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GCSE CHEMISTRY > C7.1 - C7.6 Energy Changes✔️ > Flashcards

C7.1 - C7.6 Energy Changes✔️ Flashcards

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1
Q

What is an exothermic reaction?

A

Chemical reactions which transfer energy to the surroundings often heating them up they are a very common type of reaction - causes a rise in temperture

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2
Q

What is an endothermic reaction?

A

Chemical reaction which transfers energy from the surroundings to the reacting chemicals - causes a fall in temperture

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3
Q

Examples of exothermic reactions?

A

-combustion of methane gas - realeses energy into surroundings
-neutralisation reactions between acids and alkalis

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4
Q

Examples of endothermic reactions?

A

-citric acid and sodium hydrogencarbonate
-photosynthesis

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5
Q

How do you measure exothermic reactions?

A

Measure rise in temperature

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6
Q

How do you measure endothermic reactions?

A

Measure fall in temperature

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7
Q

What reactions,products have a higher energy content than the reactants?

A

Endothermic - products have higher energy content than the reactants so energy is transferred from the surroundings

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8
Q

Method for investigating temperature change of a chemical reaction - classify it as exothermic or endothermic ?

A

-measure 25 cm^3 of sodium hydroxide - pour into polystyrene cup.

-stand cup in 250cm^3 beaker.

-measure temp of sodium hydroxide every 30 seconds while stirring.

-after 2 mins add 25cm^3 of hydrochloric acid and stir record temp every 30 s for 10 min

-repeat
repeat with copper(II) sulfate and iron fillings
repeat with potassium hydroxide and nitric acid

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9
Q

Equipment for investigating temperature change of a chemical reaction - classify it as exothermic or endothermic ?

A

-measuring cylinder
-polystyren cup - less heat loss
-thermometer
-stopwatch
-stirrer
-hydrochloric acid
-sodium hydroxide

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10
Q

Results for investigating temperature change of a chemical reaction - classify it as exothermic or endothermic ?

A

-all exothermic - showed increase in temperture - energy transfered from reacting chemicals heat up surroundings

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11
Q

Conclusion for investigating temperature change of a chemical reaction - classify it as exothermic or endothermic ?

A

All three exothermic but copper sulfate most exothermic reaction - largest temperature change

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12
Q

uses of exothermic reactions?

A

-hand/body warmers
-self heating cans

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13
Q

Uses of endothermic reactions?

A

-cold packs
-chill cans of drinks

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14
Q

How does a reusable hand warmer work?

A

Based on formation of crystals from solutions of salts - supersatured solution is prepared by dissolving as much salt as possible in hot water - solution then allowed to cool - metal disk in pack when pressed releases small metal particles starts of crystalisation - crystal transfer energy to surroundings - work for 30 min

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15
Q

How does a 1 time handwarmer work?

A

-uses energy transferred to surrounding in the oxidation of iron - iron turns into hydrated iron(III) oxide in exothermic reaction - sodium chloride used as catalyst - lasts for hours but can not be reused

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16
Q

How are exothermic reactions used in self-heating cans?

A

Press button on base of can - breaks seal lets water and calcium oxide mix - exothermic reaction can begin

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17
Q

Reaction used to heat up food/drink in self-heating cans?

A

Calcium oxide + water - calcium hydroxide

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18
Q

How do chemical cool packs work?

A

Endothermic reaction used - ammonium nitrate and water kept seperate in the pack when squeezed-bag inside water pack breaks - releases ammonium nitrate - usually work for about 20 min- only used once - same used for cooling cans/drinks

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19
Q

What is activation energy?

A

Minimum amount of energy needed to start a reaction

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20
Q

Explain why bond breaking is an endothermic process?

A

Energy has to be supplied to break chemical bonds - energy taken in from surroundings - so its endothermic

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21
Q

Explain why bond making is an exothermic process?

A

When new bonds are formed - energy is transferred to surroundings - energy is released as it has to be stabalised

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22
Q

Endothermic reaction profile ?

A

Products at higher energy level then reactants as reactants react to form products - energy transferred from the surroundings - surroundings get colder

23
Q

Exothermic reaction profile?

A

Products at lower energy level of the reactants - when reactant form products energy transffered to surrounding - surrounding gets hotter

24
Q

What is bond energy?

A

The energy needed to break bonds between two atoms

25
Q

How do you know if it as endothermic reaction?

A

Energy released when new bonds are formed is less then energy absorbed when bonds are broken

26
Q

How do you know if it as exothermic reaction?

A

Energy released when new bonds are formed is greater then energy absorbed when bonds are broken

27
Q

What is energy needed for in a reaction?

A

Energy must be supplied to break the bonds between atoms in the reactants

28
Q

What happens to energy when bonds are formend?

A

When new bonds are formend between atoms in a chemical reaction energy is released

29
Q

What happens to energy when bonds are broken?

A

Energy has to be supplied to break chemical bonds this means it is an endothermic process - energy is taken in from the surroundings

30
Q

Equation for caculating overall energy change?

A

Energy change = break - make

31
Q

What can determine the voltage produced between two metals?

A

The greater the difference in reactivity between the two metals used the higher the voltage produced

32
Q

What happens when two metals , conected by a wire are dipped in a salt solution?

A

The more reactive element will donate electrons to the less reactive metal which forms a simple electrical cell - because metals tend to loose electrons to form ions

33
Q

Word equation for the reaction of hydrogen and oxygen?

A

Hydrogen + oxygen - water

34
Q

Benefits of using hydrogen fuel?

A

does not produce carbon dioxide so there is reduced impacts on global warming and less greenhouse gases being released

35
Q

Issues with using hydrogen fuel?

A

-problems with storing it safely
-supplying the hydrogen to burn in car engines
-making hydrogen using electrolysis - requires electricity from non renewable fossil fuels which release carbon dioxide

36
Q

Explain what a fuel cell is?

A

Electrical cell thats supplied with a fuel and oxygen and uses energy from the reaction between them to produce electrical energy efficiently

37
Q

Issues with developing and replacing engines powered by fossil fuels with cleaner energy sources?

A

The issue is trying to match the performance,convinence and price with the petrol/diesel car and make it a economical descion for the customer

38
Q

What happens at the negative electrode or anode (opposite for fuel cells) in a fuel cell?

A

Hydrogen gas supplied as fuel to the anode - it diffuses through the graphite electrode and reacts with hydroxide ions to form water and provides a source of electrons to the circuit

39
Q

What happens when fuel enters the fuel cell?

A

The fuel becomes oxidised and sets up a potential differene within the cell an example is the hydorgen-oxygen fuel cell

40
Q

What is the positive electrode?

A

Anode - electrolysis
Cathode - fuel cells

41
Q

What is the negative electrode?

A

Cathode - electrolysis
Anode - fuel cells

42
Q

What happens at the positive electrode or cathode (opposite for fuel cells) in a fuel cell?

A

Oxygen gas supplied to the positive electrode and diffuses through the graphite and reacts to form hydroxide accepting electrons from the circuit

43
Q

equation at negative electrode - fuel cells?

A

2H₂(g) + 4OH-(aq) —— 4H₂O(I) + 4e-

44
Q

How are the electrodes adapted in fuel cells?

A

The electrodes are made of porous carbon (have lots of tiny holes) and also contain a catalyst to speed up the reaction

45
Q

Described the structure of a fuel cell?

A

In the centre there is an electrolyte (most common is potassium hydroxide) - either side there is the electrodes (negative anode on left and positive cathode on right) connected by a wire - on the outside of the electrodes there is the anode compartment and cathode compartment with each having an inlet at the top - outlet on the cathode compartment

46
Q

What is the half equation for the oxidation of hydrogen at the negative electrode (anode)?

A

H₂ - 2H(+) + 2e(-)

47
Q

What is the half equation for the reduction of oxygen at the positive electrode (cathode)?

A

O₂ + 4H(+) + 4e(-) - 2H₂O

48
Q

What happens to the H+ ions produced through the oxidation of hydrogen at the negative electrode (anode) ?

A

H+ ions in the electrolyte move to the cathode (positive electrode) where the oxygen gains the electrons transfered via the wire from the anode and react with the H+ ions to form water

49
Q

How do the electrons travel to the opposite electrodes?

A

Via wires

50
Q

Advantages of hydrogen fuel cells?

A

-do not need to be electrically recharged
-no pollutants are produced
-can be a range of sizes

51
Q

Disadvantages of fuel cells?

A

-hydrogen is highly flammable
-hydrogen is sometimes produced for the cells by non renewable ways
-hydrogen is hard to store

52
Q

What are primary cells?

A

First mass produced batteries - they cannot be recharged (the dry cells with electrodes made of zinc and carbon are non-rechargeable) and once the reactant has run out the cells stop working and has to be disposed off

53
Q

How can some batteries be reused?

A

The battery is connecetd to a power supply that reverses the reactions that occur at each electrode which regenerates the origional reactants

GCSE CHEMISTRY (21 decks)

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