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GCSE Chemistry > Energy changes > Flashcards

Energy changes Flashcards

1
Q

2What is meant by the law of conservation of energy?

A

Energy is conserved in chemical reactions and cannot be created or destroyed. The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place.

This explains why if a reaction transfers energy to the surroundings the product molecules must have less energy than the reactants, by the amount transferred.

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2
Q

What is meant by an exothermic reaction?

A

A reaction that transfers energy to the surroundings so the temperature of the surroundings increases.

EXOTHERMIC
- temp of surroundings increases
- temp of reaction increases

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3
Q

What is meant by an endothermic reaction?

A

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

ENDOTHERMIC
- temp of surrounding decreases
- temp of reaction decreases

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4
Q

Examples of exothermic reactions

A
  • Combustion reactions
  • Neutralisation reactions
  • Oxidation reactions
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5
Q

Examples of endothermic reactions

A
  • Reaction of citric acid and sodium -hydrogen carbonate
  • Thermal decomposition
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6
Q

How are endothermic reactions used in every day life?

A
  • Sports injury packs are based on endothermic reactions
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7
Q

How are exothermic reactions used in every day life?

A
  • Hand warmers
  • Self-heating cans for food and drink
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8
Q

Draw the reaction profile for an exothermic reaction.

A
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9
Q

Draw the reaction profile for an endothermic reaction and an exothermic reaction.

A
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10
Q

Why do the products have lessenergy than the reactants on the reactant profile of an exothermic reaction?

A
  • Energy has been transferred from the reactants to the surroundings.
  • So products will have less energy than reactants by that amount
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11
Q

The difference in energy between the reactants and the products is:

A

The amount of energy taken in/transferred to surroundings.

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12
Q

Why do the products have more energy than the reactants on the reactant profile of an endothermic reaction?

A
  • Energy has been transferred from the surroundings to the products
  • So reactants will have less energy than products by that amount.
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13
Q

Where is the activation energy on a reaction profile

A

The peak/max point

  • Measure from reactant to peak
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14
Q

What is meant the activation energy?

A

The minimum energy particles must have to react.

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15
Q

Explain the concept of energy in reactions.

A

For particles to react energy is required so they can collide.

The minimum amount of energy particles must have to react = activation energy.

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16
Q

Making bonds?

A
  • Exothermic

Releases energy, transferred to surroundings

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17
Q

Breaking bonds?

A
  • Endothermic

Energy is required, taken in.

remember: BREAKing bonds ENDothermic

Break up = End

18
Q

What does the energy value of a chemical bond tell us?

A

The energy required to break that bond.

19
Q

True or false?

During a chemical reaction:
* energy must be supplied to break bonds in the reactants
* energy is released when bonds in the products are formed.

A

True

20
Q

REMEMBER:

A

In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds (which is why -).
In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds (which is why +)

21
Q

Describe a practical to investigate the variables that affect temperature changes in a chemical reaction.

A
  • AIM; investigate temp change in a neutralisation reaction of HCL(hydrochloric acid) and sodium hydroxide.

In this experiment we add increasing volumes of sodium hydroxide to HCl, and each time we measure the max temp reached.

IDV - Sodium hydroxide
DV - maximum temperature reached
CV - volume of hydrochloric acid, concentrations of both the hydrochloric acid and sodium hydroxide solution.

  • Use a measuring cylinder to measure 30cm^3 of dilute hydrochloric acid.
  • Transfer this into a polystyrene cup and stand the cup inside a beaker (the beaker stops it from falling over).
  • Use a thermometer to measure the temp of the acid and record in a table
  • Use a measuring cylinder to measure 5cm^3 of sodium hydroxide solution
  • Transfer this to to the polystyrene cup
  • Immediately place a lid with a hole over the cup and put thermometer thru the hole in the lid
  • Ensure the bulb of the thermometer is in the solution
  • Use thermometer to gently stir solution
  • Bc reaction is exothermic energy is released so temp of solution should increase
  • Observe temp rise on thermometer, when reading stops increasing measure the highest temperature reached.
  • Rinse out and dry the polystyrene cup and repeat experiment using 10cm^3 sodium hydroxide solution.
  • Continue to repeat this experiment several more times and each time increase the volume of sodium hydroxide solution by 5 cm^3, until we reach a maximum of 40 cm^3
  • Repeat all of this 1 more time, and calculate a mean for each volume of sodium hydroxide solution.
  • At the end plot a graph of your results after recording results in a table.

OBSERVATIONS:
- As volume of sodium hydroxide solution is increased, max temp reached increases
- This is because there are more reactant particles of sodium hydroxide to react with hydrochloric acid
- Because this is an exothermic reaction more energy is released
- So max temp reached increases.

  • At a point max temp begins to decrease
  • So much NaCl added, not enough HCl
  • Greater volume of solution
  • Energy released in now more spread out within the greater vol of solution
  • So less energy transferred to surroundings.
22
Q

Why do we use a polystyrene cup with a lid during RP4?

A
  • to reduce heat loss to the sides and the bottom bc polystyrene is a good thermal insulator
  • lid reduces heat loss to the air
23
Q

At a point, when we continue to add NaCl to HCl max temp transferred to surroundings begins to decrease, as shown on graph. suggest why.

A
  • So much NaCl added, not enough HCl (1)
  • Greater volume of solution (1)
  • Energy released in now more spread out within the greater vol of solution (1)
  • So less energy transferred to surroundings (1) - which is why max temp transferred to surroundings falls.
24
Q

Give an example of an electrolyte

A

A solution of an ionic compound

An electrolyte is a liquid that can conduct electricity.

25
Q

How can electricity be produced using an electrolyte?

A
  • Place two different metals in an electrolyte.
  • Connect both metals with a voltmeter.
  • A potential difference will be produced because an electric current will flow through the wire.
  • This is because chemical reactions will occur on the surface of the metals.
  • So electricity will be produced.

^^SET UP OF A CELL

26
Q

Draw a diagram of a cell

A
27
Q

Why can a cell only produce electricity for a limited amount of time?

A

Chemicals in cell will eventually run out.

28
Q

How can a simple cell be made?

A

By connecting two different metals in contact with an electrolyte with a voltmeter.

29
Q

True or false, cells can only be produced if we use metals with different reactivities?

A

True - the size of the potential difference produced depends on the difference in reactivity (ie big diff or small diff).

The larger the difference in reactivity, the greater the pd so the greater the current produced.

30
Q

What does the voltage produced by a cell depend on?

A

Type of electrode (and the diff in their reactivity).
Type of electrolyte.

31
Q

What is meant by a battery?

A

A battery is two or more cells connected in series.

Batteries produce greater voltages than cells.

32
Q

What is meant by a non-rechargeable battery?

A

A battery where chemical reactions stop when one of the reactants has been used up resulting in no more electricity being produced.

The chemical reactions in non-rechargeable batteries cannot be reversed and so cannot be recharged.

33
Q

What is meant by a rechargeable battery?

A

Batteries that can be recharged because the chemical reactions can be reversed when an external electrical current is supplied.

34
Q

State an example of a non-rechargeable battery.

A

An alkaline battery.

35
Q

Draw a diagram for a fuel cell.

A
36
Q

Draw a diagram for a battery.

A
37
Q

Draw a diagram for a cell.

A
38
Q

How do fuel cells produce energy?

A
  • They are supplied by an external source of fuel (eg hydrogen)
  • and oxygen or air.
  • The fuel is oxidised and releases electrons.
  • to produce a potential difference and an electric current.
  • In a hydrogen fuel cell, the only waste produce is water.
39
Q

Fuel cells; half equations

A

At negative electrode:

2H2 -> 4H+ 4e-

(hydrogen molecule splits into hydrogen ions and electrons - the electrons pass through the wire to produce an electrical current.)

At positive electrode:

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

(Oxygen molecules combine with electrons and hydrogen ions produced at negative electrode)

So in a hydrogen fuel cell the hydrogen is combining with oxygen to be oxidised.

40
Q

What occurs in a hydrogen fuel cell?

A

hydrogen is combining with oxygen to be oxidised.

Electrons released from hydrogen pass into the wire to produce an electrical current.

41
Q

Overall equation for hydrogen fuel cell:

A

2H2 + O2 -> 2H2O

42
Q

Advantages and disadvantages of hydrogen fuel cells compared to rechargeable batteries:

A
  • Hydrogen fuel cells produce electricity for as long as hydrogen is provided while rechargeable batteries run out and need to be recharged.
  • Hydrogen fuel cells do not get less efficient the longer they run while rechargeable batteries store less electricity the more charging cycles they go through and will eventually have to be replaced.
  • Hydrogen fuel cells can be a source of drinkable water eg on space craft.

PROBLEMS
- Hydrogen fuel cells run on hydrogen which is an explosive gas - this is difficult to store safely, while dangerous fuels aren’t required for rechargeable batteries (bare in mind that some rechargeable batteries can catch fire if not manufactured correctly)

  • Hydrogen fuel cells produce a relatively low pd/voltage so lower current, so several are needed together to produce a larger electrical current, while rechargeable batteries can produce a greater pd than hydrogen fuel cells.

GCSE Chemistry (9 decks)

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