Combustion reactions are exothermic. They release energy, and heat up the surroundings.

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Presentation transcript:

Combustion reactions are exothermic. They release energy, and heat up the surroundings.

When we study enthalpy changes in reactions like this, the reactants and products are the system.

Everything else, including the bench top, air, Bunsen burner, thermometer and hand are the surroundings.

In an exothermic reaction, the enthalpy of the system decreases.

In an endothermic reaction, the enthalpy of the system increases.

Enthalpy changes vary according to the conditions.

This symbol means an enthalpy change under standard conditions.

298 K or 25 ºC. Sometimes other temperatures are specified.

Standard pressure is 1 atmosphere or 1.01 x 10 5 Nm −2.

Solutions have a standard concentration of 1 mol dm −3.

A stated temperature, usually 298 K, and 1 atmosphere pressure.

What is the standard state of H 2 O?

It is not a solid or a gas under standard conditions.

The standard state of H 2 O is liquid.

The standard enthalpy of formation of an element in its standard state is zero.

The standard state of hydrogen, H 2, is a gas.

The standard state of oxygen, O 2, is a gas.

Hydrogen and oxygen react together to make water.

The standard state of water is liquid. This is the equation for the enthalpy of formation of water.

Enthalpies of formation can be negative or positive.

Standard enthalpies of combustion always have negative values, as combustion is an exothermic reaction.

This is a bomb calorimeter, which is used for making accurate measurements of energy changes.

Glucose will burn completely in excess oxygen.

The glucose is ignited electrically.

It burns completely to form carbon dioxide and water.

Corrections would be made to the final enthalpy value: the conditions are not standard and the pressure is not constant.