Reaction profile
Breaking and making bonds
The capacity of a system to do work or the quantity required for mechanical work to take place. Measured in joules (J). For example, a man transfers 100 J of energy when moving a wheelbarrow. is transferred when The chemical link that holds molecules together. are broken or are formed. During a chemical reaction:
- energy is taken in to break bonds in the A substance that reacts together with another substance to form products during a chemical reaction.
- energy is released when new bonds are made in the A substance formed in a chemical reaction.
The difference in the amounts of energy involved determines the type of reaction. A reaction is:
- Reaction in which energy is given out to the surroundings. The surroundings then have more energy than they started with so the temperature increases. - if more energy is released in forming bonds in the products than is taken in when breaking bonds in the reactants
- Reaction in which energy is taken in. - if less energy is released in forming bonds in the products than is taken in when breaking bonds in the reactant
Reaction profile diagrams
A Chart showing how the energy of reactants and products changes during a reaction. shows how the The capacity of a system to do work or the quantity required for mechanical work to take place. Measured in joules (J). For example, a man transfers 100 J of energy when moving a wheelbarrow. in the A substance that reacts together with another substance to form products during a chemical reaction. and A substance formed in a chemical reaction. changes during a reaction. It includes the The minimum energy required for a chemical reaction to take place., the minimum energy needed for a reaction to start. The activation energy is shown as a 鈥榟ump鈥 in the line, which:
- starts at the energy of the reactants
- is equal to the difference in energy between the top of the 鈥榟ump鈥 and the reactants
The overall change in energy in a reaction is the difference between the energy of the reactants and products.
Exothermic reactions
The diagram shows a reaction profile for an exothermic reaction.
Question
Describe how you can tell from a reaction profile that a reaction is exothermic.
In the reaction for an exothermic reaction, the overall energy change is negative. You can tell this because the products have less energy than the reactants, and the arrow showing the overall change in energy points downwards.
Endothermic reactions
The diagram shows a reaction profile for an endothermic reaction.
Question
Describe how you can tell from a reaction profile that a reaction is endothermic.
In the reaction for an endothermic reaction, the overall energy change is positive. You can tell this because the products have more energy than the reactants, and the arrow showing the overall change in energy points upwards.