C7.1 - C7.6 Energy Changes✔️

Question 1

What is an exothermic reaction?

Answer 1

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

Question 2

What is an endothermic reaction?

Answer 2

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

Question 3

Examples of exothermic reactions?

Answer 3

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

Question 4

Examples of endothermic reactions?

Answer 4

-citric acid and sodium hydrogencarbonate
-photosynthesis

Question 5

How do you measure exothermic reactions?

Answer 5

Measure rise in temperature

Question 6

How do you measure endothermic reactions?

Answer 6

Measure fall in temperature

Question 7

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

Answer 7

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

Question 8

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

Answer 8

-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

Question 9

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

Answer 9

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

Question 10

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

Answer 10

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

Question 11

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

Answer 11

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

Question 12

uses of exothermic reactions?

Answer 12

-hand/body warmers
-self heating cans

Question 13

Uses of endothermic reactions?

Answer 13

-cold packs
-chill cans of drinks

Question 14

How does a reusable hand warmer work?

Answer 14

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

Question 15

How does a 1 time handwarmer work?

Answer 15

-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

Question 16

How are exothermic reactions used in self-heating cans?

Answer 16

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

Question 17

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

Answer 17

Calcium oxide + water - calcium hydroxide

Question 18

How do chemical cool packs work?

Answer 18

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

Question 19

What is activation energy?

Answer 19

Minimum amount of energy needed to start a reaction

Question 20

Explain why bond breaking is an endothermic process?

Answer 20

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

Question 21

Explain why bond making is an exothermic process?

Answer 21

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

Question 22

Endothermic reaction profile ?

Answer 22

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

Question 23

Exothermic reaction profile?

Answer 23

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

Question 24

What is bond energy?

Answer 24

The energy needed to break bonds between two atoms

Question 25

How do you know if it as endothermic reaction?

Answer 25

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

Question 26

How do you know if it as exothermic reaction?

Answer 26

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

Question 27

What is energy needed for in a reaction?

Answer 27

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

Question 28

What happens to energy when bonds are formend?

Answer 28

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

Question 29

What happens to energy when bonds are broken?

Answer 29

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

Question 30

Equation for caculating overall energy change?

Answer 30

Energy change = break - make

Question 31

What can determine the voltage produced between two metals?

Answer 31

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

Question 32

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

Answer 32

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

Question 33

Word equation for the reaction of hydrogen and oxygen?

Answer 33

Hydrogen + oxygen - water

Question 34

Benefits of using hydrogen fuel?

Answer 34

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

Question 35

Issues with using hydrogen fuel?

Answer 35

-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

Question 36

Explain what a fuel cell is?

Answer 36

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

Question 37

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

Answer 37

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

Question 38

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

Answer 38

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

Question 39

What happens when fuel enters the fuel cell?

Answer 39

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

Question 40

What is the positive electrode?

Answer 40

Anode - electrolysis Cathode - fuel cells

Question 41

What is the negative electrode?

Answer 41

Cathode - electrolysis Anode - fuel cells

Question 42

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

Answer 42

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

Question 43

equation at negative electrode - fuel cells?

Answer 43

2H₂(g) + 4OH-(aq) ------ 4H₂O(I) + 4e-

Question 44

How are the electrodes adapted in fuel cells?

Answer 44

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

Question 45

Described the structure of a fuel cell?

Answer 45

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

Question 46

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

Answer 46

H₂ - 2H(+) + 2e(-)

Question 47

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

Answer 47

O₂ + 4H(+) + 4e(-) - 2H₂O

Question 48

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

Answer 48

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

Question 49

How do the electrons travel to the opposite electrodes?

Answer 49

Via wires

Question 50

Advantages of hydrogen fuel cells?

Answer 50

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

Question 51

Disadvantages of fuel cells?

Answer 51

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

Question 52

What are primary cells?

Answer 52

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

Question 53

How can some batteries be reused?

Answer 53

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