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In today's T-Level Chemistry lesson, we are going to study A twelve point three, Enthalpy Changes.

By the end of this lesson, you should be able to define H and delta H, describe energy conservation in exothermic and endothermic reactions, read enthalpy diagrams, and explain what a catalyst does and does not change.

Enthalpy, H, is the energy content of a system at constant pressure. It is a state quantity. On a diagram you compare the reactant level with the product level.

The enthalpy change, delta H, is H of the products minus H of the reactants. It can be positive or negative. It is commonly quoted in kilojoules per mole.

H and delta H are not interchangeable. Delta H is not simply the energy of the products.

Energy is conserved. It is not created or destroyed. It is transferred between the reacting system and the surroundings.

In an exothermic change, energy is transferred from the system to the surroundings. Products have lower enthalpy than reactants, so delta H is negative. The surroundings often feel warmer.

Watch the exothermic profile. Reactants sit higher. Products sit lower. The pathway rises to a peak. Activation energy is the energy from the reactant level to that peak, not from zero on the y-axis. Delta H is the downward difference from reactants to products.

In an endothermic change, energy is transferred from the surroundings into the system. Products have higher enthalpy than reactants, so delta H is positive. The surroundings often feel cooler.

Watch the endothermic profile. Products sit higher. Activation energy is still from the reactant level to the peak. Delta H is the upward difference.

A reaction that transfers forty five kilojoules to the surroundings is exothermic, so delta H is negative.

An exothermic reaction still needs activation energy. A negative delta H does not mean the reaction starts with no barrier.

A catalyst provides an alternative pathway with a lower activation energy. Watch both pathways on one diagram.

The catalysed peak is lower. The reactant enthalpy and the product enthalpy stay the same, so delta H is unchanged.

Correct the misconceptions. Exothermic still needs E a. A catalyst lowers E a, not delta H. Endothermic does not destroy energy. Negative delta H does not mean negative energy. And delta H is not always positive.

If a diagram shows an exothermic reaction with products above reactants, the levels are the wrong way round. If an endothermic reaction is labelled delta H less than zero, the sign is wrong. If a catalyst is drawn with a lower product enthalpy, only the peak should have changed.

Examination-style question.

Part a. Define delta H. Two marks.

Delta H is the change in enthalpy between products and reactants. Delta H equals H products minus H reactants.

Part b. How does an exothermic enthalpy diagram show the energy change? Two marks.

Products are lower in enthalpy than reactants. Energy is transferred to the surroundings, so the difference is negative.

Part c. How does an endothermic change appear? Two marks.

Delta H is positive. Energy is transferred from the surroundings into the system.

Part d. Explain the effect of a catalyst on the profile. Three marks.

The catalyst provides a lower E a pathway. Reactant and product levels are unchanged, so delta H is unchanged.

Before we finish, complete the model. H is the energy content of a state. Delta H is the change between states. Exothermic, delta H negative. Endothermic, delta H positive. A catalyst changes the pathway peak, not reactant H, product H, or delta H.

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