MID SEMESTER TEST

1. Pure substance x is solid at room temperature. if the substance is heated to 230 C is melted gradually. if then cooled to room temperature, the liquid can not be frozen.
a. is it possible x of an element or a compound. explain it!
b. Does it chemical change occured? if so can it be said undergo an endotherm changing, based on the information provided?
c. can it be said that the liquid is an element, based on the information provided.

2. When a candle that weighs 10 g is burned in oxygen, carbon dioxide and water vapor formed by combustion the weight more than 10 g. was this case match with the law of conservation of mass. Explain it!

3. When carbon burns in oxygen under limited number, it will form two gaseous compounds. Suggest the way to differentiate the two compounds with one another.

4. after Mendeleev compiled the periodic table, he concluded that the atomic weights of certain elements was wrong ruling, and this conclusion was apparently correct. How Mendeleev was able to predict the several atomic weights were wrong? why his predictions are not always right. Explain it!

5. When an aqueous solution of Merkuri chloride is added an aqueous solution of silver nitrate, a white solid forms. Identify the white solid and write the balanced equation for the reaction that occurs.

Answer :

1.a. According to me,The pure substance X is a compound,because the boiling point of the substance is very high, this possibility is an ionic compound. when frozen at room temperature the substance can not be frozen, because freezing of these compounds are also very high.so the pure substance x is a compound.

b.yes it can be said of any change in the chemical reaction.because after cooled to room temperature, a substance that is formed can not be restored to its original form or substance can be said to have a chemical change to form a new type of substance and its chemical composition of substances originally were. such as burnt paper and sugar are heated.can be said that the reaction that occurs is the endothermic reaction because the reaction is accompanied by heat transfer from the environment to the system (heat absorbed by the system from its environment) and is characterized by a decrease in ambient temperature around the system.

c.the liquid can be regarded as an element,because pure substance x is a compound that will break down into smaller compounds or elements when heated to a certain temperature (called the decomposition temperature).

2.when the candle burned with oxygen to form 10 grams of gas, namely carbon dioxide and 10 grams of water vapor, and formed a number of energy (heat and light). if the whole combustion accommodated and weighed, surely its mass equals the mass plus the mass of oxygen candles in use on burning it. reaction of candle burning:

2C20H42 (s) +61O2I (g )----> 40CO2 (g) +42 H2O (g)
10 grams 10 grams
substances before the reaction mass = mass of substance after the reaction

Law of Conservation of Mass
"The mass of the substance before the reaction is equal to mass substance of reaction"

3.Carbon forms two well-known oxides, carbon monoxide , CO, and carbon dioxide , CO2.
• Carbon monoxide.

Carbon monoxide is produced when graphite (one of the naturally occurring forms of elemental carbon) is heated or burned in a limited amount of oxygen.The reaction of steam with red-hot coke also produces carbon monoxide along with hydrogen gas(H2).(Coke is the Impure carbon residue resulting from the burning of coal.) This mixture of CO and H 2 is Called a water gas and is used as an industrial fuel. In the laboratory, carbon monoxide is prepared by heating formic acid, HCOOH, or oxalic acid, H 2 C 2 O 4 , with concentrated sulfuric acid, H 2 SO 4 .The sulfuric acid removes the elements of water (ie, H 2 O) from the formic or oxalic acid and absorbs the water produced. Because carbon monoxide burns readily in oxygen to produce carbon dioxide,

2CO + O 2 ® 2CO 2

it is useful as a gaseous fuel.It is also useful as a metallurgical reducing agent at high temperatures because it reduces many metal oxides to the elemental metal. For example, copper(II) oxide, CuO, and iron(III) oxide, Fe 2 O 3 , both are reduced to the metal by carbon monoxide.

Carbon monoxide is an extremely dangerous poison.Because it is an odourless and tasteless gas, it gives no warning of its presence.It binds to the hemoglobin in blood to form a stable compound.that is so that it cannot be broken down by body processes.When the hemoglobin is combined with carbon monoxide, it cannot combine with oxygen; this destroys the ability of hemoglobin to carry oxygen to all essential parts of the body.Suffocation can occur if sufficient amounts of carbon monoxide are present to form complexes with the hemoglobin.

•Carbon dioxide.
Carbon dioxide is produced when any form of carbon or almost any carbon compound is burned in an excess of oxygen.Many metal carbonates liberate CO 2 when they are heated.For example, calcium carbonate (CaCO 3 ) produces carbon dioxide and calcium oxide (CaO).

CaCO 3 + heat ® CO 2 + CaO

The fermentation of glucose (a sugar) during the preparation of ethanol, the alcohol found in beverages such as beer and wine, produces large quantities of CO 2 as a by-product.

C 6 H 12 O 6 ® 2C 2 H 5 OH+ 2CO 2

glucoseethanol

In the laboratory CO 2 can be prepared by adding a metal carbonate to an aqueous acid; eg,

CaCO 3 + 2H 3 O+ ® Ca²+ "" + 3H 2 O+ CO 2 .

Carbon dioxide is a colourless and essentially odourless gas That is 1.5 times as dense as water. It is not toxic, although a large concentration could result in suffocation simply by causing a lack of oxygen in the body.All carbonated beverages contain dissolved CO 2 ; hence the name carbonated.One liter (1:06 quarts) of water at 20 ° C dissolves 0.9 liter of CO 2 at one atmosphere, forming carbonic acid (H 2 CO 3 , discussed below in the section Oxyacids and Their salts), the which has a mildly acidic (sour) taste.Solid CO 2 sublimes at normal atmospheric pressure.Thus, solid CO 2 , Called dry ice, is a Valuable refrigerant That is always free of the liquid form. Carbon dioxide is also used as a fire extinguisher, because most substances do not burn in it, and it is readily available and inexpensive.Water containing as little as 2.5 percent CO 2 extinguishes a flame.

4. Dmitry Ivanovich concluded that the properties of elements are periodic function of the relative atomic mass. That is, if the elements are arranged according to the relative atomic mass increases, then the specific properties will be repeated periodically. Mendeleev placed elements with similar characteristics in a class of vertical columns called groups. Mendeleev emptied several places. This is done to determine the similarity of traits in the classroom. Mendeleev also predicted the nature of the elements that have not known it. The forecast is based on the known properties of other elements, which is adjacent either horizontally or upright. elements are expected to be found, and it accords well with the predictions of Mendeleev. One example is a germanium (Ge), which was discovered in 1886, which by Mendeleev named ekasilikon. The weakness of this theory still contains elements of the larger mass is located in front of the element whose mass is smaller. Co: Tellurium (te) = 128 in kiriIodin (I) = 127. This is because the elements that have similarity properties are placed in one class. The weakness of this theory is the atomic mass correction. Previous atomic mass. In a previous atomic mass = 76-113. Be apart, from 13.5 to 9. N 120-240.

Mendeleev was right to put tellurium in the same group with sulfur and oxygen; however, strict order of atomic weights According to the best information available Had he would have required iodine (127) to come before tellurium (128).e was Suspicious of this apparent inversion of atomic weight order; as it Happens, the atomic weights Had Mendeleev available to him agree with the currently accepted values.

5.HgCl2 + 2AgNO3 ---> Hg (NO3) 2 + 2AgCl
The resulting white powder is silver nitrate.