Chemistry - energy changes

What are exothermic reactions?

Exothermic reactions are reactions that transfer energy to their surroundings (usually as heat or sometimes light)

Why do the reactants have more energy than the products in an exothermic reaction?

more energy is released forming new bonds in the products than is absorbed to break the original reactant bonds

What happens to the temperature of the surroundings in an exothermic reaction?

it increases

What happens when bonds break?

energy is absorbed (endothermic)

What happens when bonds form?

energy is released (exothermic)

Examples of exothermic reactions

- Combustion

- Neutralisation

- respiration

- forming new bonds

Uses of exothermic reactions

Self-heating cans, hand warmers, burning fuels

What is the energy profile diagram for an exothermic reaction

What is the energy transfer in exothermic reactions?

The chemical energy store decreases, and the thermal energy of the surroundings increases

What are endothermic reactions?

Endothermic reactions are reactions that take in energy from their surroundings

What is activation energy?

energy needed to start a reaction

What happens when reactants have less energy than the activation energy?

They will just bounce off eachother

Why do the products have more energy than the reactants in an endothermic reaction?

more energy is absorbed breaking old bonds in the reactants than is released to make new bonds

Why do endothermic reactions absorb energy?

the energy required to break the bonds in the reactants is greater than the energy released when new bonds form in the products

Why do exothermic reactions release energy?

because the total chemical energy stored in the bonds of the products is less than the energy stored in the bonds of the reactants

What happens to the temperature of the surroundings in an endothermic reaction?

it decreases

Examples of endothermic reactions

- Thermal decomposition

- photosynthesis

Uses of endothermic reactions

ice packs

What is the energy profile diagram for an endothermic reaction

What is the energy transfer in endothermic reactions?

energy is transferred from the surroundings (such as heat or light) to the chemical system

What is bond energy?

energy required to break a bond

What is bond energy measured in?

kJ/mol

Formula for the energy change of a reaction

Overall energy change = energy put in - energy taken out

- if the overall energy change is negative, the reaction is exothermic

- if the overall energy change is positive, the reaction s exothermic

What are chemical cells?

a device that produces electricity from a chemical reaction

What are the components of a chemical cell?

- Two different metal electrodes

- An electrolyte

How do chemical cells work?

1. The negative anode loses electrons

2. The positive cathode gains electrons

3. Electrons flow through the external circuit from one electrode to the other

What is the anode in chemical cells?

The more reactive metal loses electrons (oxidation) and becomes the negative terminal

What is the cathode in chemical cells?

The less reactive metal receives electrons (reduction) and acts as the positive terminal.

What is the purpose of the electrolyte?

allows ions to move freely between electrodes, completing the circuit and enabling the redox reactions that produce electrical energy

What kind of reactions do chemical cells use?

convert chemical energy into electrical energy using redox reactions

What happens when there is a high difference in reactivity of the electrodes in a chemical cell?

it produces a higher voltage

How do rechargeable batteries work?

- they use reversible reactions

- A current is supplied, which forces electrons to flow in the opposite direction around the circuit and reverses the reaction.

How do chemical cells generate voltage?

The chemical reactions at the electrodes create a difference in charge between them. This charge difference generates an electric current when the electrodes are connected by a wire

How can chemical cells generate a higher voltage?

multiple cells can be connected in series, forming a battery. When connected in series, the total voltage of the battery is the sum of the voltages of the individual cells.

What are non-rechargeable batteries used for?

remote controls and toys

What are rechargeable batteries used for?

smartphones, tablets, and electric vehicles.

What is a fuel cell?

An electrical cell that is supplied with a fuel and oxygen, and uses energy from the reaction between them to produce electrical energy efficiently.

What is the product of hydrogen fuel cells?

water

What is the reaction at the cathode of a hydrogen fuel cell?

- reduction of oxygen

- O₂ + 4H⁺ + 4e⁻→2H₂O

What is the reaction at the anode of a hydrogen fuel cell?

- Hydrogen gas is split into protons and electrons

- H₂ → 2H⁺ + 2e⁻

What is the anode in hydrogen fuel cells?

the negative electrode where hydrogen gas is introduced and oxidized, splitting into protons and electrons

What is the cathode in hydrogen fuel cells?

the positive electrode where oxygen is reduced, combining with protons and electrons to produce water

How do hydrogen fuel cells work?

1. Hydrogen gas (H₂) goes into the anode. It is split into H⁺ ions and electrons

2. The electrons cannot pass through the electrolyte, so they travel through an external wire. This flow of electrons creates an electric current

3. The H⁺ ions pass through the electrolyte to the other side

4. Oxygen enters the cathode. It reacts with the H⁺ ions and returning electrons to form water

What are the advantages of hydrogen fuel cells?

- Continuous supply (doesn't need recharging like a battery)

- no pollutants are produced

- lasts longer than batteries

What are the disadvantages of hydrogen fuel cells?

- Hydrogen is flammable

- Storing and transporting hydrogen is difficult

- Producing hydrogen can require lots of energy (and may use fossil fuels)