Energy Changes

Question 1

What are some examples of exothermic reactions?

Answer 1

-burning fuels (combustion)
-Neutralisation reactions (acid and an alkali)
-Many oxidation reactions e.g. adding sodium to water

Question 2

What are some uses of exothermic reactions?

Answer 2

-some hand warmers use the oxidation of iron in air
-self heating cans of coffee or hot chocolate use exothermic reactions of chemicals in their bases

Question 3

What are some examples of endothermic reactions?

Answer 3

Thermal decomposition e.g. heating calcium carbonate causes it to decompose into calcium oxide and carbon dioxide.

Question 4

What is a use of endothermic reactions?

Answer 4

Sports injury packs, allowing it to become instantly cooler without needing to go in a freezer

Question 5

What is the biggest problem with taking energy measurements? How can this be fixed?

Answer 5

-the amount of energy lost to surroundings
-put a polystyrene cup into a beaker of cotton wool for insulation and put a lid on

Question 6

How can you test the effect of acid concentration on the energy released in a neutralisation reaction between hydrochloric acid and sodium hydroxide?

Answer 6

-put 25cm^3 of 0.25mol/dm^3 hydrochloric acid and sodium hydroxide in separate beakers
-place them in a water bath set to 25 degrees
-when they warm to that temperature put them into a polystyrene cup with a lid surrounded by cotton wool
-take the temperature every 30 seconds and record the highest temperature
-repeat the experiment with 0.5mol/dm^3 and 1mol/dm^3

Question 7

How does energy relate to the breaking and forming of bonds?

Answer 7

Energy must be supplies to break bonds (endothermic) and released when new bonds are formed (exothermic)

Question 8

How do bond energies dictate endothermic and exothermic reactions?

Answer 8

-In exothermic reactions the energy released by forming bonds is greater than the energy used to break them
-In endothermic reactions the energy used to break bonds is greater that the energy released by forming them

Question 9

How can you use the known bond energies of a compound to calculate the overall energy change for a reaction?

Answer 9

Overall energy change = sum of energies needed to break bonds in the reactants - energy released when forming the new bonds in the products

Question 10

Why can’t you compare the overall energy changes of reactions without knowing the exact numerical values?

Answer 10

Because you won’t know the exact difference between their overall energy changes.

Question 11

What is an electrochemical cell?

Answer 11

A basic system made up of two different electrodes in contact with an electrolyte?

Question 12

What are the components of an electrochemical cell?

Answer 12

-two electrodes that can conduct electricity (usually metals)
-a liquid (electrolyte) that contains ions to react with the electrodes
-charge difference between electrodes from the reactions between them and the electrolyte

Question 13

What factors affect the voltage of an electrochemical cell?

Answer 13

-type of metal -different metals react differently with the same electrolyte, causing a charge difference (voltage)
-difference in the reactivity of the electrodes
-the electrolyte used - different ions react differently with the electrodes

Question 14

Why are some batteries non-rechargeable?

Answer 14

The contain cells which use irreversible reactions. Once one of the reactants is used up, they don’t produce any more charge and have to be replaced.

Question 15

How do rechargeable batteries work?

Answer 15

The reactions in the cell can be reversed by connecting it to an external electrical current. E.g. charging your phone

Question 16

What is a fuel cell? What is an example of one?

Answer 16

An electrical cell that’s supplied with a fuel and oxygen and uses energy from the reaction between them to efficiently produce electrical energy. E.g. a hydrogen-oxygen fuel cell which produces clean water and releases energy.

Question 17

How do hydrogen-oxygen fuel cells work? (Electrolysis)

Answer 17

-electrolyte is often an acid
-hydrogen goes the negative electrode and oxygen goes to the positive one
-hydrogen is oxidised to produce H+ ions which move to the positive electrode
-oxygen is reduced to give O2 molecules
-O2 molecules react with H+ ions
-electrons flow through external circuit from the negative to positive electrode (this is an electric current)

Question 18

What are the potential advantages of using hydrogen-oxygen fuel cells to power electric vehicles?

Answer 18

-less pollutants like greenhouse gases, nitrogen oxides, sulfur dioxide, carbon monoxide
-fuel cells are less pollutant to dispose of than other batteries
-no limit to how many times they can be recharged, unlike current batteries of electric vehicles
-less expensive
-store more energy (need recharging less often)

Question 19

What are the potential disadvantages of using hydrogen-oxygen fuel cells to power electric vehicles?

Answer 19

-hydrogen is a gas so it takes up more space to store
-hydrogen is explosive when mixed with air so could potentially be dangerous
-hydrogen fuel is often made from hydrocarbons (from fossil fuels) or by electrolysis of water (which uses electricity that normally comes from fossil fuels)