Energy Changes

Describe the signs that show a chemical reaction has taken place.

Colour change, temperature change, gas given off (bubbling/fizzing), precipitate (solid) formed

State the law of conservation of mass.

The total mass of reactants is equal to the total mass of products. 

Describe what happens to atoms in a chemical reaction.

Atoms are not created or destroyed, they are only rearranged to form a new substance.

exothermic reaction meaning

An exothermic reaction is one that transfers energy to the surroundings. 

what temperature change does an exothermic reaction result in?

Exothermic reactions result in a temperature increase in the surroundings.

examples of exothermic reactions

Combustion, neutralisation and many oxidation reactions are exothermic reactions.

Exothermic reactions can also be used in real life, in handwarmers and self-heating cans.

endothermic reaction meaning

An endothermic reaction is one that takes in energy from the surroundings. 

what temperature change does an endothermic reaction result in?

Endothermic reactions result in a temperature decrease in the surroundings. 

examples of endothermic reactions

Endothermic reactions include thermal decompositions and the reaction of citric acid and sodium hydrogencarbonate

Endothermic reactions can also be used in real life, in sports injury packs

is photosynthesis an exothermic or endothermic reaction? & why

Endothermic

• The bonds in glucose store a lot of chemical energy, so building them needs an input of energy.

• Chlorophyll absorbs light energy and uses it to drive the reactions.

• Photosynthesis is endothermic because the energy needed to make glucose is greater than the energy released when bonds in CO₂ and H₂O are broken.

• Photosynthesis is endothermic because it requires energy from light to convert low‑energy molecules into high‑energy glucose.

is aerobic respiration an exothermic or endothermic reaction? & why

exothermic

• When glucose is broken down, the products (CO₂ and H₂O) have very stable bonds. Forming these stable bonds releases energy.

• The energy released is transferred to ATP, which cells use for movement, active transport, protein synthesis, etc.

• Aerobic respiration is exothermic because more energy is released when new bonds form in the products than is required to break the bonds in the reactants.

A chemical reaction can be identified by…

 a colour change, a solid being formed, a gas being given off or a temperature change

For a chemical reaction to occur, the particles of the reactants have to…

collide with each other with enough energy, otherwise the reaction cannot occur

activation energy meaning

The minimum amount of energy that particles need to have in order to react

catalyst meaning

a substance that speeds up a chemical reaction without being used up

Catalysts & activation energy

Catalysts work by providing an alternative reaction pathway, with a lower activation energy, so that more particles are able to react with each other in a given time.

Energy Level Diagram - Exothermic

In an exothermic reaction, energy is transferred to the surroundings. 

This means the products have less energy than the reactants:

Energy Level Diagram - Endothermic

In an endothermic reaction, energy is transferred from the surroundings. 

This means the products have more energy than the reactants:

the reaction profile for an exothermic reaction

the reaction profile for an endothermic reaction

Activation Energy and Catalysts on a reaction profile

Suggest what it means for a reaction to be ‘highly exothermic’

The reaction transfers a lot of energy to the surroundings.

When chemical bonds are broken, energy is…

needed

When chemical bonds are made, energy is…

released

The process of breaking chemical bonds is …thermic as it…

endothermic , requires energy to be taken in from the surroundings

The process of making bonds is ….thermic, which

exo, releases energy

A reaction is exothermic overall if…

the amount of energy released when making the new bonds in the products is greater than the amount of energy that was taken in to break the bonds in the reactants.

A reaction is endothermic overall if…

the amount of energy released when making the new bonds in the products is less than the amount of energy that was taken in to break the bonds in the reactants. 

bond energy meaning

A bond energy is the amount of energy needed to break up one mole of a given covalent bond.  

overall energy change of the reaction meaning

The difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed