Energy changes

Question 1

What is a reversible reaction?

Answer 1

It is a reaction which proceeds in two directions –> forward and reverse direction. The reactants combine to form products. The products can form reactants.

Question 2

exothermic reaction definition

Answer 2

An exothermic reaction is a reaction in which the amount of energy taken in is less than the amount of energy released (given out). Net energy is released. The enthalpy change is negative.

Question 3

EquilibRium

Answer 3

The rate of the forward reaction is equal to the rate of reverse reaction. When equilbrium is reached, the concentration of the reactants and products remain constant.

Question 4

Haber process equation

Answer 4

• 3H2 + N2 (reversible) 2NH3

- iron oxide catalyst

Question 5

Conditions for reversible reactions

Answer 5

• Rf = Rr
• closed system
• Le Chatelier’s principle

Question 6

When equilibrium is reached, the concentration

Answer 6

of the reactants and products remain constant

Question 7

Uses of ammonia

Answer 7

• fertiliser industry:crop production
• explosive industry- TNT (Trinitrotuolene)
• household detergents
• It is a very weak alkali- it could neutralise acid in soil
• smelling salts: ammonium carbonate- medical use of ammonia- alfactory nerve will sting - to help people who have fainted

Question 8

endothermic reaction definition

Answer 8

a reaction during which the amount of energy absorbed (taken in) is greater than the amount of energy released. Net energy is absorbed. The enthalpy change is positive.

Question 9

conditions for a reversible reaction

Answer 9

• rate of forward reaction = rate of reverse reaction
• closed system
• Le Chatelier’s principle

Question 10

Heat of reacion (delta H) enthalpy change definition

Answer 10

The difference between the energy of the reactant (s) and the energy of the product (s).

Question 11

Activation energy definition

Answer 11

The minimum energy needed to start a chemical reaction. It is the energy needed to break the ‘bonds’ of the reactants.

Question 12

catalyst

Answer 12

-It is a chemical substance that could SPEED UP/ increase the rate of a chemical reaction, without itself undergoing any chemical change.

Question 13

Catalyst properties (3)

Answer 13

• a catalyst only changes the rate but not the yield
• catalysts lower the activation energy and provide an alternative energy pathway
• advantages: energy costs are lowered. Less electricity is needed. Less fossil fuels are burned. Environmentally friendly.

Question 14

Industrial catalysts and uses

Answer 14

• iron/iron oxide: Haber-Bosch process (NH3)
• nickel: hydrogenation (margarine and chocolates)
• vanadium pentoxide (V2O5): sulfuric acid (H2SO4 conc)
• platinum (Pt): nitric acid (HNO3 conc)

Question 15

What are bond energies used for?

Answer 15

To determine whether a chemical reaction is exothermic or endothermic

Question 16

When equilibrium is reached, the

Answer 16

concentration of the reactants and products remain constant

Question 17

Applications of exothermic reactions

Answer 17

• hand warmers
• self-heating cans
  eg:
• many oxidation reactions: respiration
• combustion reactions: burning fuel (methane + oxygen–> carbon dioxide + water)
• many neutralisation reactions (hydrochloric acid + sodium hydroxide –> sodium chloride + water)

Question 18

Applications of endothermic reactions (much less common than exothermic reactions)

Answer 18

• sports injury packs: inside the pack is a bag of water and a substance such as ammonium nitrate. When the pack is squeezed, the water bag bursts and the ammonium nitrate dissolves in the water in an endothermic process.
  eg:
• calcium carbonate (heat) –> calcium oxide + carbon dioxide
• ## Reaction of acids with metal hydrogencarbonates (citric acid and sodium hydrogencarbonate are in sherbet sweets and react in your mouth in an endothermic reaction.

Question 19

Energy must be supplied to do what?

Answer 19

• To break the bonds in the reactants

Question 20

Energy is released when what?

Answer 20

• when bonds in the products are made

Question 21

simple cell

Answer 21

• It is a source of electrical energy
• simplest design consists of two electrodes made from metals of different reactivity immersed in an electrolyte and connected to an external voltage.

Question 22

zinc and copper eg- simple cell

Answer 22

• zinc is the more reactive metal and forms ions more easily, readily releasing electrons
• the electrons give the more reactive electrode a negative charge and sets up a charge difference between the electrodes
• the electrons then flow around the circuit to the copper electrode which is now the more positive electrode
• the difference in the ability of the electrodes to release electrons causes a voltage to be produced.
• the greater the difference in the reactivity of the metals, the greater the voltage produced.
• The electrolyte ysed also affects the voltage.

Question 23

Why does the electrolyte used also affect the voltage?

Answer 23

Different ions react with the electrodes in different ways.

Question 24

non- rechargeable batteries (eg: alkaline batteries)

Answer 24

• electrochemical cells include the familiar batteries used in everyday appliances and cars
• they work by connecting two or more cells in series, which combine to give a larger overall voltage.
• over time, the electrodes degrade as the reactions that occur there are irreversible.
• cells produce a voltage only until one of the reactants is used up and when this occurs the battery dies or goes flat.
• the products formed cannot be reverted back into reactants as the reaction is irreversible and the battery must be replaced.

Question 25

rechargeable batteries

Answer 25

• the reactions are reversed by connecting the cells to an external electrical supply
• this reverses the chemical reactions taking place, allowing the cycle to be repeated.

Question 26

Fuel cell

Answer 26

• It is an electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode
• fuel cells are becoming more common in the automotive industry to replace petrol or diesel engines
• as the fuel enters the cell it becomes oxidised which sets up a potential difference or voltage within the cell
• different electrolytes and fuels can be used to set up different types of fuel cells
• eg: hydrogen-oxygen fuel cells

Question 27

Hydrogen-oxygen fuel cell

Answer 27

-combines both elements to release energy and water
-the cell consists of an electrolyte which is usually phosphoric acid and porous carbon electrodes coated with a catalyst
-hydrogen enters at the anode where it is oxidised and oxygen enters at the cathode where it is reduced
Anode reaction: 2H2–> 4H+ + 4e-
Cathode reaction- 4H+ + O2 + 4e- –> 2H20
overall reaction: 2H2 + O2 –> 2H20
The electrons move around the external circuit from the cathode to the anode
-this movement of electrons is used to drive an electric motor.