Energy changes and chemical changes

The beaker becomes warm, meaning the temperature of the surroundings (the beaker and its contents) has increased.

When the surroundings get hotter, energy is being released from the chemical reaction into the surroundings. A reaction that releases energy to the surroundings is called an exothermic reaction.

The reaction is exothermic — energy is released to the surroundings, causing the temperature to rise. The prefix exo means 'out', reminding us that energy exits the reaction.

We need to interpret a temperature increase in the surroundings and link it to the correct type of reaction.

Eliminate 'endothermic, absorbing energy from the surroundings' — endothermic reactions make the surroundings get colder, not warmer. The beaker is warm, so this is wrong.

Eliminate 'the reaction has not yet started' — a temperature change is itself strong evidence that a chemical reaction is occurring. Also, a colour change is not required for a reaction to be happening.

Eliminate 'reactants have more stored energy than the products did before the reaction' — while this is true for exothermic reactions, it does not explain the temperature change and is not what the observation directly tells us.

The only correct answer is 'the reaction is exothermic, releasing energy to the surroundings' — a warm beaker means energy is being released, which is the definition of an exothermic reaction.

Energy is absorbed when bonds break, and energy is released when bonds form. The type of reaction depends on which process requires more energy.

For an endothermic reaction, the energy absorbed to break the bonds in the reactants is greater than the energy released when new bonds form in the products. The reaction must draw extra energy from the surroundings.

For an exothermic reaction, the opposite is true — the energy released forming bonds in the products is greater than the energy needed to break bonds in the reactants, giving a net release of energy.

The correct answer is: 'when the energy needed to break bonds in the reactants is greater than the energy released when new bonds form in the products' — this describes the energy balance for an endothermic reaction.

We need to identify the bond energy condition that makes a reaction endothermic (absorbs energy from surroundings, making them cooler).

Eliminate 'energy released forming bonds in products is greater than energy needed to break bonds in reactants' — this describes an exothermic reaction, not endothermic.

Eliminate 'activation energy is very high compared to energy released' — activation energy is the energy needed to start a reaction; it does not determine whether a reaction is exothermic or endothermic overall.

Eliminate 'no bonds are broken and only new bonds are formed' — every chemical reaction involves both bond breaking and bond forming. This is not a real scenario.

The correct answer is that the energy to break reactant bonds is greater than the energy released forming product bonds — the reaction must absorb extra energy from the surroundings, making it endothermic.