Chemistry 5 - Energy Changes

Question 1

Exothermic and endothermic reactions

Answer 1

Energy is moved around in chemical reactions

In an exothermic reaction heat is given out 
Burning fuels - combustion 
Neutralisation reactions ( acid + alkali
Oxidation reactions
Hand warmers 
Self heating cans of hot chocolate

In an endothermic reaction heat is taken in
Much less common than exothermic
Reaction between citric acid and sodium hydrogencarbonate
Thermal decomposition
Sports injury packs

Question 2

More exothermic and endothermic reactions

Answer 2

Energy transfer can be measured
You can measure the amount of energy released by a chemical reaction by taking the temperature of He reactants, mixing them in a polystyrene cup and measuring the temperature at the end of the reaction
The biggest problem with energy reactions is the amount of energy lost to the surroundings
You can reduce it a bit by putting the cup into a beaker of cotton wool to give more insulation and putting a lid on the cup to reduce energy lost by evaporation
This works for neutralisation reactions or reaction between metals and acids or carbonates and acids

Question 3

Bond energies

Answer 3

Energy must always be supplied to break bonds:

• energy must be applied to break existing bonds - so bond breaking is an endothermic process
• energy is released when new bonds are formed - so bond formation is an exothermic process

Bond energy calculations:
Overall energy change = energy required to break bonds - energy released by forming bonds

Question 4

Cells and batteries

Answer 4

Chemical reactions in a cell produce electricity:
An electrochemical cell is a basic system made up of two different electrodes in contact with an electrolyte. The two electrodes are usually metals ( conduct electricity ). Electrolyte is a liquid containing ions which react with the electrodes. This reaction sets up a charge difference between the electrodes. If connected by wire the charge is able to flow and electricity is produced

The voltage of a cell depends on many factors :

• different metals will react differently with the same electrolyte - causing the charge difference
• the bigger the difference in reactivity between the electrodes the bigger the voltage of the cell
• the electrolyte ( different ions in solution will react differently with metal electrodes)
• a battery can be formed by adding cells in series ( the voltages of the cells are combined increasing the voltage)

Non rechargeable batteries:
Chemical reaction at electrodes is irreversible e.g alkaline battery ( one of the reactants gets completely used up
Rechargeable batteries: reaction can be reversed by connecting to external electric circuit

Question 5

Fuel Cells

Answer 5

Fuel cells use fuel and oxygen to produce electrical energy
When fuel enters cells it becomes oxidised and sets up a potential difference within the cell

Hydrogen-Oxygen fuel cell involve a redox reaction:
The electrolyte - solution of potassium hydroxide
The electrodes - porous Carbon with a catalyst
Hydrogen goes into anode compartment
Oxygen goes in cathode compartment
At the negative electrode hydrogen loses electrons to form H+ ions ( oxidation )
At the positive electrode oxygen gains electrons from the cathode and reacts with H+ ions from the electrolyte to make water ( reduction ).
Overall reaction - hydrogen + oxygen > water

Hydrogen-Oxygen fuel cells could be used in vehicles

• don’t produce as many pollutants as other fuels
• batteries store less energy than fuel cells
• electric vehicles have to dispose of their batteries
• a limit to how many times you can recharge an electric battery