C7 - Energy Changes

Question 1

Describe what an exothermic reaction is

Answer 1

• They transfer energy to their surroundings as heat energy which increases the temperature of the surroundings
• The products have less energy than the reactants
• Energy changes are shown as negative

Question 2

Examples of exothermic reactions

Answer 2

• Combustion
• Certian oxidation reaction
• Some neutralisation reactions

Question 3

Pracitcal uses of exothermic reactions

Answer 3

• Hand warmers
• Self heating food cans

Question 4

Describe what an endothermic reaction is

Answer 4

• They take in energy from their surroundings which decreases the temperature of the surroundings
• The products have more energy than the reactants
• Energy changes are shown as positive

Question 5

Examples of endothermic reactions

Answer 5

• Thermal decomposition
• reaction of citric acid with sodium hydrogen carbonate

Question 6

What is the activation energy

Answer 6

• When particles collide, they must have sufficient energy to react
• Th activation energy is the minimum amount of energy required that particles must have to react

Question 7

What do bond energies state

Answer 7

• Breaking chemical bonds requires energy so is endothermic
• Making chemical bonds releases energy so is exothermic

Question 8

Variables for the neutralisation exothermic reaction of HCl and NaOH

Answer 8

• Independent - volume of sodium hydroxide
• Dependent - max temp reached
• Control - voulme of HCl and concentration of both HCl and NaOH

Question 9

Method for experiment for exothermic reaction of HCl and NaOH

Answer 9

• Measure 30cm3 of dilute HCl using measuirng cylinder and pour in a polystyrene cup
• Stand the cup inside a beaker to stop it falling over
• Use a thermometer to measure the initial temp of the HCl
• Pour 5cm3 NaOH into the cup and put a lid with a thermometer and stir
• The temperature will increase and record the temp change
• Rinse out cup and repeat with increasing volumes of NaOH
• Repeat whole thing to find a mean

Question 10

Explain the results of the exothermic experiment with HCl and NaOH

Answer 10

• As the volume of NaOH increases, the max temp increases as more particles react with the HCl
• After a certain point, the temperature decreases due to there being too much NaOH(in excess) and some that doesn’t react
• This means the energy released is spread out into a larger volume and the cold NaOH absorbs the heat energy lowering the temp

Question 11

Describe exothermic and endothermic reactions in terms of bonds energies

Answer 11

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

Question 12

How is a simple cell set up and what happens

Answer 12

• Put two different metals in an electrolyte ( conductive solution)
• There will be a voltage between them and a current will flow through the wire
• This is due to the chemical reaction occuring on the electrodes
• Electrons pass from the more reactive metal to the less reactive metal generating a flow of electrons

Question 13

Key ideas about cells

What affects voltage, are cells indefinite

Answer 13

• Cells can only produce electricity for a set amount of time as the chemicals run out
• Electricity can only be produced if metals with different reactivities are used
• The greater the different in reactivity, the greater the voltage
• Th electrolyte also affects the voltage

Question 14

What is a battery

Answer 14

Two or more cells connected in series and this produces a greater voltage than a single cell

Question 15

How can rechargeable batteries be recharged

Answer 15

• The chemical reactions that occur in them are reversible

Question 16

Why can some batteries not be recharged

Answer 16

• in alkaline batteries, the reactants run out producing no more electricity
• This is due to the reactions being non-reversible

Question 17

What is a fuel cell

Answer 17

• A reaction vessel where a fuel e.g. hydrogen is supplied and reacted with oxygen in the air
• These chemical reactions produce a potential difference

Question 18

Full, balanced equation for the reaction inside a hydrogen fuel cell

Answer 18

• 2H2 + O2 —> 2H2O
• The hydrogen is being oxidised

Question 19

What happens at the negative electrode in a hydrogen fuel cell

Answer 19

• Hydrogen molecules split into hydrogen ions and electrons
• Half equation: 2H2 —-> 4H+ + 4e-
• Electrons pass through wire giving current

Question 20

What happens at the positive electrode in a hydrogen fuel cell

Answer 20

• Oxygen atoms combine with hydorgen ions and electrons
• Half equation: O2 + 4H+ + 4e- —> 2H2O

Question 21

Advantages of hydrogen fuel cells

Answer 21

• Produce electricity as long as hydrogen is supplied and don’t need to be recharged
• Don’t get less efficient the longer they run
• Can be a source of drinking water
• Only waste product is water - not pollutant
• More reliable as they don’t have moving parts
• Don’t have a finite life like lithium ion batteries

Question 22

Disadvantages of hydrogen fuel cells

Answer 22

• Hydrogen has to be kept under high pressure which is difficult to store
• Hydrogen can be explosive
• Lots are needed together to produce a large enough potential difference
• Very expensive

Question 23

What do catalysts do

Answer 23

• Increase the rate of reaction
• By providing an alternative reaction pathway witha lower activation energy
• This increases the likelihood of successful collisions

Question 24

Difference in processes between electrolysis and in chemical cells

Answer 24

• Electrolysis used electricity to produce chemical reactions
• Chemical cells use chemical reactions to produce electricity