ENERGY CHANGES

What is an exothermic reaction

Transfers energy to the surroundings so that the temperature of surroundings increases

3 examples of exothermic reactions

• Combustion

• Neutralisation reactions

• Many oxidation reactions

uses of exothermic reactions

• Self heating cans

• Hand warmers

What’s an endothermic reaction

Takes in energy from the surroundings shown by a decrease in temperature of the surroundings

2 examples of endothermic reactions

• Citric acid and sodium hydrogencarbonate

• Thermal decomposition

Uses of endothermic reactions

Sport injury packs

What’s the problem of energy measurements and how to fix?

• Problem = energy lost to surroundings

• Reduce energy loss by putting polystyrene cup in a beaker of cotton wool to give insulation, and putting lid on top to reduce energy lost by evaporation

How can you measure energy released by chemical reaction

you can measure the energy releaser by chemical reaction in solution by taking temperature of reagents mixing them in a polystyrene cup and measuring the temp of the solution at the end of the reaction

practical for temperature change

1. Put 25cm cubed mole/dmcubed of HCL and NaOH in separate beakers

2. Place beakers in water bath to set 25 degrees until both at same temp

3. Add HCL followed by NaOH to a polystyrene cup with lid

4. Repeat steps 1-4 using 0.5 mol/dmcubed of HCL

when can chemical reactions only occur

• When teacting particles collide with each other and with sufficient energy

What is activation energy

Minimum amount of energy that particles must have to react

why is bond breaking endothermic

energy must be supplied to break existing bonds

why is bond making exothermic

energy released when new bonds formed

What’s a electrochemical cell

A basic system made up of 2 different electrodes that are in contact with an electrolyte

Why are electrodes metals usually?

So they can conduct electricity

What’s an electrolyte

Liquid containing ions which reavt with electrodes

What do cells contain which react to produce electricity

Chemicals

What’s a battery

Consists of 2 or more cells connected together in series to provide greater voltage

what makes a voltage of a battery bigger

the bigger the difference in reactivity of the electrodes

How do you get a bigger voltage in a battery

Combine batteries

Why do reactions stop in a non reusable battery

When one reactant is used up

Which battery is non rechargeable

Alkaline batteries

Why can some cells and batteries be recharged?

Chemical reactions are reversible when an external current is supplied

What’s a fuel cell

an electrical cell thats supplied with a fuel and oxygen and uses energy from the reaction to produce electrical energy efficiently

what happens when the fuel enters the cell

it is oxidised and sets up a p.d within cell

what does the hydrogenoxygen fuel cell involve

redox reaction

in a HO fuel cell what goes to the anode and what happens

hydrogen, is oxidised and produces h+ ions in the electrolyte move to the cathode

in the HO fuel cell what goes to the anode

oxygen gains electrons from cathode and racts with h+ ions,to make water. = reduction

half equation at the anode

H₂ → 2H⁺ + 2e⁻

half equation at the cathode

O₂ + 4e⁻ + 4H⁺ → 2H₂O

whats the overall reaction

2H₂ + O₂ → 2H₂O,

advantages of HO fuel cells

• dont produce as many pollutants

• batteries in electric vechiles are rechargable but theres a limit to how many times they can be recharged before needing replacing

• batteries are more expensive to make than fuel cells

• batteries store less energy than fuel cells so would need to be recharged more often

disadvantages of HO fuel cells

• fuel cells dont produce many pollutants whereas disposing electric vechiles batteries is more pullutinf as they are highly toxic compounds

• hydrogen is a gas so takes more space to store than rechargeable battery

• its explosive when mixed with air so hard to store safely

• hydrogen fuel is made from hydrocarbons, or by electrolysis which uses electricity