C5: Energy Changes

Is energy conserved in chemical reactions?

Yes - the total amount of energy in the universe is the same before and after a chemical reaction.

Define an exothermic reaction.

A reaction that transfers energy to the surroundings, causing the temperature of the surroundings to increase.

Define an endothermic reaction.

A reaction that takes in energy from the surroundings, causing the temperature of the surroundings to decrease.

How can you tell if a reaction is exothermic from a temperature measurement?

The temperature of the reaction mixture/surroundings increases during an exothermic reaction.

How can you tell if a reaction is endothermic from a temperature measurement?

The temperature of the reaction mixture/surroundings decreases during an endothermic reaction.

Give three examples of exothermic reactions.

Combustion, many oxidation reactions, neutralisation reactions (any three).

Give two everyday uses of exothermic reactions.

Self-heating cans (e.g. self-heating coffee) and hand warmers.

Give two examples of endothermic reactions.

Thermal decomposition; the reaction between citric acid and sodium hydrogencarbonate.

Give one everyday use of endothermic reactions.

Sports injury cold packs / instant cold packs.

How would you measure temperature change in a reacting solution?

Use a polystyrene cup (insulated) as a calorimeter, add reagents, stir and record temperature every 30 seconds using a thermometer or temperature probe; note the maximum or minimum temperature reached.

What is activation energy?

The minimum amount of energy that reacting particles must have to react (to start a chemical reaction).

What does a reaction profile (energy level diagram) show?

The relative energies of reactants and products, the activation energy, and the overall energy change of the reaction, with a curved line showing the energy during the reaction.

On a reaction profile for an exothermic reaction, are the products at higher or lower energy than the reactants?

Lower - energy is released to the surroundings, so products have less energy than reactants.

On a reaction profile for an endothermic reaction, are the products at higher or lower energy than the reactants?

Higher - energy is taken in from the surroundings, so products have more energy than reactants.

On a reaction profile, where is the activation energy shown?

It is the difference in energy between the reactants and the peak (transition state) of the curve.

How do you identify a reaction as exothermic from its reaction profile?

The products are lower in energy than the reactants; the overall energy change is negative (energy is released).

How do you identify a reaction as endothermic from its reaction profile?

The products are higher in energy than the reactants; the overall energy change is positive (energy is taken in).

[HT] What happens to chemical bonds during a reaction?

Bonds in the reactants are broken (energy is absorbed) and new bonds in the products are formed (energy is released).

[HT] Is bond breaking exothermic or endothermic?

Endothermic - energy must be supplied to break bonds.

[HT] Is bond forming exothermic or endothermic?

Exothermic - energy is released when bonds form.

[HT] How do you calculate the overall energy change of a reaction using bond energies?

Overall energy change = energy needed to break bonds in reactants − energy released when bonds form in products.

[HT] If energy released by bond forming > energy needed to break bonds, is the reaction exothermic or endothermic?

Exothermic - more energy is released than absorbed, so energy is transferred to the surroundings.

[HT] If energy needed to break bonds > energy released by bond forming, is the reaction exothermic or endothermic?

Endothermic - more energy is absorbed than released, so energy is taken in from the surroundings.

[HT] Using bond energies: H₂ + Cl₂ → 2HCl. Bond energies: H-H = 436 kJ/mol, Cl-Cl = 242 kJ/mol, H-Cl = 431 kJ/mol. Calculate the energy change.

Energy in (breaking): 436 + 242 = 678 kJ. Energy out (forming): 2 × 431 = 862 kJ. Energy change = 678 − 862 = −184 kJ (exothermic).

[HT] A bond energy calculation gives an energy change of +150 kJ/mol. What does the positive sign indicate?

The reaction is endothermic - energy is absorbed from the surroundings.

[CO] What is a chemical cell?

A device that uses a chemical reaction to produce electricity.

[CO] What factors affect the voltage produced by a simple cell?

The types of electrode (metals used) and the type of electrolyte used.

[CO] How is a simple cell made?

By connecting two different metals in contact with an electrolyte; the greater the difference in reactivity of the metals, the greater the voltage produced.

[CO] What is a battery?

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

[CO] What is the difference between a non-rechargeable and a rechargeable cell/battery?

In a non-rechargeable cell, the chemical reaction stops when one reactant is used up and cannot be reversed. In a rechargeable cell, the reaction is reversible by applying an external electrical current.

[CO] Give an example of a non-rechargeable battery.

An alkaline battery (e.g. a standard AA cell).

[CO] What is a fuel cell?

An electrochemical cell that uses a continuous supply of fuel (e.g. hydrogen) and oxygen to produce electricity; the fuel is oxidised electrochemically.

[CO] What is the overall reaction in a hydrogen fuel cell?

Hydrogen is oxidised by oxygen to produce water. (2H₂ + O₂ → 2H₂O)

[CO] Give one advantage of hydrogen fuel cells over rechargeable batteries.

Hydrogen fuel cells produce only water as a by-product and can run continuously as long as fuel is supplied, without needing recharging downtime.