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6.02 ChemLive Exothermic and Exothermic Reactions Exothermic Reactions • Exothermic chemical reactions cause their surroundings to warm up by giving off heat. • Chemical bonds are formed during exothermic reactions. • Exothermic reactions are usually spontaneous. – This is a reaction that takes place on its own without an external force and another reaction needed to drive it. • Example: Lighting a match. When the head of the match is struck, it results in the spontaneous release of stored energy (heat) from the reactants into the surroundings. Potential Energy Diagram-Exothermic • The reactants contain more stored energy than the product because energy from external sources is not required, but given off. This gives the product more stability due to the lower amount of energy needed. Endothermic Reactions • Energy is drawn in from the external environment, causing its surroundings to loose heat, or "cool down." • Chemical bonds are broken with endothermic reactions. • Most endothermic reactions are not spontaneous. – A non spontaneous reaction is a reaction that doesn't occur naturally and it normally tends to favor the reactants of a chemical reaction. • Example: Photosynthesis – sunlight + 6CO2(g) + H2O(l) = C6H12O6(aq) + 6O2(g) Potential Energy Diagram- Endothermic • The endothermic chemical reaction creates a product that has a higher energy level than the original materials, causing the reactant's stored energy to decrease. Exothermic/Endothermic Examples Exothermic Reactions Endothermic Reactions rusting iron cooking an egg condensation of rain from water vapor evaporation of water formation of snow in clouds conversion of frost to water vapor making ice cubes melting ice cubes burning sugar producing sugar by photosynthesis Measuring Heat reaction reaction Exothermic reaction, heat given off & temperature of water rises Endothermic reaction, heat taken in & temperature of water drops 6.02 Endothermic and Exothermic lab activity • Hands-on Assignment Choices –Choice One: Steel Wool and Vinegar –Choice Two: Baking Soda and Vinegar Choice One: Steel Wool and Vinegar • Materials: – – – – one piece of steel wool vinegar thermometer (if you have one) glass jar with lid, or disposable plastic cup (not polystyrene—also referred to as Styrofoam™) with lid/plastic wrap – newspaper (Use it to cover the countertop next to the kitchen sink. This could get a little messy!) Choice One: Steel Wool and Vinegar • Procedure: – Place the steel wool in a jar or cup and cover with vinegar. – Let the steel wool soak in the vinegar for one minute. – If you have a thermometer, measure the temperature of the vinegar and steel wool. – Squeeze the vinegar out of the steel wool and pour the vinegar from the jar into the sink. – Place the steel wool immediately back into the jar/cup and seal the lid. You should be able to see it start rusting over the next few minutes. – Place your hands on the outside of the container after sealing the lid and record any temperature change that you observe. – When you think the temperature change is complete, open the jar. If you have a thermometer, measure the temperature of the steel wool. – When you are done making observations, it is safe to dispose of the steel wool in the trash. Be sure to wash the glass jar and thermometer before putting them away. Choice One: Steel Wool and Vinegar Observations to help you with your 6.02 quiz that follows the activity: 1.What was the initial temperature in the jar______ degrees Celsius. 2.What was the temperature after the reaction? _____degrees Celsius. 3.What was the change in temperature? Increase or decrease? 4.Based on information from Part I and from observations, was this reaction Exothermic or Endothermic? An increase would be exothermic. A decrease would be endothermic 5.Are chemical bonds forming or breaking? Observation- What do you see happening? 6.In this experiment, the acidic vinegar dissolved a protective coating off the steel wool, allowing the oxidation of the steel to begin. This process is called rusting. In this reaction, iron (III) is combined with oxygen gas to form iron oxide. Write the balanced chemical reaction for the rusting of iron. __Fe+__O2 __FeO3 Choice Two: Baking Soda and Vinegar • Materials: – one disposable plastic (not polystyrene—also referred to as Styrofoam™) cup – one disposable plastic spoon (plastic fork or knife may be used) – thermometer (if you have one) – measuring cup and spoon – newspaper (Use it to cover the countertop next to the kitchen sink. This could get a little messy!) – baking soda – vinegar Choice Two: Baking Soda and Vinegar • Procedure – Measure two tablespoons of baking soda and pour it into the clean, dry plastic cup. – Measure out one cup of vinegar. – If you have a thermometer, place it in the measuring cup of liquid and write down the initial temperature. – Grasping the plastic cup near the bottom with one hand, pour the liquid into the plastic cup with your other hand. – Continue to grasp the cup as you stir the solution gently with the plastic spoon. Observe any temperature change felt through the plastic cup. – If you have a thermometer, record the temperature change as you stir the solution very gently. – When you are done making observations, it is safe to pour the solution down the sink and to dispose of the plastic cup and spoon in the trash. Be sure to wash the measuring spoon, measuring cup, and thermometer before putting them away. Choice Two: Baking Soda and Vinegar Observations to help you with your 6.02 quiz that follows the activity: 1.What was the initial temperature in the jar of vinegar?______ degrees Celsius. 2.What was the temperature after the reaction with baking soda? _____degrees Celsius. 3.What was the change in temperature? Increase or decrease? 4.Based on information from Part I and from observations, was this reaction Exothermic or Endothermic? An increase would be exothermic. A decrease would be endothermic 5.Are chemical bonds forming or breaking? Observation- What do you see happening? 6.In this experiment, the acidic vinegar reacted with baking soda producing sodium acetate and carbonic acid. The carbonic acid quickly decomposes into carbon dioxide and water. The chemical reactions involved: HC2H3O2 + NaHCO3 ===> NaC2H3O2 + H2CO3 H2CO3 ---> H2O + CO2 •You are ready to take the 6.02 quiz