5. Energy Changes

What happens to energy during chemical reactions?

It’s conserved

Amount of energy in universe at beginning and end

Same

Energy levels of products and reactants in EXO

Product molecules must have less energy than reactants by amount transferred

What is an exothermic reaciton?

Transfers energy to the surroundings, so temp of surroundings increases

What are some examples of exothermic reactions?

Combustions, Oxidation, Neutralisation

Self-heating cans, hand warmers

What is an endothermic reaction?

Takes in energy from surroundings so temp of surroundings decreases

Product molecules must have more energy than reactants

What are examples of endothermic reactions?

Thermal Decomposition, Reaction of citric acid and sodium hydrogencarbonate

Some sports injury packs

When can chemical reactions occur?

When reacting particles collide with a sufficient energyW

What is activation energy?

Minimum amount of energy that particles must have to react

What do reaction profiles show?

Relative energies of reactants and products

Activation Energy

Overall Energy Change

Exo reaction profile

Energy of reactants higher than energy of products (products have less energy than reactants because heat is released to surroundings)

Endo reaction profile

Energy of products higher than energy of reactants (energy from surroundings taken in)

Energy during reactions?

• Energy must be SUPPLIED to BREAK bonds in reactants

• Energy is RELEASED when bonds are FORMED

Bond energy calculations

Sum of energy taken in to break bonds - sum of energy released to form bonds = overall energy change

Energy taken in to break MORE THAN (>) energy released when formed

ENDO : overall energy taken in

Energy taken in to break LESS THAN (<) energy released when formed

EXO (overall energy released)

What reacts to produce electricity in cells?

Chemicals

What does voltage produced by a cell depend on?

A number of factors like type of electrode and electrolyte

How can a simple cell be made?

By connecting two different metals in contact with an electrolyte

What are batteries?

Two or more cells connected together in series to provide a greater voltage

Non-rechargeable cells and batteries

• Chemical reactions stop when one of the reactants has been used up

• Alkaline batteries are non-rechargeable

Rechargeable cells and batteries

• Can be recharged because the chemical reactions are reversed when an external electrical current is supplied

What are fuel cells?

• Supplied by external source of fuel (e.g Hydrogen) and oxygen or air. The fuel is oxidised electrochemically within the fuel cells to produce a p.d

• Overall reaction hydrogen fuel cell involves the oxidation of hydrogen to produce water

• Hydrogen fuel cells offer a potential alternative to rechargeable cells and batteries

Qualities of Hydrogen Fuel Cells

• Fuel cells can be used constantly provided that fuel keeps being put in

• Hydrogen is a gas so needs to be stored at high pressure and so is harder to transport

• Only produces water when burnt

Qualities of rechargeable batteries

• Can be recharged by reversing reaction, so fuel doesn’t need to keep being supplied

• Hard to dispose of (non-biodegradable)

• Will eventually stop working

Equations for each half-cell (Fuel Cells)

Anode (positive electrode): H2 (g) → 2e- + 2H+(aq)

Cathode (Negative electrode): 4H+(aq) + O2(g) + 4e- → 2H2O(g)