Chemical Reactions: Fast and Slow Interpreting data
The table below shows the temperature changes that take place when substances are dissolved in water. Copy the table and fill in if the reaction is endothermic or exothermic.
Name of substance to be dissolved in water
Potassium chloride Calcium chloride
Sodium hydrogencarbonate Sodium carbonate
Starting
temperature (°C) 25 25 25 25
Energy Profile Diagrams
Energy transfers in chemical reactions are shown in energy profile diagrams. These diagrams show the energy stored in the reactants compared to the energy stored in the products.
Exothermic Energy Reactants Products Energy Endothermic Products
Temperature after dissolving (°C)
12 33 15 29
Endothermic or exothermic reaction?
12.2
Reactants Time z Fig. 12.2.4 z Fig. 12.2.5
Figure 12.2.4 shows the energy profile diagram for an exothermic reaction. Notice that the products have less energy than the reactants. This is because energy has been given out.
Figure 12.2.5 shows the energy profile diagram for an endothermic reaction. Notice that the products have more energy than the reactants. This is because the extra energy needed to form the products has been taken in from the surroundings.
Endothermic or exothermic?
Carry out an investigation to find out if the following reactions are exothermic or endothermic: zz zz zz
Sodium hydroxide and dilute hydrochloric acid Sodium hydrogencarbonate and citric acid Calcium chloride and water
zz zz zz
Calcium chloride and baking soda and water Vinegar and sodium carbonate Water and Epsom salts
1. Some students suggest that a polystyrene cup should be used when the reactants have been mixed together, rather than a beaker. Why would a polystyrene cup allow for more accurate results?
2. Should you consider putting a lid on the polystyrene cup? Why?
3. As you are unsure of how each of these substances will react with each other, how can this investigation be carried out safely?
4. Draw the results table you will use to collect information during this investigation. 207 Time
