products, the activation energy, and the overall energy change of a reaction.

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13
Q

What are reaction profiles also known as?

A

Energy level diagrams

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14
Q

On a reaction profile what do we draw a curved line to represent?

A

The energy as the reaction proceeds.

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15
Q

For the reaction profile for an exothermic reaction, should the ‘reactants’ or the ‘products’ line be higher up?

A

The reactants because they have more energy

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16
Q

For the reaction profile for an endothermic reaction, should the ‘reactants’ or the ‘products’ line be higher up?

A

The products because they have more energy.

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17
Q

What are bond energies?

A

The amount of energy required to break one mole of a particular covalent bond.

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18
Q

What three things happen in a chemical reaction?

A

1-The bonds of the reactant molecules are broken

2-The atoms rearrange themselves

3-The bonds of the product molecules are formed

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19
Q

Is breaking bonds an endothermic or exothermic process?

A

Endothermic (bendo)

20
Q

Is making bonds an endothermic or exothermic process?

A

Exothermic (mexo)

21
Q

Why is breaking bonds an endothermic process?

A

It requires energy which is gained from the surroundings.

22
Q

Why is making bonds an exothermic process?

A

It releases energy to the surroundings.

23
Q

Are bond energy values negative or positive?

A

Always positive

24
Q

△H

A

Change in energy

25
Q

The bond energy of a H-Cl bond is 431kJ. What does this mean in terms of energy?

A

-Forming one mole of H-Cl bonds releases 431kJ of energy

-Breaking one mole of H-Cl bonds requires 431 kJ of energy

26
Q

How do we calculate the overall energy change of a reaction?

A

(sum of) energy required to break the bonds of a reactants - (sum of) energy released by forming bonds of products

or

energy of reactants - energy of products

27
Q

Does -△H show an exothermic or endothermic reaction?

A

Exothermic reaction as the energy released from forming new bonds is greater than the energy needed to break existing bonds.

28
Q

Does +△H show an exothermic or endothermic reaction?

A

Endothermic reaction as the energy needed to break existing bonds is greater than the energy released from forming new bonds.

29
Q

What is thermal decomposition?

A

Heating a substance causing it to decompose .

For example, calcium carbonate decomposes into calcium oxide (quicklime) and carbon dioxide.

CaCO₃→CO₂+CaO

30
Q

Why must energy be supplied in a chemical reaction?

A

To break bonds in the reactants.

31
Q

When is energy released in chemical reactions?

A

When bonds in the products are formed.

32
Q

What are bond energies used to calculate?

A

The energy needed to break bonds and the energy released when
bonds are formed.

33
Q

You can’t compare the overall energy changes of reactions unless you know what?

A

The numerical differences in bond energies.

Else there could be a situation in which less energy is needed to break certain bonds because they are weaker, but less energy is released forming new bonds because they’re also weaker so we wouldn’t know.

34
Q

What do we label the x-axis on reaction profiles?

A

Progress of reaction

35
Q

What do we label the y-axis on reaction profiles?

A

Energy

36
Q

What must we do when drawing reaction profiles?

A

-Make sure the arrows touch the top and bottom of each line

-Draw a dotted line out from the reactants line

37
Q

What must we label in reaction profiles?

A

The axis, the products and reactants (name them!), the activation energy, the overall energy change.

38
Q

What does the difference in height between the products and reactants on a reaction profile represent?

A

The overall energy change during the reaction per mole.

(the energy taken in or out per mole)

39
Q

Where is energy transferred in an endothermic reaction?

A

To reacting chemicals, causing the temperature of the surroundings to decrease.

40
Q

Where does the energy transferred to the surroundings come from? (exothermic reaction)

A

The reacting chemicals.

41
Q

What does boiling a reusable hand warmer do?

A

Re-dissolves the crystals, so that the hand warmer is ready to be activated once more.

42
Q

A molecule of ethane and a molecule of iodine react. What needs to happen for the reaction to occur?

A

-The ethane and iodine molecules must collide.
-With more energy than the activation energy.

43
Q

Why is the line for the products higher than the line for the reactants in an endothermic reaction?

A

In endothermic reactions, energy from the surroundings is transferred to the reacting chemicals, so the energy of the products will be higher than the reactants.

44
Q

How can catalysts increase the rate of reactions?

A

By lowering the activation energy, increasing the likelihood of successful collisions.

45
Q

What is the unit most commonly used for measuring bond energies?

A

kJ/mol

Chemistry (11 decks)

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