Energy Changes

exothermic reactions give energy out to the surroundings.

• energy is transferred from the chemical store of the reactants to the thermal energy store of the products - the products thus has less chemical energy.

• final temperature of surroudings is higher than start temperature

• combustion, neutralisation, most oxidation

endothermic reactions take heat in from the surroundings.

• energy is transferred from the thermal store of the reactants to the chemical energy store of the products - the products thus has more chemical energy.

• final temperature of surroundings is lower than start temperature

• thermal decomposition, citric acid + sodium hydrogencarbonate

these are reaction profiles. they show the relative energies of reactants and products.

the unit for energy is KJ/mol

activation energy is the minimum amount of energy needed for a reaction to occur. make sure when you draw reaction profiles the arrow for it touches the top of the curve.

enthalpy/energy change is the amount of heat energy released/gained in a reaction - in other words, it’s the difference in energy between products and reactants.

Bonds

each covalent bond has a different bond energy - amount of energy needed to make/break a bond.

bond breaking reactions are endothermic → energy must be supplied to break bonds

bond making reactions are exothermic → energy released when bonds form

the amount of energy needed to break a bond is the same as the amount needed to make it.

enthalpy change = reactants - products

negative for exothermic - thermal energy released forming new bonds (products) larger than energy needed to break bonds (reactant).

positive for endothermic - energy needed to break bonds (reactants) is greater than energy to make them (products)

you need to be able to perform bond energy calculations given bond energies. remember to count every bond an element has.

Required practical: investigating energy changes

Method

1. place a polystyrene cup in a beaker for support

2. measure 30cm³ of HCl using a measuring cylinder, and pour this into the cup.

3. put the thermometer in the cup. hold it at all time so it doesn’t tip and break

4. measure the temperature of the HCl and record this in a table

5. add sodium hydroxide in 5cm³ portions, measure the temperature of the mixture and record in a table. repeat until you’ve added 40cm³ total.

6. repeat steps 2-3 3 times to make the results more reliable, and find the average temperature

7. plot the average temperature on a graph. draw two lines of best fit, one with a positive gradient and one with a negative gradient. where they cross is the exact point of neutralisation.

Cells and batteries

cells contain chemicals that react to produce electricity. you can make a simple cell by connecting two different metals in contact with an electrolyte.

potential difference produced by a cell is dependent on a number of factors, including the electrodes and electrolytes

you don’t need to know exactly how cells work. just know that electrons (current) flow from the more reactive metal electrode to the less reactive one through a wire.

oxidation of electrolyte at more reactive metal electrode → electrons

difference in reactivity between the electrodes affects potential difference (larger difference = greater potential difference)

batteries consist of two or more cells connected together in series to provide a greater voltage.

when cells stop working, it’s usually because they are non-reversible and have used up all of one reactant - the electrodes literally increase and decrease in size due to the oxidation and reduction - so there’s no flow of electrons that causes things to work. alkaline batteries are non-reversible

rechargeable cells, on the other hand, can be recharged because their chemical reactions are reversed when an external electrical current is applied.

Fuel cells

fuel cells are supplied by an external source of fuel (commonly hydrogen), oxygen and air.

the fuel gets oxidised electrochemically within the fuel cell, which creates a potential difference that drives charge around the cell. at the anode, hydrogen is oxidised. at the cathode, oxygen and hydrogen are reduced to produce water. you need to know the half equation for this:

• H₂ → 2e^- + 2H⁺

• O₂ + 4H⁺ + 4e^- → 2H₂O

• that simplifies to O₂ + 2H₂→ 2H₂O when you cancel out the electrons and the hydrogen ions

hydrogen fuel cells are a potential alternative for rechargeable cells and batteries in cars, for example.