Presentation is loading. Please wait.

Presentation is loading. Please wait.

Energy changes in chemical reactions. When the head of a match is struck, energy is released. Some of this energy heats the surroundings and some is released.

Similar presentations


Presentation on theme: "Energy changes in chemical reactions. When the head of a match is struck, energy is released. Some of this energy heats the surroundings and some is released."— Presentation transcript:

1 Energy changes in chemical reactions

2 When the head of a match is struck, energy is released. Some of this energy heats the surroundings and some is released as light.

3 Energy is released in chemical reactions when the reactants have more chemical energy than the products.

4 The chemical energy of a substance is the potential energy stored within it. This stored chemical potential energy is called the heat content or enthalpy of the substance and is given the symbol H.

5 System and surroundings System – the collection of atoms or molecules involved in a chemical reaction. Surroundings – anything around, or in contact with the system eg air, or the solvent.

6 System and surroundings If the enthalpy of a system decreases during a chemical reaction, then that same amount of energy must be released to the surroundings. This will cause the temperature of the surroundings to increase.

7 System and surroundings Note that the enthalpy is potential energy in the system, but it is transferred to the surroundings as heat.

8 What will happen to the temperature of the surroundings in each of these cases?

9 Endothermic reaction  H positive heat absorbed from surroundings, causing a drop in the temperature of the surroundings Exothermic reaction  H negative heat released to the surroundings, causing an increase in the temperature of the surroundings

10 Ba(OH) 2 (s) + 2 NH 4 SCN (s)  Ba(SCN) 2(s) + 2 NH 3(g) + 2 H 2 O (l) This reaction is endothermic. The system gains enthalpy, and the surroundings lose heat.

11 The change in enthalpy can be shown in the equation; or written after the equation;


Download ppt "Energy changes in chemical reactions. When the head of a match is struck, energy is released. Some of this energy heats the surroundings and some is released."

Similar presentations


Ads by Google