Topic 5- Energy changes (copy)

What is an exothermic reaction

A reaction which transfers energy to the surroundings

What are examples of exothermic reactions

Combustion, neutralisation and oxidation reactions

What are some uses of exothermic reactions

Hand warmers, self-heating cans of hot chocolate and coffee

What is an endothermic reaction

A reaction which takes in energy from the surroundings eg decomposition reactions

What is a use of endothermic reactions

Cold packs

What is a reaction profile

A graph that showed how the energy in a reaction changes as the reaction progresses

How do you find the overall energy change of a reaction profile

Distance between reactants and products

How do you find the activation energy on a reaction profile

Distance between reactants and peak of graph

How do you know if a reaction is exothermic or endothermic on a reaction profile

More energy in reactants than products = exothermic, more energy in products than reactants = endothermic

Why is bond breaking an endothermic process

Energy must be supplied to break existing bonds

Why is bond formation an exothermic process

Energy is released when new bonds are formed

What is the reaction if Bond formation>bond breaking

exothermic reaction

What is the reaction if Bond breaking>bond formation

Endothermic

What is the formula for overall energy change

Energy change = energy of bond breaking - energy of bond making

What is a cell

A system that contains chemicals which react and produce electricity. It contains two different (metal) electrodes in contact with an electrolyte

What is an electrolyte

A liquid that conducts electricity and contains ions which reacts with electrodes

How does a cell work

The chemical reactions between the electrodes and electrolyte set up a charge difference between the electrodes. If electrodes are connected by a wire the charge is able to flow and electricity is produced

Cells can only produce electricity if we use metals with…

…different reactivities

What happens if there’s no difference in reactivity between the two electrodes

The potential difference if the cell will be zero

What does the potential difference of a cell depend on

Identity of the metal electrodes, electrolyte and concentration of ions in electrolyte

Why will there be a low voltage if the concentration of ions in electrolyte is low

As the ions in the electrolyte react, their concentration will slowly decrease and so will the voltage of the cell

What is a battery/ how is it formed

A battery contains two or more cells connected in series to produce a greater voltage

Why can a cell only produce electricity for a certain period of time

Eventually the chemicals in the cell run out and the reaction stops

Why are alkaline batteries non rechargeable

The reactants in batteries run out so no more electricity is produced; reaction is irreversible

Why can you recharge rechargeable batteries

Reaction can be reversed when we apply an electrical current.

What is a fuel cell

A type of electrochemical cell that converts the chemical energy of a fuel and oxygen into electrical energy that can be used to power things

What happens in a hydrogen oxygen fuel cell

Hydrogen and oxygen react together to form water

What is often used as the electrolyte in a hydrogen oxygen fuel cell

Potassium hydroxide

What is used as the electrodes in a hydrogen oxygen fuel cell

Porous carbon with a catalyst

What happens at the anode (negative electrode) in hydrogen oxygen fuel cells

Hydrogen loses electrons to produce H+ ions, this is oxidation. H+ ions in the electrolyte move to the cathode (positive electrode)

What happens at the cathode (positive electrode) in hydrogen oxygen fuel cells

Oxygen gains electrons from the cathode and reacts with H+ ions in the electrolyte to make water, this is reduction

What is the electric current in a hydrogen oxygen fuel cell

The flow of electrons through an external circuit from the anode to the cathode

What is the overall reaction in hydrogen oxygen fuel cell

Redox

What is the half equation at the anode

H2 —> 2H+ + 2e-

What is the half equation at the cathode

O2 + 4H+ + 4e- —> 2H2O

What are the pros of hydrogen oxygen fuel cells

Fuel cell vehicles don’t produce as many pollutants as other fuels, only require hydrogen and oxygen, last longer than batteries, less polluting to dispose of

What are the cons of hydrogen oxygen fuel cells

Hydrogen is a gas so takes up more space to store than fossil fuels or batteries, hydrogen is explosive when mixed with air making it difficult to store safely, making hydrogen fuel requires energy which usually comes from fossil fuels

high cost, the lack of a widespread hydrogen refueling infrastructure, and challenges with hydrogen production and storage