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CHEM 012 Chapter Notes - Chapter 4: Iron(Ii) Sulfate, Green Leaves, Calcium Carbonate

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22 Mar 2016

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In a physical change, the substances present remain chemically the same: no new substances are formed. Any mixtures produced are usually easy to separate. The major feature of a chemical change, or reaction, is that new substance(s) are made during the reaction. Many reactions, but not all of them, are difficult to reserve. During a chemical reaction, energy can be given out or taken in: when energy is given out, the reaction is exothermic, when the energy is taken in, the reaction is endothermic. There are many mire exothermic reactions then endothermic reactions. Synthesis (or direct combination) reaction occur where two or more substances react together to form just one product: iron + sulphur iron (ii) sulphate. However, there is one very important synthesis reaction, which is endothermic: namely photosyntheses. This reaction is essential for life on earth. It takes place in green leaves of plants and requires energy from sunlight.

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Chemical Reactions and Equations Learning objectives To observe physical and chemical properties associated with chemical changes To be able to give evidence for the occurrence of a chemical reaction To write a balanced chemical equation To identify areaction as a combination, decomposition, precipitation, single or double replacement, or combustion reaction ntroduction When a substance undergoes a physical change, it changes its appearance but not its composition. For example, when ice melts to form water, it undergoes a physical change from solid to liquid. In a chemical change, the composition of a substance is changed to give a new substance with different physical and chemical properties. For example, when silver tarnishes, the shiny silver (Ag) changes to a dull gray silver sulfide (AgaS). Not only has the chemical formula changed from Ag to Agus, but also the physical properties (shininess) of the substance have changed. Some examples of physical and chemical changes are listed in Table 1 below. Table 1. Comparison of Physical and Chemical Changes Examples of Physical changes Examples and Evidence of Chemical Changes Change in state (melting, freezing, evaporating Formation of a gas (bubbles) subliming) Formation of a solid (precipitate) Change in size Tearing Change in Color Heat is given off or absorbed A solid dissolving in a solvent In a chemical reaction, atoms in the reactants are rearranged to produce new combinations of atoms in the products. However, the total number of atoms of each element in the reactants is equal to the total number of atoms in the products. This is true according to the Law of Conservation of Mass, which states that the total mass remains constant during a chemical reaction. In other words, matter cannot be created or destroyed in a chemical reaction. In a chemical equation, the reactants are shown on the left and the products are on the right. An arrow in between indicates that a chemical reaction occurs. Finally, the physical state of each compound is designated in parentheses after the chemical formula. Solid, liquid, gas, and aqueous (dissolved in water) are shown as(s,(n,(g), and (aq), respectively.

it is relatively easy to change the temperature of a substance tha

Experiment 11 ENTHALPY OF REACTION (HESS'S LAW) IDEAS Both the physical and biological worlds are characterized by a complex interplay of chemical reactions. In all chemical reactions, heat is released or absorbed. Combustion is one of the commonly observed chemical reactions accompanied by the evolution of heat energy, which may be harnessed of for such as the heating of homes, or which may be converted to other forms energy, such as mechanical energy in the steam engine or the internal combustion engine. One of the pioneering studies on the heat of chemical reactions was made by Lavoisier and ace in 1 Investigating the heat associated with various reactions, including combustion, they showed that each reaction was associated with a specific amount of heat and that the heat evolved in chemical reaction is equal to the heat absorbed in the reverse reaction. Lavoisier had proposed the modern theory of combustion as the combination of a substance with aygen. He also suggested the hypothesis that respiration is merely slow combustion. With Laplace, Lavoisier attempted to compare the quantity of heat given off by a burning candle with that given off by a live chicken. The quantity of heat was determined by measuring the quantity of ice that melted, in an insulated container, in the presence of the candle or the chicken. Little progress was made in thermochemistry until about fifty years later, when G. Hess (1802- 18500 studied heats of reaction in greater detail and stated the law a840) that bears his name: The heat evolved or absorbed in a chemical reaction is the same whether the process is accomplished in one step or in many. The law suggested, therefore, that both the reaction and the thermodynamic data could be treated algebraically. Although it was an empirical law when Hess proposed it, it was theoretically justified a few years later when the principle of conservation of energy was proposed. Hess's law can be illustrated by the following problem. Calculate the change in heat (AH) for the following reaction: 1. Mgo(s) 2H+(aq) Use the following information to solve the problem: AH- -35 kl/mole AH -56 kJ/mole 3, H+(aq) oHraq)