C7 Energy Changes

Question 1

What is the conservation of energy principle?

Answer 1

• Energy is conserved in chemical reactions
• The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place

Question 2

What is an exothermic reaction? Give examples

Answer 2

• A reaction where energy is transferred to the surroundings so that the surroundings tempurature increases
• For example: combustion, oxidation reactions and neutralisation (acid + alkali) reactions
• Negative sign of energy change

Question 3

What is an endothermic reaction? Give examples

Answer 3

• A reaction where energy is taken in from the surroundings so the surroundings temperature decrease
• For example: thermal decomposition, reaction of citric acid and sodium hydrogencarbonate.
• Positive sign of energy change

Question 4

What is activation energy?

Answer 4

Minimum amount of energy that particles need to react

Question 5

What is a reaction profile?

Answer 5

A graph which shows the relative energies of reactants and product, as well as activation energy of the reaction

Question 6

What occurs in a chemical reaction in terms of bond energies? Describe exothermic and endothermic reactions in terms of bond breaking/forming.

Answer 6

• Energy is supplied to break bonds in the reactants
• energy is released when bonds in the products are formed
• Exothermic - energy released from forming bonds is greater than that needed to break the bonds#
• Endothermic - energy needed to break bonds is greater than energy released making them

Question 7

What is the equation to find enthalpy charge in terms of bond energies?

Answer 7

Energy of reaction = sum of bonds broken - sum of bonds made

Question 8

What must the product molecules have if a reaction transfers energy to the surroundings?

Answer 8

The product molecules must have less energy than the reactants, by the amount transferred

Question 9

What are some everyday uses of exothermic reactions?

Answer 9

• Self-heating cans
• Hand warmers

Question 10

What are some sports injury packs based on?

Answer 10

Endothermic reactions

Question 11

What is the burning of gas an example of?

Answer 11

• The burning of fuels, such as the combustion of methane gas, is an obvious example of exothermic reactions
• When methane (the main gas present in natural gas) burns, it gets oxidised and releases energy to its surroundings

Question 12

How do single use hand warmers use exothermic reactions to heat you up?

Answer 12

• Makes use of the energy transferred to the surroundings in the oxidation of iron
• Irons turns into hydrated iron (III) oxide in an exothermic reaction
• The reaction is similar to rusting
• Sodium chloride (common salt) is used as a catalyst
• This type of hand warmer is disposable
• It can be used only once but lasts for hours

Question 13

How do multiple use hand warmers make use of exothermic reactions to warm you up?

Answer 13

• A small metal disc in the plastic pack is used to start the exothermic change
• When you press this a few times, small particles of metal are scraped off
• These start off the crystallisation
• The crystals spread throughout the solution, transferring energy to the surroundings in an exothermic change
• They work for about 30 minutes
• To reuse the warmer, you put the pack into boiling water to redissolve the crystals
• Once it has cooled down, the pack is ready to activate again

Question 14

How are exothermic reactions used in self heating cans?

Answer 14

• They make drinks like hot coffee without any external heating device (e.g. a kettle)
• The reaction used to transfer energy to the food or drink is usually: calcium oxide + water -> calcium hydroxide
• You press a button in the base of the can
• This breaks a seal and lets the water and calcium oxide mix
• Then the exothermic reaction begins

Question 15

How are endothermic reactions used for sports injuries?

Answer 15

• Endothermic reactions can be used to cool things down
• For example, chemical cold packs usually contain ammonium nitrate and water
• When ammonium nitrate dissolves, it absorbs energy from its surroundings, making them colder
• These cold packs are used as emergency treatment for sports injuries
• The decrease in temperature reduces swelling and numbs pain
• The ammonium nitrate and water are kept separate in a pack
• When squeezed or struck, the bag inside the water pack breaks, releasing ammonium nitrate
• The same endothermic change can also be used to chill cans of drinks

Question 16

When can chemical reactions only occur?

Answer 16

When reacting particles collide with each other and with sufficient energy

Question 17

What does the reaction profile of an exothermic reaction look like?

Answer 17

Question 18

What does the reaction profile of an endothermic reaction look like?

Answer 18